The task is "Wake up and do as you please." — so I have freedom to create a new Three.js demo in /space/demos/. The last demo was 045-critical-point (an Ising ferromagnet laboratory). Let me check the current state of the demos folder to see what exists and then create a new, impressive demo.

Let me start by installing shell tools and exploring the space.

Let me explore the current state of the demos folder.

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Let me look at a recent demo card in the index, and check the structure of a recent demo (045) to see conventions. Then I'll decide what new demo to create — number 046.

Let me think about what demo to create. Recent demos:

• 044-photon-garden
• 045-critical-point (Ising model laboratory)
• 043-hyperfold
• 042-neon-tilt
• 041-squish-lab
• 040-lumen-lab
• 039-kinetic-theory
• 038-sandpile
• 037-epicycle
• 036-wavefunction

I need something fresh. Ideas for demo 046:

1. Reaction-diffusion 3D — done partially by morphogenesis (030)? 030-morphogenesis probably covers reaction-diffusion. Skip.
2. Cloth/soft-body — 041-squish-lab may be soft-body physics. Skip.
3. Double pendulum / chaotic pendulum gallery — "chaos" partially covered by 019-attractorium (strange attractors).
4. Game of Life 3D / cellular automata — could be cool: a GPU-based 3D cellular automaton with rules like 5766 (Amoeba, Builder, etc.), voxel cubes instanced, glowing. Might be too similar to morphogenesis/critical point.
5. Wormhole / space warping — 006-event-horizon covers black hole.
6. Fireflies swarm sync — 026-synchrony probably covers synchronization (Kuramoto?). 020-murmuration is boids. Skip.
7. Terrain/erosion sim — 021-terraforge likely terrain. Skip.
8. Raymarched fractal — 003-mandelbulb covers it.
9. Tensegrity / rope physics — partially covered?
10. Electric field line explorer / charged particles — 040-lumen-lab might be optics lab. 034-magnetosphere covers charged particle in magnetic field.
11. Diffusion-limited aggregation (DLA) — crystal growth... 023-frostline is probably frost/snowflake. Hmm.
12. Voronoi shatter / procedural destruction — fun but heavy.
13. N-body galaxy collision — 005-gravity sandbox covers N-body.
14. Hyperbolic geometry / Escher-like — 043-hyperfold might be hyperbolic folding.
15. Ocean waves FFT — 009-azure-odyssey maybe ocean. 022-cloudsea. Skip.
16. Double-slit / wave interference tank — 036-wavefunction is quantum. 032-wavefront maybe wave propagation. Hmm.
17. Plasma globe / electric arcs — 028-teslasphere covers it.
18. L-system 3D plants / generative garden — 044-photon-garden is maybe photon mapping? "Photon garden" hmm.
19. Flocking fireflies + light trails — done boids.
20. Space elevator / orbital mechanics game — hmm.
21. Fire simulation — 029-pyrosymphony likely fire audio.
22. Bubble raft / soap film minimal surfaces — cool: soap film physics with minimal surfaces?
23. Ray-traced glass caustics — 040-lumen-lab is optics.
24. Pinball / marble machine — 031-galton-cascade is Galton board.
25. Gravity wells spacetime fabric visualization — classic: a mesh deforming under masses. Might be partially covered by gravity sandbox/event horizon, but a "spacetime fabric" trampoline with rolling marbles is distinct enough... Still similar to 005/006.
26. Knot theory / torus knots morphing — meh.
27. Voxel engine minecraft-like — heavy.
28. Asteroid mining game — 008-void-runner is a game.
29. Snake 3D — meh.
30. Lorenz weather lab — attractors done.
31. Turing machine / rule 110 CA gallery — could do "elementary cellular automata" as falling sand-style rows building pyramids of patterns. Actually a beautiful idea: Cellular automata tapestry — elementary CA (rule 30, 90, 110, 184) rendered as growing woven fabric in 3D, with audio. Distinct from morphogenesis (reaction-diffusion continuous).
32. Superconductor levitation / magnetic levitation sim — 045 covered ferromagnetism.
33. Traffic flow simulation (Nagel-Schreckenberg) — could be cool: 3D city with cars, but big.
34. Ant colony / pheromone trails — 011-physarum covers slime mold (similar).
35. Fourier epicycles drawing — 037-epicycle probably covers epicycles/fourier. Skip.
36. Double pendulum with trail art + perturbation split — chaos visualized. Partially attractors. A "Pendulum Wave" machine is gorgeous: pendulum wave + 3D. Hmm, synchrony (026) might be pendulum wave/metronome sync.
37. Voronoi planet / procedural planet — terraforge covers terrain.
38. Raymarched metaballs goo — ferrofluid (025) covers metaballs.
39. Chladni plates — 027-cymatics covers it.
40. Tides / moon orbital sim — hmm.
41. Electric guitar string / wave equation plucked strings with FFT audio — audio-heavy. 007-sonic-reactor is audio.
42. Gel/SPH fluid — 004-fluid-dynamics, 041-squish-lab. Skip.
43. Origami / paper folding — hyperfold (043) likely folding. Skip.
44. Quantum tunneling / orbital clouds — 036-wavefunction. Skip.
45. Solar system orrery with real ephemeris — could be lovely, but 005 gravity sandbox & 037 epicycle adjacent. Hmm.
46. Tesseract / 4D polytopes explorer — 043-hyperfold might be 4D folding. Risky.
47. Light clock / special relativity — relativistic effects visualization (Terrell rotation, aberration, Doppler). Distinct! "Relativistic fly-through" with latticework city. Complex to do accurately but doable: approximate with vertex shader? Real relativistic rendering is heavy. Skip.
48. Forest fire CA + percolation — cool: percolation threshold demo. Similar to sandpile (038)? Sandpile is abelian sandpile (Bak-Tang-Wiesenfeld). Forest fire/percolation adjacent. Hmm.
49. Kuramoto oscillators on 3D surfaces — synchrony covers.
50. Musical sequencer + physics — neon-tilt (042) is a tilt maze game? Not sure.
51. Game of Life on a torus with glow — GoL in 3D torus surface? Could be pretty but CA-ish.
52. Hopalong / Gumowski-Mira orbits — attractors again.
53. Flocking + predator-prey ecosystem sim — "Ecosystem" with agents, energy, reproduction, graphs. Murmuration is boids, but an ecosystem with multiple species, food webs, population charts is different and juicy. Could do a 3D terrarium: grass grows, herbivores eat, predators hunt. Population dynamics chart. Instanced critters. That's a nice "wow" demo.
54. Crystal growth (snowflake) sim — frostline covers.
55. Vortex methods / smoke rings — fluid-adjacent.
56. Pendulum wave machine — check 026-synchrony content first.
57. Turing patterns on 3D animals — morphogenesis. Skip.
58. Mandelbrot deep zoom — mandelbulb covers fractals.
59. Tensegrity bridge builder — bridge-building physics game: nodes + beams, stress visualization (like Poly Bridge). That's juicy: verlet physics, stress colors, destruction. Distinct from existing demos! "Bridge Lab" — build trusses over a canyon, test with traffic, see stress colors, collapse. Interactive game-like. Could be great.
60. Marble run builder — galton adjacent.
61. Rocket landing game (lunar lander with thrust vectoring) — game. 008-void-runner is a game (runner). A physics-y game is fine.
62. Tidal locking / three-body figure-8 — gravity.
63. Kaleidoscope shader — simple.
64. Particle life — "Particle Life" (cluster-based emergent behavior with attraction/repulsion matrices) — VERY cool and distinct! Thousands of colored particles with species interaction matrices producing emergent amoeba-like structures. GPU compute (though WebGL needs transform feedback or texture-based). Distinct from physarum (trail-based). Could do CPU with spatial hashing for ~2000 particles, or GPU. Emergent "cell" structures look alive. Nice.
65. Conway's Game of Life but continuous (SmoothLife) — distinct-ish from morphogenesis? Morphogenesis is probably reaction-diffusion (Gray-Scott). SmoothLife visually similar. Skip.
66. GravityAssist / Kerbal-style orbital toy — gravity adjacent.

Let me check what some ambiguous demos actually are: 026-synchrony, 030-morphogenesis, 043-hyperfold, 044-photon-garden, 041-squish-lab, 018-threshold, 033-gossamer, 016-skitter, 010-abyssal-bloom, 012-powder-alchemy.

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The demos so far: 001 cube, 002 galaxy, 003 mandelbulb, 004 fluid, 005 gravity, 006 black hole, 007 audio reactor, 008 game, 009 ocean journey, 010 abyssal bloom (deep sea), 011 physarum, 012 powder (falling sand), 013 clockwork, 014 neon rain, 015 aurora, 016 skitter (bugs), 017 coral, 018 threshold, 019 attractors, 020 murmuration (boids), 021 terraforge (terrain), 022 cloudsea (clouds), 023 frostline (frost/snowflake growth), 024 koi pond, 025 ferrofluid, 026 synchrony (sync oscillators), 027 cymatics, 028 teslasphere (arcs), 029 pyrosymphony (fire + audio), 030 morphogenesis (reaction-diffusion), 031 galton, 032 wavefront (wave sim), 033 gossamer (spider web), 034 magnetosphere, 035 cloud chamber, 036 wavefunction (hydrogen orbitals), 037 epicycle (fourier), 038 sandpile, 039 kinetic theory (gas), 040 lumen lab (optics), 041 squish lab (soft body), 042 neon tilt (pinball), 043 hyperfold (4D polytopes), 044 photon garden (path tracing), 045 critical point (Ising).

For demo 046, great candidates that don't overlap:

1. Bridge/truss builder with stress sim — structural engineering game. Distinct! Poly-Bridge-like: build trusses, run a train/truck over, watch stress colors + collapse. Uses verlet/XPBD physics. Very interactive, game-like, and educational.
2. Particle Life — emergent multi-species particle systems. Visually alive, distinct from physarum (which uses pheromone trails) — particle life uses direct attraction/repulsion. Mesmerizing.
3. Ecosystem terrarium — agents + food web.
4. Double pendulum chaos gallery — attractors adjacent.
5. Knot/tensegrity — meh.
6. Ant colony — physarum adjacent.
7. Turing machine music — meh.
8. Cellular automata tapestry — sandpile is CA but visually different genre; a 3D voxel CA gallery (Amoeba, Builder, Coral rules) is quite pretty. But 038 sandpile + 030 morphogenesis + 012 powder alchemy (falling sand CA) already cover a lot of CA territory. Skip.

I think "Particle Life" would be gorgeous but it's somewhat in the family of boids/physarum. The bridge builder is very interactive and different — physics game. The last few demos were heavy on physics-lab themes (Ising, path tracing, 4D, pinball, soft-body, optics, gas). A structural-engineering game with collapse dynamics would be fresh and fun.

Alternatively: "Tensegrity / rope bridge". Or "Marble machine / Rube Goldberg builder" — 031 galton is a fixed cascade; a marble-run builder is a builder game.

Let me go with the truss bridge builder: "Bridge Works" / "Load Test" — a bridge engineering sandbox:

• A canyon with two cliffs, water below.
• Grid-based build mode: click/drag to place beams (road deck + steel members), auto-cost, budget.
• Materials: steel, wood, cable? Maybe road + steel + cable.
• Test mode: a truck/train drives across; physics via XPBD: nodes as particles, beams as distance constraints with breaking stress; stress colorization (green→red), real-time collapse into the river with splashes.
• Sound: creaks proportional to stress, metallic snaps, splash, rumble as truck crosses, success fanfare.
• Extras: wind, earthquake test? Maybe a "quake" button that shakes the canyon. Multiple levels (gap widths, anchor points). Preset bridges (arch, suspension, cantilever...). Cost + stress readouts, "max load" report.

Physics: 2D-ish bridge in a vertical plane but rendered in 3D with nice depth — actually, let me do the bridge as a full 3D structure? Full 3D truss builder is complex for UI. Classic Poly Bridge is 2D. I could do 2.5D: two parallel 2D trusses (left and right side of the deck) — physically simulate one 2D plane ×2? Simpler: simulate in a 2D plane, render extruded in 3D with two mirrored frames + deck + cross bracing for looks. That's a good balance.

Physics implementation: XPBD particles for nodes, distance constraints for beams. Beams break when strain exceeds threshold. Truck: a vehicle with two wheels as particles connected by rigid constraints, wheels interact with "road" beams via collision (wheel vs beam segment). Simpler: truck moves along the road deck nodes — i.e., the truck applies downward load at deck nodes nearest to its position, and advances if the deck under it is intact; falls otherwise. Hmm, but proper wheel-on-beam collision is cooler: wheels are circles colliding with beam segments (line segments), which handles the truck falling into gaps and driving over deformed decks. I'll implement circle-vs-segment collision between wheels and road beams.

To keep it tractable:

• 2D physics plane (x = horizontal along bridge, y = vertical), rendered in 3D.
• Nodes: particles with mass, gravity.
• Beams: distance constraints with compliance (XPBD), each has material properties (stiffness, break strain, cost, mass).
• Anchors: fixed nodes at cliff edges (and possibly intermediate anchor points in the canyon, e.g., rock pillar in some levels).
• Road beams: beams flagged as drivable; wheels collide with them.
• Truck: chassis node + two wheels (particles), springs between, motor force propelling forward at speed, load mass.
• Breaking: monitor strain each frame; if |strain| > breakStrain for N frames, remove constraint, spawn debris particles + sound + spark.
• Water: y < waterLevel → node sinks with drag, splash particles + sound. Debris splashes.
• Success: truck reaches far side → fanfare, confetti? report: cost, max stress, deflection.
• Failure: truck falls in water → splash, "structure failed".

Editor UX:

• Build grid: snap to grid (e.g., 1m). Drag from node to node to create beam; click beam to delete (right-click or X mode); choose material: Road (deck), Steel, Wood, Cable. Auto-create nodes.
• Modes: Build / Test / Erase; keyboard shortcuts.
• Budget display; cost per meter per material.
• Presets: "Suspension", "Arch", "Truss classic", "Cable-stayed", "Drawbridge"? Generate procedurally.
• Levels: gap width variants, with/without central rock pillar anchor.
• Extras: wind gusts slider? Quake button (shake anchors) — fun destruction. Undo/reset. "Stress view" toggle (heat map on beams always on actually). Camera: orbit around, but keep bridge plane central.

Rendering in 3D:

• Canyon: two cliffs (procedural rock, displaced boxes/extruded shapes), river below with animated shader, sky, maybe distant mountains, fog.
• Bridge: each 2D beam rendered as two 3D beams at z=±deckHalfWidth plus cross members every node; deck rendered as box segments with road texture stripes. Beam color by stress: use vertex colors or per-instance color via InstancedMesh (each beam instance scaled between endpoints). Update instance colors each frame for stress heat.
• Wheels/truck: simple stylized truck mesh (boxy cab + wheels as torus/cylinder).
• Splashes: particle burst; debris: instanced small boxes falling.
• Bloom post-processing, vignette, grain — consistent with other demos.
• Generative audio: wind ambience, river noise, creaks (filtered noise bursts pitched by stress), snap (metallic ping), splash (noise burst), truck engine rumble, success chord.

That's a hefty but doable single-file demo. Complexity is high — maybe 2000+ lines. Let me structure it well. Given the "advanced" guideline, this fits: interactive game + physics + collapse dynamics.

Alternative simpler-but-still-wow: "Particle Life" — ~800 lines. But the bridge builder is more novel within the collection. Time budget: previous demos took ~13 min. I'll aim for efficient implementation, building the file in parts and syntax-checking with node.

Let me check 045's structure quickly for conventions (import maps, quality tiers, UI patterns) to reuse. Let me peek at its HTML head and key parts.

Good. Conventions confirmed:

• Three.js 0.170.0 via unpkg importmap
• EffectComposer + UnrealBloomPass + ShaderPass (grade) + OutputPass
• Quality tiers with autoscale
• Generative WebAudio
• Panels: HUD top-left, stats top-right, controls, chart bottom-left, help overlay
• Snapshot PNG, keyboard shortcuts, touch support

Now, demo 046: "Load Test" / "Bridge Works" — a truss-bridge engineering sandbox with XPBD physics and collapse dynamics. Let me design it carefully, then write the file in parts.

Design: "Bridge Works" (046)

Core concept

A canyon crossing challenge: build a bridge between two cliffs over a river. XPBD 2D truss physics (nodes = particles, beams = distance constraints that snap when over-strained). A test truck drives across. Real-time stress heat-map, dramatic collapse, splashes. Budget + cost. Presets + procedurally generated starter bridges. Editor with grid snap, materials (road deck, steel, wood, cable). Quake & wind tests.

Physics (2D plane, XPBD)

• Nodes: x, y, prevX, prevY (verlet) or XPBD with velocity. I'll use position-based dynamics: pos, prevPos, invMass. Gravity, damping, floor/cliff collisions, water.
• Beams: constraint between nodes a,b with restLength. XPBD with compliance α: stiffer for steel, softer for wood; cables: only tension (push apart allowed? cables resist only stretching: apply constraint only when dist > rest).
• Strain: (dist - rest)/rest. If |strain| > breakStrain (tension & compression separate thresholds maybe), beam breaks → remove, spawn debris, sound.
• Substeps: e.g., 8 substeps per frame at 60fps, classic XPBD small-step.
• Solve order: integrate → solve constraints (Gauss-Seidel over beams, random order or sorted) → collisions.
• Anchors: invMass=0 nodes at cliff edges (rock anchors). Some levels have a mid-canyon pillar anchor.
• Wheels vs beams: circle-segment collision for road beams: project wheel center onto segment, if dist < wheelR and beam is road && beam alive → push node & wheel apart (impulse split by inverse mass). Also add friction/drive: motor force along road direction.
• Truck: two wheel particles + chassis particle. Wheels: radius 0.45m. Chassis connected by springs to wheels. Motor: apply horizontal force on wheels toward target speed while either wheel touches road. Load: mass.
• Water: y < waterY → strong drag + buoyancy for nodes (they float/sink slowly), splash on entry; truck wheels submerged → test failed.
• Wind: horizontal force on beams (per node), gusty noise. Quake: oscillate anchor positions / apply global accel.

Editor

• Grid: 1m spacing; bridge plane spans x from cliffL to cliffR; y around deck level.
• Tools: Road, Steel, Wood, Cable, Erase, Move (drag node?) — keep: place beams by dragging node-to-node (click empty = create node + beam from last node? Simpler: click-drag from A to B creates/uses nearest nodes within snap radius). Right-click or Erase tool deletes beam under cursor (nearest beam within threshold).
• Materials table:
  • Road deck: cost 150/m, mass 120 kg/m... keep simple units. Break strain tension 8%, compression 10%, stiff. Required for truck to drive.
  • Steel: cost 100/m, break 6%.
  • Wood: cost 40/m, break 4%, less stiff, lighter.
  • Cable: cost 60/m, tension-only, break 12% strain (cables stretch), very light.
• Budget: level-based, e.g., 12000. Show cost, remaining.
• Max beam length: 8m (prevent silly long beams) — show red ghost if invalid.
• Undo (Ctrl+Z), Clear.
• Levels: "First Crossing" (24m gap, budget 8000), "Wide Canyon" (36m, 15000), "The Pillar" (32m gap with central rock anchor, 12000), "High Winds" (28m + wind, 10000)? Keep 4 levels.
• Test: truck drives from left cliff. Speed slider? Truck mass options (Light van / Truck / Heavy haul — heavier load = more stress). Score: success + remaining budget + max stress margin.
• Collapse detection: count broken beams; truck splash → failed.
• Quake button: shakes for 3s. Wind slider for gusts.

Rendering (3D)

• Bridge plane at z=0. Each 2D beam → 3D as two parallel members at z=±1.1m connected to deck? Simpler & prettier: render each beam as a box from a to b at z offset ±1.0 (twin frames) + deck planks on road beams across z. Cross-bracing: X struts at each node pair every other node. Use InstancedMesh: one unit box geometry, per-instance matrix (position midpoint, rotation around z from angle, scale (len, thick, thick)), plus instance color for stress. Two instances per beam (z=±1). Road beams additionally get deck plank instances (across z, width 2.6m) with lane stripe texture? Simple color/roughness.
• Stress colors: instance color lerp green→yellow→red (tension vs compression maybe blue tint for compression). Broken beams: remove instance (swap-with-last).
• Ghost preview beam while dragging (line/box, green ok / red invalid).
• Grid: faint dots/lines on build plane (only visible in build mode).
• Canyon: procedural cliffs — big displaced geometry: use PlaneGeometry with noise displacement rotated to vertical? Easier: build cliff from extruded boxy noise via custom BufferGeometry? Simplest good look: use several stacked noise-displaced boxes / or a Shape+Extrude: cross-section polygon with jagged top edge. Or use "Wall" meshes: create a heightfield strip along x,z at canyon sides. Let me do: cliff = box geometry (depth 30, width cliffW, height 25) with vertex noise displacement on its outer faces, MeshStandardMaterial vertexColors with rock gradient + strata stripes. Two of them, mirrored. Plus a central pillar for the pillar level (rock cone/cylinder with noise).
• River: plane with animated shader (flow noise, sparkles), semi-transparent, at y=waterY (-12m below deck).
• Waterfall mist? skip.
• Sky: gradient dome + sun + distant mountain silhouettes (large cones with fog). Fog for depth.
• Clouds: few billboard sprites drifting? Cheap: big soft sprites.
• Truck: small stylized low-poly truck: box chassis + cab + 2 visible wheels (cylinders) at z=0? Truck rendered at z=0 crossing. Wheels rotate by speed. Physics wheels at z=0 plane.
• Debris: InstancedMesh small boxes with velocity+spin, fade after splash, splash particles (instanced quads rising & fading), dust puffs on snaps.
• Flags/banners on towers for wind visualization? Cute: pennant flags at cliff edges flapping with wind (skinned planes or just rotating cones). Maybe simple cloth triangle flags on poles at both ends, flutter via vertex shader — nice wind readout.
• Camera: OrbitControls with damping, default 3/4 view. During test, optional follow-cam toggle tracking truck.

Audio (WebAudio, generative)

• Ambience: river noise (filtered brown noise) + wind (bandpassed noise with gust LFO).
• Creaks: stress-driven — pick random highly-stressed beam, play short creak (sawtooth with noise, pitch by strain) throttled.
• Snap: metallic crack (noise burst + highpass + ping).
• Splash: filtered noise burst with lowpass sweep.
• Truck: engine rumble (saw + sub osc, pitch by speed) while testing + horn at start? Fun: horn honk on test start.
• Success: major chord arpeggio; fail: sad descending tone.
• Quake: low rumble.
• Mute toggle + volume.

UI

• HUD top-left: title "BRIDGE WORKS", level name, status line.
• Stats top-right: budget bar (used/total), cost, beams count, max stress %, live "integrity" bar, truck status.
• Toolbar bottom-center: material buttons (Road/Steel/Wood/Cable/Erase), Test (space), Reset bridge, Undo, load select, truck weight select, wind slider, quake button, help.
• Toasts for events ("Beam snapped!", "Truck reached the far side! + score").
• Help overlay (H): controls list.
• Chart? Maybe a live stress histogram or load-deflection curve... Could add small canvas: deck deflection over time during test. Nice touch: "deflection vs time" chart bottom-left. Keep simple: deflection sparkline during test.
• Snapshot PNG button. Adaptive quality tiers (pixel ratio, bloom, debris counts). Touch support: tap-drag beams, buttons.

Modes & flow

• BUILD mode: edit, physics paused (or settled static), ghost beam preview, grid visible.
• TEST mode: physics live, truck spawns & drives; can pause/reset; auto-return to build after outcome? Keep running; R resets truck keeping bridge; B back to build (repairs? No—broken stays broken until repaired: beams broken during test must be rebuilt or "Repair all" button restores). Simpler & friendly: exiting test restores bridge to pre-test state (snapshot). That's standard (Poly Bridge does that). Yes: on Test, snapshot structure; on stop/back-to-build, restore.
• Also "Drive" manual mode? Skip — auto-drive with speed set by weight choice.

Scope check: this is big. Aim ~1600-2000 lines total. Let me be efficient and write it in 4-5 chunks, then validate JS with node --check after extracting module script, plus check GLSL braces.

Key correctness concerns:

• XPBD constraint solve with breaking.
• Wheel-beam collision robustness (swept? use substeps 8×60Hz = 480Hz physics; truck speed ≤ 6 m/s → 1.25cm/step, fine vs wheel radius 0.45m).
• Cables tension-only.
• Ghost preview picking: mouse ray → plane z=0 intersection → snap to grid.
• Beam/node dedup: nodes keyed by grid coords "gx,gy"; beam key sorted node ids; prevent duplicates & zero-length.
• Editor max span 8m, must connect within build zone x∈[cliffL-2, cliffR+2], y∈[-14, 14]?

Level geometry:

• Deck level y=0. Cliff top y=0. Gap from x=-gap/2 to +gap/2. Cliffs extend outward. Anchors: at cliff edges provide anchor nodes at y=0 and y=-3, x=±gap/2 (and maybe x=±(gap/2+2)). Anchor nodes have invMass 0. Pillar level: anchor nodes at x=0, y=-8, -4, 0 on top of rock pillar from river.
• River y = -11.
• Build zone y ∈ [-10, +12], x ∈ [-gap/2-6, gap/2+6].

Truck: start x = -gap/2 - 6 on cliff road (cliff top has road surface — wheels collide with road beams only; so include fixed "ground road" segments on cliffs: static road beams from x=-gap/2-12..-gap/2 at y=0 (anchors both ends? mid nodes anchored too) — give each cliff a static road strip as anchored road beams. Same right side x=gap/2..gap/2+12. Truck drives left→right at target speed (per weight: van 5 m/s 1.5t, truck 4 m/s 4t, hauler 3 m/s 8t). Success when chassis x > gap/2 + 4.

Physics units: meters, kg, seconds. Gravity -9.81. Node mass: from connected beams/2 (recompute on build change), clamp min 20kg. Beam mass per m: steel 25, wood 12, road 40, cable 4. XPBD compliance α: steel 2e-7, wood 8e-7, road 1e-7, cable 1e-6 (tension only). Actually tune: with substeps 8, XPBD effective stiffness is high; compliance values need experimentation but I'll pick plausible and rely on PBD-ish behavior (can also just use small compliance ≈ 0 for near-rigid). Use compliance 0 for road/steel (rigid), 1e-8 wood? Hmm — risk: too rigid = jitter. Classic PBD distance projection (compliance 0) works fine for bridges; wood slightly compliant 5e-7, cable 2e-6. Break strains: steel tension 0.055 compression 0.075 (buckling), wood 0.04/0.05, road 0.09/0.12, cable 0.14 tension only. Hmm rigid PBD makes strain values jumpy; but with truck loads strains stay small. Visual stress metric: use constraint force estimate = |Δx| * something... Better: stress shown = |strain| smoothed (EMA) normalized by break strain. Fine.

Node count limits: cap nodes at ~400, beams ~800. Grid snap 1m; gap 24-36m; fine.

Solver perf: 800 beams × 8 substeps = 6400 projections/frame — trivial. Wheels collision vs road beams: broadphase by x range. OK.

Debris: on beam break, spawn 2-3 debris chunks instanced boxes with velocity; cap 300. Splash: particle burst at water entry for nodes/debris/truck.

Rendering beams: InstancedMesh count = beams*2 (twin frame) + planks for road. Rebuild instance list when topology changes; update colors every frame (stress) and matrices when nodes move. Node positions change every frame during test → recompute matrices for all instances each frame: 1600 matrix compositions — fine.

During BUILD mode, physics static: nodes at rest positions (no gravity settle). Actually fun: run physics in build mode too but nodes pinned? No — bridges sag on load only during test; in build mode keep static (positions as placed). During test, run sim.

Quake: applies sinusoidal horizontal acceleration to all nodes (and anchors?) for 3s. Wind: per-beam force normal to beam? Apply at nodes: force = windPressure × beamLength × sin(gustNoise(t + phase)). Keep simple: global horizontal force on each node proportional to exposed length, gusts via summed sines.

Failure/success logic:

• Truck rear wheel or chassis y < waterY+0.5 → "SPLASH — test failed" (after 1s in water).
• Chassis x > gap/2+4 → success → score = remainingBudget bonus + integrity.
• Broken beam count > 0 → warning toast each snap with count.
• If truck stuck (speed < 0.2 for 4s while in gap) → "stuck" fail option to stop.

Chart: deflection of mid-deck node (max downward y of any road node) vs time during test — small canvas 220×90, plus stress histogram? Keep deflection line + max-stress line (two colors).

Camera follow toggle (F): lerp camera target to truck.

Let me also add preset bridges (auto-build): "King Post Truss", "Pratt Truss", "Arch", "Suspension", "Cable-stayed". Auto-generate within budget. Presets build into current level if affordable & fits geometry. These give instant gratification.

Preset generation (parametric by gap G, deck y=0):

• Pratt truss: deck road beams across, verticals every 3m up to height 4, diagonals toward center on top chord, top chord steel. Standard.
• Arch: parabolic arch below deck y = -h*(1-(2x/G)^2) with hangers to deck; anchors at cliff bases (x=±G/2, y=-3 anchor). Hmm anchors at (±G/2, 0) fine.
• Suspension: towers at x=±G/2-? Actually towers at cliff edges (x=±(G/2+1)?). Towers must stand on cliffs: tower base anchored at x=±(G/2+2)? Build zone extends 6m onto cliffs. Tower at x=±(G/2+1.5), height 8; main cable parabola between tower tops sagging to y=1.5 at mid; hangers every 3m; cable continues to anchor at x=±(G/2+5). Deck road across. Cables as cable material, towers steel, hangers cable.
• Cable-stayed: single tower at left cliff x=-(G/2+1.5) height 10, stays fan to deck points every 4m; plus symmetric second tower right. Deck road.
• King post: for small gaps: deck + A frames.

These are doable parametrically — add beams via same addBeam API (with dedup).

Sounds good. Extra flourish: "Hydraulic press" mode? No, enough.

Name: "Bridge Works"? Or "Load Test — Bridge Engineering Sandbox". Title: "Load Test". Folder: 046-load-test. Hmm "Bridge Works" is clearer. Let me go with 046-bridge-works, title "Bridge Works".

Let me now write the code. I'll write in parts to /tmp then assemble to /space/demos/046-bridge-works/index.html. Actually simpler to write directly in chunks with heredocs? The read_write tool writes whole files or ranges. I'll write part files and cat them together (like previous run did), then extract and node --check the module.

Part plan:

• p1: HTML head + CSS + body panels + importmap + script start: imports, config, state, audio engine.
• p2: physics core (nodes/beams store, materials, build API, presets, levels).
• p3: solver (substeps, constraints, collisions, truck, water, wind, quake), stats (stress EMA, deflection).
• p4: three.js scene (cliffs, river shader, sky, flags, bridge instancing, truck mesh, debris, splashes, ghost, grid).
• p5: UI wiring (toolbar, editor input, raycast picking, modes, chart, toasts, help, snapshot, quality tiers, keyboard/touch), main loop.

Given message size limits, I'll write ~5 chunks each maybe 300-450 lines. Total ~2000 lines. Let me be disciplined and compact but readable.

Let me define precise data structures:

Nodes stored in Map key ${gx},${gy} (grid ints). Node: {id, gx, gy, x,y, px,py, invM, anchor, alive}. Grid 1m. Positions = grid coords in meters (x from -24..24 etc).

Beams Map key ${aId}|${bId} sorted. Beam: {id, a, b, mat, rest, alive, strain, stressEMA, brokenAt}.

Physics arrays: I'll keep object arrays and iterate — 800 objects × 8 substeps is fine in JS.

Solver:

Wheels vs road beams: for each wheel (particle w with radius R), for road beams with |beam.midX - w.x| < beam.len/2+R+margin: compute closest point on segment; if dist < R: push wheel out along normal n by (R-dist) * (wb... wheel is heavy: give wheel invM relative to beam nodes) — apply positional correction split between wheel and beam nodes (each node weighted by distance along segment). Also friction: relative tangential velocity damp → motor: add tangential force to reach target speed: after collision, wheel drive: vx += (targetV - vxAlong)*grip. Grip 0.5.

Truck: wheels w1,w2 (mass mEach = truckMass0.35), chassis c (mass 0.3M) connected: chassis to each wheel by rigid distance? Use spring constraints (soft distance) + chassis upright: simpler: chassis connected to wheels with two distance constraints forming triangle (w1-w2 dist fixed 2.2, w1-c 1.6, w2-c 1.6). Rigid-ish with compliance 0. Wheels have rotation for rendering: angle += vx/R*h.

Wheels are nodes too (in nodes array with special flag, invM from truck mass). Chassis same. They participate in gravity & water. Chassis collision vs road beams too (small radius 0.3) so truck body rests if wheels break through? Fine add chassis collision too.

Anchor nodes: invM=0.

Node mass recompute: for each beam alive: mass/2 to each node. invM = anchor||truck? careful: truck nodes fixed mass. Keep flag.

Strain/stress per substep: strain = (dist-rest)/rest; stressEMA = lerp(stressEMA, |strain|, 0.05) computed once per frame (not substep) for display; breaking check per substep: tension: strain > breakT → break; compression: -strain > breakC → break (wood/steel buckle). Add tiny random per-beam break threshold variation ±8% for organic failures. On break: alive=false, spawn debris + snap sound + toast counter, remove instance.

Also over-stressed creak: beams with stressEMA > 0.5*break → random creak sounds, and beam "shiver" visual? skip shiver.

River shader: plane 200×40 with fragment: flow noise stripes + fresnel sparkles; uniform time. Plus underwater tint fog: scene.fog handles.

Cliffs: I'll build with BoxGeometry(8, 26, 60, 6, 10, 6) displaced by noise on CPU, vertex colors rock strata. Positioned x=∓(gap/2+4), top at y=0 → center y=-13. Left cliff occupies x<-gap/2: center x = -(gap/2 + 4). z spans 60. Looks fine with noise on the gap-facing face.

Also road surface on cliffs: dark strip boxes at y=0.02 spanning z∈[-1.4,1.4], x from edge outward 12. With center line dashes (texture via canvas?). Simple: dark box + white small boxes dashes. Truck path y=0 at wheel contact → wheel center y=R when on deck.

Static road on cliffs as collision: implement as "ground beams" — road material beams with anchors at both ends and mid nodes anchored: from x=-(gap/2)-12+? Let me create them at build: for x in [-gap/2-12 .. -gap/2] step 3: road beams, all nodes anchored (invM 0). Same right. These are part of structure but flagged fixed=true, not counted in cost, not breakable (set breakT huge). They also render as beams — but visually the cliff road strip box covers them. Actually to avoid double render weirdness, render ground road beams as planks only (no twin frames). Add flag.

Presets build between the gap; must clear existing player beams first (keep fixed ground).

Budget: level.budget; cost = Σ beam cost*len (non-fixed). Over-budget: prevent adding beam (toast "Over budget").

Undo: stack of ops {type:'add', beam} {type:'del', beam} (player only).

Test flow:

• Press TEST (Space/T): require at least deck continuity? Not required—truck will fall, that's the fun. Snapshot: serialize player beams (mat, a, b keys) & nodes grid keys. mode='test'; spawn truck; physics on; deflection chart reset.
• STOP (R or button): restore snapshot (remove broken changes), mode='build', truck removed, debris cleared.
• Auto: on success/fail show toast + score, keep sim running (watch wreck), user presses R to return.

During test, editor disabled (clicks ignored or allow "emergency repair"? no).

Quake button during test: shake. Wind slider active during test (and gentle in build? only test).

Level select changes geometry → rebuild cliffs, ground, clear player structure, reset budget, camera maybe.

OK also " integrity" readout: % of player beams unbroken.

Scoring: on success: score = 1000 + remainingBudget0.1 - brokenCount50; show maxStress %. Persist best per level in localStorage? Fine, small.

Alright — also need a nice "help" overlay and touch: touch drag creates beams (two-finger orbit?). OrbitControls handles one-finger rotate which conflicts with drawing. Solution: editor drawing only when pointer is over build plane AND mode=build AND using mouse/touch with a "Draw" toggle? Poly-bridge on touch: default one finger = draw when starting on empty plane; orbit via two fingers; a toggle button switches one-finger behavior. Simpler: desktop: mouse drag draws (left button), orbit with right-drag / ctrl+drag; set OrbitControls.mouseButtons LEFT=null? I'll set OrbitControls to use RIGHT for rotate, MIDDLE pan, wheel zoom; LEFT reserved for editor. Touch: two-finger orbit/pinch (controls default with touches.TWO), one-finger = draw. Set controls.touches = { ONE: null, TWO: THREE.TOUCH.DOLLY_ROTATE }. That works: touch-action none.

Also add a subtle "build plane" visual: translucent vertical plane at z=0 spanning build zone with grid shader, visible only in build mode.

Ghost: while dragging show beam from start node to cursor snap, green/red; on release create.

Erase: click near beam (<0.5m) removes nearest player beam; drag-erase multiple.

Node move? skip.

Alright, audio: single AudioEngine with master gain; functions: creak(stress), snap(), splash(size), rumble(start/stop engine), horn(), success(), fail(), quakeRumble(), wind/river ambience loops. Standard WebAudio nodes.

Quality tiers: pixelRatio, bloom on/off, debris cap, grid density. Autoscale by fps EMA.

Snapshot: renderer.domElement.toDataURL → download link. preserveDrawingBuffer true.

Chart: 2D canvas bottom-left (deflection + max stress lines, autoscale). Visible during test.

Toasts: stacked divs bottom-right? Existing demos use toast top-center. I'll do bottom-center small stack.

Help overlay: toggle H with controls list & goal.

Deep-link? skip.

Now — writing time. I must be careful with size; I'll write compact but clear code. Let me draft the entire file mentally then emit chunks.

Key globals:

State:

Node: {key,gx,gy,x,y,px,py,vx,vy,m,invM,anchor,fixed(in ground),truck:0|1,radius (wheels),alive:true, splashed:false}

Actually wheels/chassis not in grid map — separate arrays but same solver loop over "dynNodes" = nodeArr (grid) + truckNodes. I'll keep single array P (particles) for solver; grid map only for editor lookups.

Beam: {id, key, a:P, b:P, mat, rest, comp(α), breakT, breakC, cable, alive, stress, fixed, instA, instB indices}

Build API:

Mass recompute: for grid nodes: m = 8 (base joint) + Σ beam.mass*len/2; invM = anchor?0:1/m. Truck nodes separate masses.

Anchors: level defines anchor points: e.g., First Crossing: (±G/2,0),(±G/2,-3)? but -3 inside cliff... nodes at cliff face x=±G/2 y∈{0,-2,-4} anchored; also ground road nodes anchored. Anchor flag set when node created at those coords: maintain Set of anchor keys per level; getNode checks set.

Pillar level adds anchors at (0,0),(0,-2),(0,-4),(0,-6)? pillar top at y=0? pillar from water: rock cone rising to y=0 at x=0 → anchors (0,0),(0,-2). Hmm make pillar top y=-2 to force using it as support below deck: anchors (0,-2),(0,-4). Good.

Levels:

1. "First Crossing": G=20, budget 9000, wind 0. Anchors y 0,-2,-4 both sides.
2. "Long Span": G=30, budget 15000, wind 0.15.
3. "The Pillar": G=30, budget 12000, pillar at x=0 (anchors (0,-2),(0,-4)), wind 0.1.
4. "Storm Pass": G=24, budget 11000, wind 0.7 default slider high, gusts strong. Quake suggested.

Ground road extent: 12m each side: x from ±(G/2) to ±(G/2+12) step 2 anchored road beams (fixed). Truck start x=-(G/2)-8. Success x > G/2+6.

Cliff boxes: width 14 (x from ±G/2 to ±(G/2+14)), height 26 (y -26..0), z 40.

Presets (use addBeam with silent budget? They should respect budget: check affordability first, toast if can't afford):

• pratt(): deck across (road every 2m), posts every 4m up h=4, top chord, diagonals V-inverted. Beams: deck: (x,0)-(x+2,0) road; verticals steel; top chord steel; diagonals steel.
• arch(): parabola y = -5*(1-(2x/G)^2) sampled every 2m: arch segments wood/steel; hangers from arch to deck at same x (vertical beams); deck road every 2m. Ends at (±G/2,0) anchors. Arch below deck down to -5 (above water -11 ✓).
• suspension(): towers at x=±(G/2+1.5)?? that's on cliffs outside gap... then cables wouldn't hold deck. Real suspension: towers AT the cliff edges x=±G/2 (anchor base), rise to y=9; main cable from (±(G/2)+? , 1) anchor back on cliff (x=±(G/2+5),y=0.5) up over tower tops sagging to y=2 mid; hangers every 2m from cable to deck. Deck road every 2m. Towers: two steel verticals? single column of segments steel + cross brace. Cable sampled parabola every 2m.
• cableStayed(): tower at x=±G/2 height 10 (steel), stays from tower top & mid to deck points every 3m into span (cable), deck road + deck steel under? deck road every 2 plus longitudinal steel top? keep: road deck + stays.
• kingpost() small: deck road every 2; king post triangles: at center post up 3m, diagonals to quarter points; wood material cheap.

Preset code must handle dedupe (addBeam skips duplicates silently) & cost check at end (if over budget → rollback? simpler: compute list first, sum cost, if > remaining → toast & abort).

Editor interactions:

• pointerdown (left / one-finger) in build mode: intersect plane z=0 → snap gx,gy (round); if erase tool → try remove at point & set erasing drag; else begin drag: startKey=getNode? Don't create node yet (avoid orphan nodes): store start snap; show ghost.
• pointermove: update ghost end snap; validity: within zone, span ≤ MAX_SPAN, not same point, cost ok.
• pointerup: if valid → addBeam(start,end,currentTool) with undo push; if invalid (same point & tool road?) ignore. Click without drag on existing beam with draw tools = nothing.
• Erase drag: pointermove with erase removes beams near cursor (throttle).
• Hover highlight: nearest beam within 0.6 shown highlighted (erase) — use emissive color on instances? We'll have beam instance color array; highlight by temp color override. Keep simple: only ghost + cursor ring at snap point. Add small ring mesh at snap cursor. Good enough.

Raycast: ray from camera through pointer; intersect plane z=0: t = -origin.z/dir.z.

Camera default: position (G0.35, 7, G0.75) look at (0,-1,0). Controls target (0,-1,0), maxPolarLimit etc.

Follow cam: if follow && truck: target lerps to (truckX, 0.5, 0), camera maintains offset.

Rendering bridge:

• twinFrame: for each non-road beam: two boxes at z=±0.9, thickness mat.thick, length rest→current dist; road beams: deck plank box (len × 0.12 × 2.4) at z=0 + optional edge beams twin at z=±1.2 thickness 0.18 (visual side girders, also colored).
• Cables: cylinder-ish thin boxes (use same box, thin) — fine, plus slight sag? no.
• InstancedMesh: capacity 4096; per frame rebuild: iterate beams alive → compose matrix at midpoint, quaternion from angle in xy-plane, scale (len, thick, thick or plank dims). Use dummy Object3D. Color = stressColor(stress/break) via instanceColor. For wood maybe slight per-beam hue jitter (store rand).
• Rebuild every frame when mode=test (positions move) or on change in build (cheap enough to rebuild every frame anyway: ≤ ~2400 instances × compose = ok).
• instanceColor needsUpdate each frame. OK.

Stress color: t = clamp(stressEMA / breakThreshold) → gradient: 0 #3ddc84 green →0.5 #ffd54a →0.8 #ff8a3c →1 #ff3b30; plus base tint mix with material color at low stress: mix(matColor, heatColor, smoothstep(0.05,0.6,t)). Broken → not rendered.

Node joints: small spheres instanced at grid nodes that have beams — nice detail, capacity 1024, color steel-gray. Render only in build mode? Render always (small).

Debris: instanced boxes cap 300: each {x,y,z,vx,vy,vz,rx...,life}. Spawn at break: 2 pieces along beam with beam material color; physics simple gravity + water splash. Update in main loop (not substep).

Splashes: instanced sprite planes camera-facing: spawn ring of 10-16 droplets with upward velocities, gravity, fade scale; plus expanding ring mesh (torus? flat ring sprite) — do droplets + quick white foam quad. Cap 200 droplets.

Truck mesh: group at (chassis pos): body box (2.6×1.1×1.6) colored per weight; cab box; wheels: 4 cylinders (visual at z=±0.8 for each physics wheel) rotate by wheel.angle. Update from physics each frame; tilt by chassis angle relative wheels midpoint.

Flags: two pennants at cliff edges on poles: pole cylinder + flag = plane geometry 1.2×0.7 with vertex shader flutter (uniform windStr, time). Nice wind indicator.

River: PlaneGeometry(160, 46) rot -90° x at y=WATER_Y, custom ShaderMaterial: base deep blue-green, moving voronoi-ish sparkles via fract sin noise, flow direction +x, foam near cliffs? plus subtle opacity. Additive highlights. Also big soft "mist" sprites above water in gap.

Sky: large sphere with gradient shader (horizon warm → zenith deep blue), sun disc glow; directional light matching; hemisphere light. Mountains: 6 large cones far with fog. Fog: scene.fog = new THREE.Fog(0x0b1220, 60, 220)? Match sky horizon color.

Clouds: 8 sprites (canvas radial gradient) drifting slowly.

Post: composer bloom 0.55, grade pass (vignette+grain+chromatic+ slight warm). Same pattern as 045 (copy approach).

Lights: DirectionalLight (sun) with shadows? Shadows for truck/deck onto water? Shadow map 2048, only bridge + truck cast; cliffs receive? Shadows might be costly with instancing updates but fine. Enable PCFSoft; castShadow on instanced mesh true; receive on water? water is shader — skip receive; add a dark "shadow catcher" plane? Skip shadows entirely → use AO-ish darkening under bridge? Keep shadows OFF for reliability & perf; use hemisphere + directional + rim. Hmm, visuals benefit from shadows... The guidelines say good lighting & shadows. Enable renderer.shadowMap, sun casts, bridge instances cast onto cliffs & water? Water shader material won't receive. Cliffs receive. Truck casts. Deck planks cast onto water? no receiver. It's fine: cliff faces receiving bridge shadows will look great in morning light. Keep shadow map 2048, camera box around scene (±(G/2+16), y -16..16). Cost ok.

Audio implementation (compact):

Throttle creaks via lastCreak time. In solver loop each frame: find max stressed beam; if stress>0.45break → maybe creak.

Chart: canvas 240×110: two series: deflection (cyan, meters), maxStress % (orange), rolling 600 samples at 60fps (10s). Draw axes minimal. Only update in test.

HUD stats rows: Level, Budget bar used, Cost, Beams, Broken, Max stress, Integrity, Deflection, Truck (state), Wind. Toast area. Toolbar buttons: material select (4 + erase), Test/Stop, Undo, Repair? (reset), Presets dropdown (5), Level dropdown (4), Weight dropdown (3), Wind slider, Quake, Follow, Mute, Help, Snapshot. That's a lot of buttons — arrange toolbar bottom center with small grouped buttons + selects styled.

Keyboard: 1-5 tools (road, steel, wood, cable, erase), Space test/stop, U undo, X clear, Q quake, F follow, M mute, H help, P snapshot, L cycle level, B back to build(stop). Esc close help.

Quality tiers:

autoscale by fps EMA <42 down / >57 up after 8s.

Resize handler. preventDefault context menu. localStorage best scores.

Now, potential perf: matrix rebuild each frame fine.

Edge cases: beam breaking during iteration — mark alive=false, skip; removal from arrays deferred (splice after loop or filter at frame end). Instances rebuilt from alive only.

When a beam breaks, its nodes may drop below water → splash per node once (flag). Also nodes fully unsupported just hang (no static check needed — physics handles).

Truck spawning: place wheels at y = 0.55 (=R + small), x0 = -(G/2)-8, w2 = x0-2.2? facing right: front wheel ahead. chassis above between. Zero velocity. Motor target speed by weight; accelerate gently (traction limit). If no contact under wheels (gap) truck falls — physics.

Truck-ground: ground road beams exist on cliffs (fixed). Contact normal push. Also wheel vs cliff top when beam broken? The cliff box top at y=0: add simple ground collision for wheels: if |x|≥G/2 && y<R → y=R (prevents falling through cliff even if visuals). But then truck could drive across cliff with broken ground beams — fine (cliff is solid rock).

Also nodes vs cliff top: if |x|≥G/2 && y<0 → y=0? That would make collapsing debris hover on cliff top — acceptable (rock). Keep y≥0.02 with friction damp. But careful: build zone allows y as low as -2/-4 anchors at cliff FACE x=±G/2 exactly (|x|=G/2 boundary). Use strict >: collision only if |x| > G/2 + 0.01. Anchors at exactly ±G/2 are fine.

Hmm wait: arch preset ends at (±G/2, 0) anchors; hangers to deck. Arch nodes below y<0 within gap (|x|<G/2) — no collision since |x|<G/2. Good.

Pillar collision: rock pillar at x=0 from y=-11..-2 radius ~2.2: nodes colliding: if |x|<2.2 && y< -2 + ... treat as cylinder: push out horizontally? Keep simple: skip pillar collision (beams pass through visually rarely; anchors hold). Acceptable. Actually arch in pillar level could intersect pillar top visually — minor.

Failure detection: if truck.chassis.y < WATER_Y+1 && !failed → failed (splash big, sad sound, toast). Success: chassis.x > G/2+6 && chassis.y > -1 → success (horn + chord, confetti debris? maybe golden splash particles). Stuck: if mode test && !failed && !success && t>3 && avgSpeed<0.15 for 3.5s → toast "Truck stuck — bridge held but impassable" (count as fail? call it stalemate) — allow user to stop.

Also "weight on bridge": truck mass matters via wheel node masses pushing deck.

Deflection metric: min y of road-material beam midpoints (deck sag) relative 0: defl = -min(0, minY). MaxStress: max stressEMA/break over alive beams.

Sounds solid. Now write it. I'll aim for clarity & compactness. Also remember: NO template literals inside GLSL strings? Template literals are fine in JS generally (045 extracted module passed node check). But careful when extracting module script for node --check: import statements with bare specifiers pass syntax check (node --check only parses; import ... from 'three' parses fine as module? node --check file.mjs handles ESM syntax if .mjs extension. Yes previous run used app.mjs.)

One more consideration: keep GLSL in template literals — fine.

Extraction: sed between '' and '' (the last script tag; importmap script also exists — extract after the module marker line).

Let me write the files now. I'll create /tmp/bridge/ parts p1..p6 and cat to /space/demos/046-bridge-works/index.html.

Also update /space/demos/index.html: insert new card after "".

Write Part 1: HTML + CSS + panels.

Let me carefully write CSS for toolbar with groups, selects, sliders, toggle states; toast stack; help overlay; chart panel; level dialog? Keep toolbar organized:

Top-left HUD: title + level name + status. Top-right stats panel. Bottom-left chart (test only). Bottom-center toolbar (two rows: materials row, actions row). Bottom-right: toasts stack? Toasts usually top-center; I'll do top-center under a small banner. Keep bottom-right for wind/quake quick controls? Put wind slider in toolbar row 2. Help overlay full-screen modal.

Buttons style consistent with panels. Mobile: toolbar wraps, smaller.

Let me write the code now. For efficiency I'll write big chunks per tool call.

Part sizes ~ let's go. I'll number beams via id counter.

Important detail — instance color heat: InstancedMesh with setColorAt. MeshStandardMaterial with vertexColors? For instance colors use material.color white and instanceColor. Use MeshStandardMaterial({metalness:0.35, roughness:0.5}) and per-instance color. Emissive heat at high stress: standard material emissive is uniform... trick: use two materials? Simpler: rely on color + bloom threshold: bright red near break will bloom slightly. To get glow, scale color >1? Instance colors clamp? They accept floats >1? InstancedBufferAttribute float can exceed 1 — with tone mapping it blooms. Multiply heat color by (1 + 1.5*smoothstep(0.7,1,t)) for bloom pop. Nice.

Truck wheels rotation: cylinder axis along z → rotate around z? Wheel rolling around its z-axis (axle) — CylinderGeometry axis is Y; rotate geometry rotateX(PI/2) → axis z. Then mesh.rotation.z -= speeddt/R? Rolling direction: forward +x → rotation.z negative. angle -= v/Rdt.

Ghost: Mesh box thin + ring at cursor. Update per pointer move only in build mode.

Grid plane: PlaneGeometry(width, height) at z=0 (vertical: it's XY plane by default! PlaneGeometry lies in XY — perfect, no rotation), shader: grid lines every 1m, fade edges, only build zone; transparent, depthWrite false, visible in build mode. Plus border.

For snapping cursor ring: RingGeometry in XY plane at z=0.05.

OrbitControls config: enablePan true (screen-space), mouseButtons: {LEFT: -1? } Setting LEFT: null might break lib? In three r170 OrbitControls, setting mouseButtons.LEFT = -1 disables? Actually assigning null works? To be safe: LEFT: THREE.MOUSE.PAN? then ctrl? Hmm I want left free for drawing. Use: LEFT: undefined → check source: if mouseButtons.LEFT is not matched, no action. In r170: switch ( event.button ) { case 0: mouseAction = this.mouseButtons.LEFT; ...} then switch(mouseAction) with cases MOUSE.DOLLY/ROTATE/PAN, default: state = STATE.NONE. undefined → default none. OK safe. touches: { ONE: undefined? touches.ONE = -1 } similar default none; TWO: THREE.TOUCH.DOLLY_PAN. Good.

But then desktop rotate = RIGHT drag (context menu suppressed) — tell users in help + hint toast on first load ("Drag with LEFT to build · RIGHT-drag to orbit · wheel to zoom").

Middle = pan.

Now the beam picking for erase & hover: distance point-to-segment in 2D over player beams. Fine.

Undo stack ops; Clear pushes bulk op? Clear = remove all player beams (push op with list for undo restore). Implement undo restore for add (remove beam) and del (re-add), and clear (re-add all). OK.

Serialization for test snapshot: list of player beams [{a:akey,b:bkey,mat}] — restore: clearPlayer(); for each add (bypass budget, no undo). Node removal: when beam removed, orphan nodes with no beams & not anchor → delete from map (keeps map clean). Track refcounts.

getNode anchor check: anchorKeys Set built per level.

Let me also compute level anchor keys:

Cliff-face anchors at x=±G/2 are ON the face; beams from them into gap. Also ground beams anchored along cliffs give road.

Wait: ground road beams: nodes at y=0, x=-G/2-12..-G/2 step 2: those within |x|>G/2 → anchored & fixed. Node at x=-G/2 exactly = also anchor (in anchor set). Good.

Truck start: x0=-(G/2)-8 (on ground road which spans to -G/2-12 ✓).

Beam instance capacity: player beams ≤ ~500, ground 12, twin frames 2×, planks 1× per road → ≤ ~1300 + joints ≤ 700 → capacities 2048 & 1024 fine.

Deflection chart samples: midRoadY: min over road beams (player) midpoint y.

Let me now write everything. I'll try to keep the JS ~1400 lines. Start writing part files.

Actually — one more mechanic for fun: after success, allow "Next weight" quick test. The weight select handles it.

Also small detail: cost display with $.

Toast function with types (info/good/bad).

First-visit hint: show help overlay on first load? Show compact hint bar that fades. I'll show help overlay initially with "click anywhere to begin" — good for audio unlock too.

Time to write. Part 1 (HTML/CSS/panels):

**********************************************************************************************ls.

**********************************************************************************************ne.

*******************************************************************************er.

Oops — I have an escaping bug: toast('Preset exceeds this level\\'s budget... — inside a normal file write that becomes \\' in the actual file → that's a backslash followed by end of string? Let me check: In the written file, the content is literal: toast('Preset exceeds this level\\'s budget ($' ...). In JS source, '...level\\' is an escaped backslash, then 's budget...' starts a new string... actually 'Preset exceeds this level\\' = "Preset exceeds this level" then s budget ($' — wait: 'Preset exceeds this level\\'s budget ($' parses as: string 'Preset exceeds this level\' (i.e. Preset exceeds this level\), then s budget ($' — that's a syntax error? No wait: after the closing quote, s is an identifier... '...\\'s...' — the \\ is an escaped backslash, then ' closes the string, then s budget ($' — invalid syntax!

I need to fix that line. Also in setupLevel there's a weird line x = x; // (loop increment handles step) — the for loop: for (let x = x0; x !== x1 + step; x += step). With sx=-1: x0=-G/2, x1=-(G/2+12), step=-2. Loop while x !== x1+step = -(G/2+12)-2. x goes -G/2, -G/2-2, ..., stops when x === x1+step = -(G/2+14). Last iteration x = x1 = -(G/2+12), adds beam from x1 to x1+step=-(G/2+14). That adds an extra beam beyond GROUND_EXT and node at -(G/2+14) which is outside build zone but that's OK for fixed ground. Hmm, but the condition x !== x1+step with exact multiples works since G/2 and steps are integers when G is even (20, 30, 24 → G/2 = 10, 15, 12 ✓). But the extra beam to -(G/2+14) is unintended. Let me rewrite that loop more clearly:

Clean. Also remove the silly x = x; line.

Also in addBeam for fixed ground beams: nodes at |x|>G/2 anchored by .fixed flag (invM=0) ✓. But makeNode checks anchorKeys only; fixed flag set after. OK.

One more issue: in removeBeam, when I delete orphan nodes — truck nodes never in nodeMap so fine.

In presetSuspension: L.push([k2(sx*(half+2),0), k2(sx*(half+5),0), 'road']) — that's a 3m road beam on the cliff — it's a player beam costing money, overlapping fixed ground beams → beamMap dedupe: fixed beams were added with keys by node ids; the preset beam connects same node positions → getNode returns same nodes → same key → dedupe returns null.

Also k2() with half-grid: presetArch uses Math.round so all keys integer ✓. presetSuspension cabY rounded ✓. half-2 etc integers when G even ✓. presetKing xm = (x0+x1)/2 could be odd multiples: x0=-10,x1=-6 → xm=-8 ✓ integer; but if step doesn't divide (G=20: -10..10 step4: x0=-10,-6,-2,2,6 → x1=-6,-2,2,6,10 ✓ all even, xm integer ✓). G=30: -15..15: x0=-15, x1=min(-11,15)=-11, xm=-13 ✓; then -11,-7;-7,-3;-3,1;1,5;5,9;9,13;13,15 → xm=14 ✓. OK integers. Arch for G=30: half=15, x from -15 step 2 ✓ integers; archY(x) rounded to nearest 0.5 then Math.round → integer ✓. Hmm: I do Math.round(archY(x0)*2)/2 then Math.round(y0) → double rounding, fine.

Suspension hangers if (y > 1) — hanger from (x,y) to (x,0) length y ≤ MAX_SPAN ✓ (max th=9 at towers... at x=±half y=th=9 → hanger length 9 > 8! At x=±half those are the tower positions; the cable at x=half-2: y = sag + (9-1.5)((half-2)/half)^2 ≈ 1.5+7.50.72=6.9 → hanger 7 OK. At x=±half exactly: loop runs x up to half → cable point at (half,9)=tower top; hanger 9m rejected by addBeam (returns null silently since >MAX_SPAN... but preset uses addBeam with MAX_SPAN check → returns null, skipped ✓ fine).

But wait — preset cost estimate vs addBeam MAX_SPAN: estimate counts all, addBeam skips >8m. Minor overestimate, fine.

Backstay k2(tx,th) to k2(ax,1): tx=±half, ax=±(half+5): length = sqrt(25+64)=9.4 > 8 → rejected! Need intermediate node: add mid node at (half+2.5?) — must be integer: half=10: tx=10, ax=15 → midpoint 12.5 not integer. Use two segments: (10,9)→(13,5)→(15,1): lengths sqrt(9+16)=5 ✓, sqrt(4+16)=4.47 ✓. mid = sx*(half+3), ymid=5. For G=20: half=10 ✓. G=30: half=15: (15,9)→(18,5)→(20,1) ✓ same offsets. G=24: half=12: (12,9)→(15,5)→(17,1) ✓. Good — replace with two-segment backstay via (half+3, 5).

Cable-stayed: stays length from (tx,10) to (tx-sx*dx,0), dx up to 12: length sqrt(144+100)=15.6 > 8 → rejected! Need cables to be allowed longer? Cables in real world span long. Option: raise MAX_SPAN for cables to 16? Editor limit per material: cable max 16m, others 8m. Let's do: MAX_SPAN_PER_MAT = {road:8, steel:8, wood:8, cable:16}. And stays every 2-3m... 15.6 ≤ 16 ✓. Update addBeam to use per-mat max. Editor ghost validity too.

Check stayed tower: tx = sx*(half-2): G=20 → tx=±8, stays to deck points x = tx - sx*dx: dx 3,6,9,12 → for sx=1: 5,2,-1,-4 ✓ within gap. For sx=-1: -5,-2,1,4 ✓. Lengths: dx=12 → sqrt(144+100)=15.6 ≤ 16 ✓.

Also arch: for G=30, rise=min(6, 6.6)=6; arch bottom at -6, above water -11 ✓. Hangers length up to 6 ✓.

Now fix p3 issues with replace operations:

1. Fix the ground road loop.
2. Fix the toast escaping.
3. MAX_SPAN per material.
4. Backstay two-segment.

Then continue with part 4 (solver) — wait, I planned solver in p4. Note markBridgeDirty, rebuildEnvironment, updateStats, toast, windSlider are referenced — will be defined later (windSlider as global from DOM id? I use windSlider.value — element id windSlider becomes global var via window; in modules, named elements are still accessible as globals via window named properties — yes, window.windSlider works even in module scope since it's a global property lookup. But safer to grab references explicitly in UI part: I'll define const windSlider = document.getElementById('windSlider') etc. in the UI part — but setupLevel references windSlider before... order of execution: setupLevel called at init after UI refs defined. Since const in module scope is hoisted in TDZ — setupLevel function body references windSlider at call time; as long as const is initialized before setupLevel() is invoked, fine. I'll ensure init order: define UI refs early in the UI section, then call setupLevel.

Let me fix p3 now.I need to fix a few issues in p3: a string-escaping bug, the ground-road loop, per-material max span (cables need longer spans for suspension/stayed presets), and the backstay geometry:

****************************************************************************************************************************************************************************ts.

Part 5: the 3D scene — renderer, camera, controls, lights, sky, cliffs, river shader, flags, bridge instanced meshes, joints, truck mesh, debris, splashes, ghost, grid, post-processing.

Key pieces:

• scene, fog, camera, renderer with shadows.
• environment build: cliffs (displaced boxes with vertex colors), cliff road strips, pillar, river (shader), sky dome (shader), sun light, hemisphere, mountains, clouds.
• bridge rendering: InstancedMesh for frames (unit box), planks, joints.
• ghost beam + cursor ring + grid plane (build mode).
• truck group.
• debris + droplets instanced.
• confetti = golden droplets.
• flags with shader.
• post: composer, bloom, grade pass.
• markBridgeDirty flag — actually I rebuild instances every frame anyway; markBridgeDirty can be a no-op that triggers immediate update... I'll implement rebuildBridgeInstances() called every frame; markBridgeDirty() can just call updateStats() & nothing else needed. I'll keep markBridgeDirty as a function that updates stats to avoid undefined errors.

Cliff geometry: I'll make a helper makeCliff(side): BoxGeometry(cliffW=GROUND_EXT+6, height=26, depth=44, segs 8,12,8), displace vertices with noise, color strata. Position: for left cliff (side=-1): the cliff occupies x from -(G/2+cliffW) to -G/2. Center x = -(G/2 + cliffW/2), y center = -13 (top at 0). Displacement: apply noise only to vertices on the gap-facing side (x near +cliffW/2 local for left cliff) and top edge variation. Simpler: displace all outer vertices with noise scaled by distance from top... Let me do generic: for each vertex, compute n = noise3(x0.15, y0.15, z0.15); offset along normal-ish (x,z direction) by n1.2, plus strata. Vertex colors: rock color lerp by y with horizontal banding sin(y*0.8). Fine.

Simple noise: implement value-noise hash function.

Pillar: cylinder-ish cone with noise at x=0 rising from water to y=-2. radius 2.5 at bottom, 1.6 at top. Render only when level.pillar.

River shader: plane 200x50 at y=WATER_Y rotated -PI/2. Fragment: animated flow noise: use fbm of sin/cos hash... keep cheap: two moving sine-noise layers + fresnel highlight + sparkle. Color deep teal → highlight. Add slight transparency? Opaque with spec highlight is fine + bloom sparkles.

Sky: SphereGeometry(400) BackSide shader gradient + sun glow + a few stars? Day scene. Horizon haze. Sun disc at direction.

Mountains: 5 cones at distance 150-260, colors fog-blended. With fog on standard material they'll blend.

Clouds: sprites with canvas radial gradient, 8 of them drifting.

Flags: pole + flag plane with onBeforeCompile flutter? Simpler: custom ShaderMaterial for flag (unlit-ish with basic shading). I'll do ShaderMaterial double-side: color amber, vertex z-wave by (x/len)ampsin(timespeed - xk) where amp ∝ wind. Position at cliff edges on poles at (±(G/2+2), 3, z=-1.5)? Put poles at the start/finish: left flag at (-(G/2+3), top y≈3) and right at (G/2+3). Actually place near anchors: x=±(G/2+1.5), pole height 4. Flag attached at top.

Anchor markers: small glowing amber octahedrons at anchor points (only those at cliff face & pillar) — visible in build mode. Instanced or individual sprites — few (6-10): use small meshes.

Bridge instancing:

• frameMesh: InstancedMesh(boxGeom, frameMat, 4096) — boxGeom unit 1×1×1.
• Per beam (alive): road: plank instance (len,0.12,2.6) at z=0 + two side girders (len,0.16,0.16) at z=±1.25; others: two frames at z=±0.95 (len,thick,thick); cables: single at z=±0.95 thinner (thick) — maybe cables just twin thin.
• jointsMesh: InstancedMesh(octahedron small r=0.16, cap 1024) at nodes with refs>0 (non-truck), z=±0.95? Joint at z=0 fine (looks like rivet plate). Slight z offset ±0.95 pairs? Keep single at z=0, scale small.
• Colors: per instance stress color.

Wait — twin frames at z=±0.95 with joints at z=0 looks odd; put joints twin too at z=±0.95. Capacity fine.

Truck group: chassis box 2.8×0.9×1.7 at y offset, cab box, wheels 4 cylinders. Position group at midpoint of wheels, rotate z by angle of chassis relative to wheels axis: angle = atan2(w1.y-w2.y, w1.x-w2.x) (pitch). Chassis pos sets body y.

Debris: instanced boxes cap ~240. spawnDebris(beam): 2 chunks: positions along beam, velocities from endpoints + random, angular vel. Update: gravity, spin, water splash (small), fade after 6s (scale down). Color = mat color.

Droplets: instanced planes (billboard: use sprites? InstancedMesh planes with lookAt per instance is heavy — instead use small spheres? Cheap: instanced tetra? Use Points with size attenuation & custom texture! Points with vertex colors + size per point via PointsMaterial size global... per-point size needs ShaderMaterial. Simpler: THREE.Points with PointsMaterial(size 0.35, map soft circle, transparent, depthWrite false, vertexColors). Positions updated per frame. Good enough and cheap. spawnSplash(x,y,v): 12 droplets upward cone + white foam color. spawnConfetti: 40 golden/colored droplets with more spread.

Grid plane: shader material on PlaneGeometry(G+2*GROUND_EXT, 24) at z=-0.05? at z=0 exactly behind beams? Put at z=-0.02, transparent grid lines 1m, major 4m, edge fade, only build mode visible. XY plane default orientation ✓ (faces +z). position (0, 1, 0) covering y -12..14 → center y=1, height 26.

Ghost: box mesh scaled like beam, amber transparent; plus ring cursor.

Post: standard composer w/ bloom + grade shader (vignette, grain, slight chromatic aberration, warm tint). Copy pattern.

Shadows: renderer.shadowMap.enabled = true; sun.castShadow; frameMesh.castShadow = true; planks cast; cliffs receiveShadow; truck cast. InstancedMesh shadows work. Shadow camera ortho bounds ±(G/2+18) etc.

markBridgeDirty(): I'll define as updating stats + nothing else (instances rebuilt per frame).

Also "rebuildEnvironment()" called from setupLevel: clears & rebuilds cliffs/river/pillar/anchors/flags per level.

Let me write part 5 now — it's big. I'll split scene into p5 (core scene + environment) and p6 (bridge/truck/debris rendering + post), then p7 UI + loop. Keep each write manageable.

Issue: in rebuildEnvironment I reference roadTopMat, dashMat, poleMat before their const declarations in module order — rebuildEnvironment is called later (at setupLevel at init), so by call time the consts are initialized. Fine (TDZ only matters at call time).

Also anchorGroup added to envGroup then anchorGroup.clear() — clear removes children, OK. But I envGroup.add(anchorGroup) every rebuild → adding same object again is a no-op re-add (moves to end). Fine.

Cliff face displacement: faceDir * x > width * 0.49 — for left cliff s=-1, face toward gap means local +x? The cliff mesh local x from -width/2..width/2. For left cliff positioned at -(G/2 + w/2 - 0.6), gap is at +x direction (toward center). So gap-facing side is local +x. faceDir=s=-1 → condition -x > w*0.49 → x < -w*0.49 → the -x face — wrong side. I should pass faceDir such that faceDir=+1 for left cliff (gap at +x). For right cliff at +(G/2+...), gap at -x. So faceDir = -s. Let me fix: makeCliffGeo(cliffW, cliffH, cliffD, -s) and displacement direction pos.setX(i, x + (n-0.5)*amp*faceDir) — pushes along face normal — sign of face normal for left cliff is +x. faceDir=-s gives +1 for left ✓.

Also cylinder height for pillar: -WATER_Y - 1.4 = 11-1.4 = 9.6 — positive ✓ (WATER_Y=-11 → -WATER_Y=11). Pillar top should reach y≈-2 (anchors at -2,-4,-6). pm.position.y = WATER_Y + 9.6/2 - 0.6 = -11+4.8-0.6 = -6.8; top = -6.8+4.8 = -2.0 ✓

Flag rotation: for right cliff (s=1) flag rotated PI so it points -x (into gap?) Actually flags point with wind; just visual. OK.

Cloud sprites: fine.

Now fix the cliff faceDir.

Part 6: bridge instanced rendering, truck mesh, debris, droplets/splashes, ghost/grid, post-processing.

Bridge rendering details:

• frameMesh InstancedMesh cap 4096 unit box. Cast shadow.
• jointMesh cap 1024 octahedron 0.15.
• Per frame rebuildBridge():
  • count instances; for each beam alive: compute mid, angle, len; color via stress.
  • road: plank (len+0.04, 0.13, 2.7) at z=0 + 2 girders at z=±1.22 (len, 0.18, 0.16)
  • non-road: two frames (len, thick, thick) z=±0.95; cables thinner (thick=0.09) — maybe 3 cables? twin fine.
  • joints: for nodes refs>0 && !truck: at (x,y,±0.95) scale 0.16 octahedra... anchor nodes too.
  • set count, instanceMatrix needsUpdate, colors.
• dummy Object3D reuse.

Color computation:

For fixed ground road beams: color darker (they're under the road strip anyway — could skip rendering fixed beams entirely! The road strip boxes cover them. Yes: skip rendering fixed beams → saves instances. But wheel contact happens on them; visually road strip is at y≈0.08 top... wheel radius 0.55 rides on beam at y=0 → wheel bottom at y=0, road strip top at 0.15 → wheel sinks 0.15 into strip visually — barely noticeable. Alternatively lift road strip top to y=0.02: box height 0.14 centered 0.08 → top 0.15. Set road strip center y = -0.05 → top 0.02. Then wheel (bottom at 0) sits on top ✓. And dashes at 0.02+0.005. Adjust: road.position.y = -0.05, dash y = 0.03.

• Ghost beam: Mesh(unit box, MeshBasicMaterial transparent) + cursor ring; update in pointer handlers; also show len & cost label? Skip label, color-coded enough.

• Grid plane shader with build-zone bounds uniform; visible=mode==='build'. Fade by distance? Simple.

• Truck group:

  • body = box(2.9, 0.75, 1.75) color spec.col, at (0, 0.45 above axle line? build group with origin at axle midpoint y=0): chassis pos - wheels mid = (0, ~1.0). Body center y = 0.55+0.75/2? Let me: wheels at group y=0 (axle), body box center (0, 1.05, 0), cab (1.05, 1.55, 0) size (0.9,0.7,1.6)? front at +x. wheels at (±1.1, 0, ±0.85) cylinders r=0.55 w=0.35 rotated X 90°. Group position = midpoint(w1,w2), rotation.z = atan2(dy,dx), then body offset y +? Simplify: group origin at axle midpoint; children positioned accordingly. Chassis physics node used only for outcome/velocity; visually body follows wheels.
  • castShadow all.

• Debris instanced mesh cap 260: each {x,y,z, vx,vy,vz, rx,ry,rz, wx,wy,wz, life, scale, col}. spawnDebris(beam): two chunks at 1/3 & 2/3 points: velocity avg of endpoints' vel + rand. Size ~ (0.5, thick0.8, thick0.8)... random box scale. Update in animate: integrate, spin, water → splash tiny + kill; life -= dt; scale *= fade at end. Compose matrix per active debris. Color set at spawn.

• Droplets Points: positions Float32Array cap 400, colors; each {x,y,z,vx,vy,vz,life}. spawnSplash(x,y,power): 14 droplets vy 2..5*power, vx random spread, white-cyan. spawnConfetti(x,y): 46 golden/orange/cyan with higher vy. Update: gravity, kill at water. Points material size 0.32, vertexColors, map soft circle (reuse cloudTex), transparent, depthWrite false, blending additive. Positions attribute dynamic. Dead droplets: move y to -9999.

• Post: composer with bloom + grade pass (vignette/grain/chromatic/warm) + output.

• markBridgeDirty(): just updateStats() (defined in p7 UI) — but p4 calls it; ensure defined before use at runtime (function declaration hoisting across module: function declarations are hoisted within the whole module — all parts concatenated into one module → function declarations hoist across parts ✓). markBridgeDirty as function declaration in p6/7 fine.

Write p6 now.

There's a leftover line in rebuildBridgeInstances for road: the "placeholder replaced below" — the setBoxInstance call then immediately overriding with plank matrix. That's sloppy: the first setBoxInstance call is redundant. Let me clean: remove the placeholder setBoxInstance line.

Also const { a, bb } = { a: b.a, bb: b.b }; — silly; simplify to const a=b.a, bb=b.b.

Also in setBoxInstance I do rotation.set twice; simplify to compute both angles properly:

• For beams in XY plane dz=0 → rotation.z = atan2(dy,dx) suffices. The dz case never happens since both endpoints at same z. Simplify function: rotation.set(0,0,atan2(dy,dx)).

Let me fix these.

***************************************************************************************************ex.

The old bad line is still in the assembled file — my replacement fixed p3.html but... wait, I fixed p3.html line 212. The assembled file has level\\'s at line 359 of app.mjs. Did my replace actually apply to the right file? It said success with the new text 'Preset exceeds this budget...'. Hmm — maybe the cat happened BEFORE... no, I edited p3 before assembling. Wait, look at the error: toast('Preset exceeds this level\\'s budget — that's the OLD text. But I replaced it and the preview showed the new text at line 212. Unless... the replace wrote to p3.html correctly. Let me check: the replace output said "affectedStartLine: 212" with new content. But app.mjs line 359 shows old content?!

Oh! I see — the replace preview showed line 212 with the NEW text. But maybe there were TWO occurrences? No, the original had one. Hmm, wait — did I re-assemble from stale parts? The cat ran just now from p1..p7 in /tmp/bridge. Let me grep p3.html to verify.

Interesting — p3.html contains level\\'s — with DOUBLE backslash. So the file literally has \\ (two chars). My original write had \\' in the JSON content which became \\'... wait the original content I wrote had \\' (in my write I typed level\\'s), which JSON-decodes to level\'s... hmm no. The write tool received the content as raw text. I typed \\' in the content which means the file got \\'? The grep output shows level\\'s — grep shows raw bytes, so the file literally contains two backslashes followed by quote. That means my write content contained \\'.

Then my replace searched for 'Preset exceeds this level\\'s budget — in the replace call, the search string was written as \\' too... The replace reported success and preview showed the new string! But grep shows old string remains. Confusing — maybe the replace matched a DIFFERENT escaping: my search text level\\'s (as I typed in the tool param) was interpreted as literal level\\'s?? And replaced with new... but then grep should not find it.

Wait, look again at the replace result: "replacements: 1 ... affectedStartLine: 212" and preview showed toast('Preset exceeds this budget ($'.... So it DID replace. But grep now shows the OLD line at line 217?! Line numbers differ: 212 vs 217. Two different lines...

OH. I see. When I wrote p3.html the FIRST time, the content may have been written with literal backslashes preserved differently... Actually the simplest explanation: my "fix" replace call itself introduced the double-backslash: my search parameter contained \\' which JSON-decodes to \'... and the file contained \'?? But grep shows \\'.

Ugh. Let me stop theorizing. The file currently contains: toast('Preset exceeds this level\\'s budget ... where \\ is literally two backslash characters in the file. In JS, '...level\\' = string ending with backslash, then s budget ($'... — syntax error as reported.

Fix it directly with replace targeting the exact current content. To match two literal backslashes in the search string, I type four backslashes in JSON. The tool params I write are taken literally though... Previous behavior suggests the tool interprets escapes. Let me use sed via shell with careful quoting instead — safer: use python or perl.

perl -i -pe 's/Preset exceeds this level\\'"'"'s budget/Preset exceeds this budget/' — messy. Use python:

python3 - <<'EOF' import re p='/tmp/bridge/p3.html' s=open(p).read() bad = "toast('Preset exceeds this level\\'s budget ($' + Math.round(cost) + ')', 'bad'); return; }"

simpler: find line containing "Preset exceeds"

lines = s.split('\n') for i,l in enumerate(lines): if 'Preset exceeds' in l: lines[i] = " if (cost > level.budget) { toast('Preset exceeds this budget ($' + Math.round(cost) + ' needed)', 'bad'); return; }" open(p,'w').write('\n'.join(lines)) print('done') EOF

Then re-assemble and re-check.

Permission denied writing via python to /tmp/bridge/p3.html (probably read-only mount for /tmp? or the file was created by a different mechanism). The shell environment may restrict direct filesystem writes outside the file tools. I'll just rewrite the whole p3.html file using the write tool with corrected content... but that's 9KB rewrite. Alternatively use the replace tool on the DEPLOYED file and skip re-assembly, plus also fix p3 for consistency. The replace tool worked before (it reported success, and one replacement did happen — the preview showed the new line!). Actually wait — the earlier replace DID report success with the preview showing the corrected line. But grep shows old content still there at line 217. The preview at the time showed line 212 with new content...

OH WAIT. I understand now! The replace tool processed my search text 'Preset exceeds this level\\'s budget — where I typed \\'. If the tool params are literal (no JSON unescaping on my side — I write raw), then the search string was level\\'s (two chars backslash-backslash? no — what I typed was backslash backslash quote). The file at that time contained... after my original write with content level\\'s — if write is literal, file contains level\\'s = two backslashes + quote. My search level\\'s matched it, replaced with new text — preview showed success at line 212. But then grep at 217 shows old text still?!

Unless the replace tool search performed escaping differently... The preview DID show the new line. And line number shifted 212→217 because my earlier edits (per-material maxLen adding lines, unlinkBeam adding lines) changed numbering. Both greps (p3:217) — hmm.

Actually wait, look at my earlier replace result: it said bytesWritten: 9074 and preview showed the fixed line at 212. Then subsequent replaces (backstay, maxLen, unlinkBeam) reported different byte counts... the maxLen replace reported 9158 and unlinkBeam 9211. If the toast fix was in, content should stay fixed. But grep now finds old text. So one of the later operations reverted it? The replace tool rewrites the whole file from a cached original?? That would explain: each replace may have been applied against a STALE copy, i.e., race condition/caching in the tool. The last write to p3 (unlinkBeam fix, bytes 9211) might have been computed from a version that still had the old toast line.

Whatever — simplest: do one final authoritative replace on the DEPLOYED file (single operation, verify after), and also on p3 (not strictly needed anymore since deployed file is what matters; but re-assembly would lose it — I'll just patch the deployed file directly and NOT re-assemble).

Patch the deployed file: search text as currently in file: contains level\\'s literally (backslash backslash quote). In my search param I'll type exactly level\\'s... that's what failed?? No — earlier replace DID succeed per its preview; the file just got overwritten by a later stale write. So typing the literal sequence works.

I'll patch /space/demos/046-bridge-works/index.html directly with replace, then re-extract & re-check.

JS syntax OK. Now verify:

1. GLSL shader structural balance (braces/parens in template literals).
2. All DOM ids referenced in JS exist in HTML.
3. All function references resolve (check for undefined function calls like toast, spawnDebris, spawnSplash, spawnConfetti, markBridgeDirty, rebuildEnvironment, updateStats, positionCameraForLevel, recomputeMasses — all defined somewhere).
4. Simulate the physics in Node to validate the bridge mechanics! I can stub browser parts: extract just the physics functions... That's complex due to THREE imports and DOM. Instead, write a standalone Node test replicating the solver core? Time is limited (50%+ used). At minimum, run a quick smoke test of the solver logic by extracting relevant pure functions.

Actually, a smarter validation: create a small Node harness that stubs THREE, DOM, etc.? Too heavy. Alternative: write a quick standalone copy of the solver math (nodes/beams/XPBD/truck) — but that duplicates code and may diverge.

Middle ground: I can load app.mjs in Node with a mocked environment: stub 'three' module etc. via import maps won't work in node. I could transform the source: remove import lines, stub THREE classes used at module top-level... The module does DOM access at top level (document.getElementById, addEventListener, renderer...). Too much stubbing.

Given time constraints, do these checks:

1. DOM ids referenced via $('...') and getElementById exist in HTML.
2. GLSL braces balance.
3. Cross-check function calls vs definitions for the main custom names.
4. Logic review of solver by re-reading.

Then deploy index update. Let me also write a small standalone physics sanity test reimplementing the core loop minimally to make sure a Pratt truss holds a truck — valuable but time is tight; the checkpoint said 50% at ~iteration 2%. I'll do a compact physics test: copy solver functions from app.mjs with stubs — actually I can do it: extract functions via regex... risky.

Quicker: write an independent mini-sim in Node mirroring the algorithm (30 lines) to validate XPBD params produce stable bridge behavior — validates parameter choices (compliance, break strains, masses) even if not the exact code. Let me do a focused one: build pratt preset programmatically (same generator logic copied), run substeps with truck, check truck crosses & max stress, and a deck-only bridge collapses. This is worth it for physics confidence. The copied logic must match: I'll copy the exact constants.

Actually simplest robust approach: transform app.mjs into something node-runnable:

• Remove the 7 import lines.
• Prepend a stub header defining minimal THREE (Object3D, Color, Vector2/3, InstancedMesh, etc.) and DOM (document.getElementById returning proxies, addEventListener noop, requestAnimationFrame noop, localStorage, performance, window...). The module calls animate() → requestAnimationFrame stub noop → fine. setupLevel(0) runs → needs DOM stubs. Then expose globals for testing: append globalThis.__test = {startTest, stepPhysics, beams, ...}. But P, beams are module-scoped; export via appending code that assigns to globalThis.

This is doable! Stub header ~60 lines. THREE usage at module top: WebGLRenderer, Scene, Fog, PerspectiveCamera, OrbitControls, lights, InstancedMesh, Object3D, Color, ShaderMaterial, Mesh, geometries, Points, CanvasTexture, Sprite, EffectComposer etc. Many classes to stub — a generic Proxy-based stub: const THREE = new Proxy({}, { get: () => class { constructor(){ return new Proxy(this, {get...(noop)}) } } }) — tricky with new THREE.Color(0xff) needing real Color for math... Colors used in beamColor etc. For physics test we don't need rendering colors to work — they just need to not crash. A generic magic class: constructor accepts anything; every property get returns another magic callable object; set ok; numeric ops? out.copy(base).lerp(_c2, x) — magic returns magic ✓. But MATS uses real new THREE.Color — magic fine.

Math.hypot etc real. The risk: somewhere needs real math on stubs at module load — e.g., new THREE.Vector3(-0.45,0.62,0.4).normalize() → magic method returns magic ✓. camera.aspect set ✓. document.createElement('canvas') → needs getContext returning magic with createRadialGradient... magic ✓. canvas.getContext('2d') magic with methods ✓.

requestAnimationFrame: noop → animate runs once then schedules noop ✓. But animate calls composer.render() magic ✓, updateStats touches real DOM via $ stubs — my document stub returns magic elements with style proxy, classList magic, textContent settable ✓. setTimeout real exists in node ✓.

localStorage stub. performance exists in node ✓. AudioContext: only on AE.init — not called (no pointer events). localStorage.getItem at module top — stub.

Then test driver appended:

But module top-level runs animate once with dt=clock.getDelta() — THREE.Clock stub magic → getDelta returns magic → Math.min(magic, 0.05) → NaN... Math.min on object → NaN → dt NaN → stepPhysics not called (mode build) ✓; updateDebris(NaN) → loops fine (empty). rebuildBridgeInstances iterates beams — ground beams exist! setBoxInstance uses _dummy magic ✓, frameMesh.setMatrixAt magic ✓. beamColor uses _c1.copy magic ✓. NaN propagates harmlessly. toast uses $('toasts').appendChild magic ✓ and setTimeout real → el.classList.add magic ✓ (element stub). document.createElement returns magic ✓. OK.

The magic stub:

Symbol.toPrimitive → 0 helps Math.min(magic,0.05) → 0 fine. But ndc.set(...) fine. new THREE.Vector2(innerWidth, innerHeight) — innerWidth undefined in node → construct magic ✓.

document stub: { getElementById: ()=>magic(), createElement: ()=>magic(), querySelectorAll: ()=>[], addEventListener(){}, body: magic() }; window = globalThis; addEventListener global noop; localStorage={getItem:()=>null,setItem(){}}; devicePixelRatio=1; innerWidth=1920; innerHeight=1080; requestAnimationFrame=()=>0; renderer... all via THREE magic.

OrbitControls etc from 'three/addons' imports — remove all import lines, define const OrbitControls=magic(), EffectComposer=magic(), etc.

Driver:

Expect success with truck (6t) on pratt for G=20 budget 10000. If physics params are sane, should pass. Also test deck-only collapse: clear, add road deck only across, startTest, run → expect fail & brokenCount>0.

But wait: startTest calls spawnTruck → AE.engineStart → this.ctx null → returns ✓; AE.horn → this.ctx null → tone returns ✓ (tone checks !this.ctx return ✓). AE.engineSet guards this.engine ✓. AE.splash → burst checks ctx ✓. toast → DOM magic ✓. spawnDebris → debris array real ✓ (new THREE.Color().clone? in spawnDebris: mcol.clone().multiplyScalar — mcol is magic... MATS[mat].col = new THREE.Color() → magic; .clone() → magic ✓ fine).

setHSL in spawnConfetti on magic ✓.

updateStats → $ magic ✓ but costSoFar() real ✓.

chart push only in animate; test driver calls stepPhysics directly ✓.

setupLevel calls rebuildEnvironment → THREE magic + document magic (createElement canvas for cloudTex at module top — magic returns prox; new THREE.CanvasTexture(c) magic ✓). makeCliffGeo: new THREE.BoxGeometry(...) magic; geo.attributes.position magic; pos.count → magic → for (i=0; i<pos.count; i++) — pos.count is magic prox; i < prox → prox coerces via Symbol.toPrimitive → 0 → loop doesn't run ✓ good. geo.setAttribute magic ✓. Mesh magic ✓. position.set magic ✓.

flagMats.push(fmat) — fmat = new THREE.ShaderMaterial({...}) magic; in animate: f.uniforms.uTime.value = t — magic set path ✓.

anchorGroup — real code: const anchorGroup = new THREE.Group() magic; anchorGroup.children.forEach — children is magic → forEach is magic callable → returns magic, doesn't iterate ✓. In animate anchorGroup.children.forEach((m,i)=>...) magic no-op ✓.

envGroup.children.length → magic → while(magic) — while loop condition on prox → prox coerces?? while(prox) uses ToBoolean → proxy of function is truthy! INFINITE LOOP in rebuildEnvironment: while (envGroup.children.length) — envGroup.children.length is magic prox → truthy forever → infinite loop!! Must handle: make magic's length return 0: in get handler, if k==='length' return 0. Also document.querySelectorAll returns [] real. envGroup.children.pop() returns magic → o.traverse?.() magic ✓.

Similarly other while loops on magic: while ($('toasts').children.length > 4) in toast — length → 0 ✓ with the fix.

Also arrays: beams real arrays ✓.

new Float32Array(...) real ✓ but not reached.

clouds loop at module top: for (i=0;i<9;i++){ const sp = new THREE.Sprite(...); sp.position.set(...); sp.scale.set(...); sp.userData.v = ...; clouds.push(sp); scene.add(sp); } — clouds real array, sp magic ✓. In animate: for (const c of clouds) { c.position.x += ...} magic ✓.

OK — with length→0 special-case the harness should work. Also hash3 real ✓ (used for mountains/clouds) fine.

controls.target.set magic; controls.update() magic ✓. camera.position.x += ... only when quakeT>0 ✓ not in driver.

riverMat.uniforms.uTime.value = t — riverMat real object? riverMat = new THREE.ShaderMaterial(...) magic ✓.

Chart: cctx = chart.getContext('2d') magic ✓. pushChart only called in animate when mode==='test' — animate runs once at module load in build mode ✓ not called in driver.

localStorage stub needed at module top (bestScores parse) ✓.

document.getElementById('levelLine').innerHTML = ... magic ✓.

windSlider.value = ... windSlider = $('windSlider') magic ✓.

One more: in p7 document.querySelectorAll('#matRow .tb').forEach(...) → return [] real ✓.

addEventListener('keydown'...) global — define globalThis.addEventListener noop... node has addEventListener on globalThis? Node 24: globalThis has EventTarget? There's addEventListener in Node? Not by default... Actually Node ≥ 15 has global EventTarget? globalThis.addEventListener doesn't exist in node (that's window). Stub: globalThis.addEventListener = () => {}.

renderer.domElement.getBoundingClientRect only in pointer handlers ✓ not called.

renderer.domElement.addEventListener — magic ✓.

OK let me also fix: const $ = id => document.getElementById(id) ✓.

Also new (window.AudioContext || window.webkitAudioContext) — only in AE.init ✓ not called.

localStorage — node has no localStorage → stub.

performance.now exists ✓. setTimeout exists ✓ (toast schedules; element stub classList magic ✓).

navigator? not used. devicePixelRatio — used: Math.min(devicePixelRatio, 1.5) → stub globalThis.devicePixelRatio=1.

innerWidth/innerHeight stub numbers ✓.

requestAnimationFrame stub → 0, so animate runs once at startup ✓.

getComputedStyle? no.

Extraction of module code: app.mjs already extracted (need re-extract after fix — app.mjs regenerated ✓ includes fix? I re-extracted after fixing deployed file → yes app.mjs is current).

Build harness:

Remove import lines: grep -v '^import ' app.mjs > body.mjs. But imports span single lines each ✓ (all single-line imports).

Driver as described. Then node harness.mjs.

Potential pitfall: module-level await? none. export? none.

Let me write the harness.

Wait — the children getter returns [] — but then envGroup.add(anchorGroup); anchorGroup.clear() — anchorGroup.clear is magic ✓. But rebuildEnvironment does envGroup.add(anchorGroup); anchorGroup.clear(); then later anchorGroup.add(m) magic ✓. In animate: anchorGroup.children.forEach → children [] → forEach no-op ✓.

But while (envGroup.children.length) → children [] → length 0 → exits ✓.

One issue: const clouds = [] real, clouds push magic sprites; animate iterates clouds (real array with magic items) ✓.

In rebuildEnvironment: const o = envGroup.children.pop() — not reached since while exits immediately ✓ (children never really accumulate since add is magic).

Also renderer.domElement — magic; app.appendChild magic ✓.

Wait — problem: scene.add(...) magic. camera.position.set(...) magic. BUT positionCameraForLevel uses camera magic ✓.

updateSunShadow: sun.shadow.camera real assignments magic ✓.

Another catch: p2 MATS creates new THREE.Color(0x7d8590) → magic ✓. beamColor: out.copy(...) magic ✓.

In p6: const dropGeo = new THREE.BufferGeometry() magic; dropGeo.setAttribute magic; but then real dropPos Float32Array ✓; dropGeo.attributes.position.needsUpdate = true — attributes magic ✓.

const _dummy = new THREE.Object3D() magic — used in setBoxInstance: _dummy.position.set(...) magic, _dummy.rotation.set(...) magic, _dummy.scale.set(...) magic, _dummy.updateMatrix() magic, mesh.setMatrixAt magic ✓.

frameMesh.count = fi — set on magic ✓; frameMesh.instanceMatrix.needsUpdate ✓.

Now driver: after module body, append:

Careful: driver references functions/vars: setupLevel, applyPreset, costSoFar, playerBeams, startTest, stepPhysics, testOutcome (module-level let — readable in same module scope ✓ since driver appended to same file), brokenCount, truck, deckSagNow, stopTest, addBeam, k2, TRUCKS, truckSpec (assignment to module let ✓).

stopTest calls removeTruck etc ✓; also calls recomputeMasses & updateStats (magic DOM) ✓.

Issue: startTest requires playerBeams().some(b=>b.mat==='road') ✓ pratt has road.

Issue: spawnTruck — AE.engineStart returns (no ctx) ✓; AE.horn returns ✓.

Potential issue in animate-once at module load: runs before driver; mode='build'; fine. But note module load runs setupLevel(0) at init → then driver setupLevel(0) again → resets ✓.

Note: toast at init → magic DOM ✓ setTimeout scheduled (3.3s) — process.exit before? Node keeps process alive until timers... driver may finish in <1s; setTimeout callbacks fire later calling el.classList magic ✓ harmless, but process.exit(0) skips them.

NaN hazard: animate runs once with dt = Math.min(clock.getDelta(), 0.05) — clock magic → getDelta() magic → coerces 0 via Symbol.toPrimitive? Math.min(magicObj, 0.05): magicObj converted to primitive → valueOf? Proxy get for Symbol.toPrimitive returns ()=>0 ✓ → min(0,0.05)=0 → dt=0. Then updateDebris(0) etc fine. stepPhysics not called. rebuildBridgeInstances with real beams ✓ magic mesh ops. fpsEMA += (1/Math.max(0,1e-4)...) → 1/1e-4=10000 → fpsEMA jumps... if (fpsEMA<42 && ...) no; else if (fpsEMA>57 && tier>0) tier=1 → upTimer+=0 ... fine no crash.

Also riverMat.uniforms.uTime.value = t — riverMat is magic (ShaderMaterial) ✓... wait, in rebuildEnvironment: if (!riverMat) — riverMat initially undefined → creates magic → assigns ✓.

gridPlane real var magic ✓.

Compile: node harness.mjs. Let me assemble: pre.mjs + body (app.mjs minus import lines) + driver.mjs.

Interesting results — the harness runs! But issues:

T1: pratt cost 0, beams 0 — applyPreset failed! Why? Preset cost check: pratt for G=20... let me compute: deck road 10 beams ×2m ×170 = 3400; verticals 6×4m steel = 2760; top chord 5×4m steel = 2300; diagonals 10×5.6m steel = 6440... total ~14900 > budget 10000 → "Preset exceeds budget" toast → aborted! And then startTest: no road → toast, mode stays build, run() does nothing (stepPhysics runs? startTest returned early; mode='build'; run calls stepPhysics anyway → but testOutcome null forever, truck null... truckX '-'. OK.

So pratt preset too expensive for level 0. Need to rebalance: either increase budget or make pratt cheaper (4m panels). Real fix: pratt with 4m panels: deck road every 2m is needed for wheel contact continuity? Wheel radius 0.55, gaps of 4m between road beams would swallow the wheel. Keep road every 2m (that's cheap: 10 × 2 × 170 = 3400). The steel above: use 4m panels with fewer diagonals. Let me recompute pratt-lite:

• deck: 10 road beams 2m = 3400
• verticals at x=-10,-6,-2,2,6,10 → 6 × 4m = 24m steel = 2760
• top chord: -10..10 step 4 → 5 × 4m = 20m = 2300
• diagonals: from each top node to center: -10top→-6,0? Classic pratt: diagonals point toward midspan at bottom. Panels: [-10,-6],[-6,-2],[-2,2],[2,6],[6,10]: diagonals: (-10,4)→(-6,0), (-6,4)→(-2,0), (2,4)→(6,0)? wait toward center: left half diagonals from top-left to bottom-right (toward center), right half from top-right to bottom-left: (-10,4)→(-6,0); (-6,4)→(-2,0); (-2,4)→(2,0)?? that's 5.66m; (10,4)→(6,0); (6,4)→(2,0); (-2,4)→(2,0) length 5.66 steel ≈ 650 each × 6 = 3900. Total ≈ 3400+2760+2300+3900 = 12360 — still > 10000.

Options: raise level 0 budget to 14000? But then hand-building is trivially affordable... Budget should feel meaningful but presets should fit. Set budgets: L0 15000, L1 22000, L2 17000, L3 16000. And trim presets a bit (height 4→3.5?). Simpler: raise budgets & keep presets. Also T3 suspension cost 15326 on budget 17500 → fits ✓ (with new 22000 plenty). Arch 9756 ✓.

Also T1 pratt after budget fix — need to verify it survives. Let me also reconsider pratt design stability: verticals+top chord+diagonals: my diagonal pattern: for x=-half; x<0; x+=4: (x,h)→(x+4,0) wait current code: L.push([k2(x, h), k2(x + 4 === 0 ? 0 : x + 4, 0), 'steel']) → for x=-10: (-10,4)→(-6,0) ✓; x=-6: (-6,4)→(-2,0); x=-2: (-2,4)→(2,0)?? x+4=2 not 0... the ternary handles only x=-4 case (x+4===0). With step 4 from -10: -10,-6,-2 → x+4 = -6,-2,2. Hmm -2→2 skips 0? nodes at 0 exist (vertical at x=2? verticals at -10,-6,-2,2,6,10 — no node at 0!). Wait verticals: for x=-half; x<=half; x+=4 → -10,-6,-2,2,6,10 ✓ (no 0). Top chord: x to min(x+4,half): -10→-6... -2→2 (skipping 0 ✓ fine since no node at 0). Diagonals left: x=-10→(-6,0), -6→(-2,0), -2→(2,0). Right: for x=half; x>0; x-=4: (x,h)→(x-4,0) → (10,4)→(6,0), (6,4)→(2,0), (2,4)→(-2,0). All triangles closed ✓. That's a valid pratt with 4m panels except center panel -2..2 has crossing diagonals both ways — fine (double intersection, strong).

Length check: diagonals (-2,4)→(2,0): 5.66 ≤ 8 ✓.

T2: bare deck = success with truckY 1.55 — the deck HELD with a 12t hauler?! broken=0. That's wrong — a 20m unsupported deck of 2m road beams should collapse. Why did it hold? Deck beams road: break strain tension 0.085 — with compliance 0 (rigid!). Road comp=0 → perfectly rigid distance constraints → the deck acts like a rigid chain; tension infinite rigidity means no strain ever exceeds... strain computed as (d-rest)/rest — rigid PBD projects fully each iteration so strain stays ~0! That's the flaw: with comp=0, constraints are infinitely stiff → measured strain is post-projection ≈ 0 → never breaks.

The strain used for breaking must be measured BEFORE projection (pre-solve residual) or computed from forces. Better: measure strain at the START of constraint processing (before applying correction) — i.e., current geometric strain. With rigid constraints after convergence it's ~0, but during load transients there will be residual. Hmm, with Gauss-Seidel order effects the last-solved beams show strain. Unreliable.

Robust approach: estimate constraint FORCE from XPBD lambda: F = λ / h² (XPBD: λ is related to impulse; force ≈ λ/h²... actually Δx applied corresponds to impulse; XPBD λ accumulated: force = λ/h² × (something with units kg·m/s²·?). In XPBD, λ has units such that C + α̃λ relationship: the constraint force F = λ / h² (units N when masses in kg, C in m). Then stress ∝ F / ( EA )... We can compute equivalent strain = F·rest/(EA)? Simpler: break when |F| > Fbreak (force threshold in Newtons) per material. F = λ/h² where λ accumulated per substep (I reset lam each substep after velocity... I reset b.lam = 0 after each substep in the loop for (const b of beams) b.lam = 0; ✓ per substep).

So per substep, after solving, force = b.lam/(h*h). With h=1/480: h²=4.34e-6. A deck under 12t truck: support forces ~ tens of kN → λ = F·h² ≈ 50000×4.34e-6 ≈ 0.22 — measurable ✓.

Breaking thresholds as FORCE: road deck section: steel-ish I-beam ~ area 0.02 m² → breaking force ~ 0.02×400e6 = 8000 kN?? Too high — a 20m span deck fails by bending/tension in bottom chord... For gameplay, tune: a 2m road beam should break when carrying more than ~15t point load mid-span with no support. Let me just pick gameplay forces: compute typical tension in bottom chord of a truss: F = M/w where M = bending moment = W·L/4 for point load mid span: W=12t=118kN, L=20m → M=590 kN·m; w=truss depth 4m → chord force 148 kN. So truss bottom chord sees ~150kN. A bare deck (no truss, depth ~0.3m beam itself): chord force = M/w with w=0.3 → ~2000 kN → breaks if threshold ~ 400-600 kN. Set break forces:

• road: 500 kN tension? Then pratt bottom chord deck at 150kN survives with margin 3x; add wind/quake/van? hauler 12t → 150kN; two trucks? only one. Quake doubles loads → 300kN still under. Hmm, want quake + hauler to threaten pratt. Set road break 260 kN? Bare deck: 2000kN ≫ 260 breaks instantly ✓. Pratt with hauler 150kN < 260 ✓ holds; with quake ~1.8× → 270 kN — snaps! dramatic ✓.
• steel member (0.22 thick box): break 300 kN (top chord compression similar magnitude as bottom chord tension; in pratt top chord compression ~150kN; diagonals less). Steel slightly stronger than road per game balance: 320kN.
• wood: 130 kN.
• cable: 200 kN (tension only). But these absolute forces scale with span & load — Long Span G=30: M = W·L/4 = 118×30/4 = 885kN·m; truss depth 4 → 221kN chord — under 260/320 ✓. Hauler on suspension: hangers carry local loads ~ deck+truck tributary... suspension worked in T3 with broken=16! (yet still "success" — it crossed while 16 beams snapped?! With rigid constraints breakage was strain-based≈random. After force-based breaking, re-test.)

Compression buckling: same force threshold × 0.8.

Also stress display: stress = |F|/Fbreak ✓ nice and stable. EMA it.

Units check via harness: print max force seen. I'll add debug in T1.

Changes to solver:

But b.lam accumulates over the single substep's iterations (1 iteration per substep here — I do one G-S pass per substep). So b.force = b.lam/h² fine.

Then breaking: tension: F>Fbreak_t; compression: -F>Fbreak_c. For cables slack (skipped when d<=rest) force=0 ✓ set b.force=0 for skipped cables.

strain for display: stress = |F|/Fthr; keep field name b.stress EMA of that.

Materials table: replace bt/bc (strain) with ft/fc (Newtons): road ft 260e3 fc 320e3; steel ft 330e3 fc 260e3 (buckling lower); wood ft 140e3 fc 110e3; cable ft 220e3 fc inf. Randomization ±7%.

beamColor: t = stress already normalized ✓ (stress field stores |F|/Fthr smoothed).

stats loop: thr = strain>=0 ? bt : bc — change to force sign. Keep b.forceRaw sign. b.force = λ/h² (signed: positive = tension? Constraint C = d-rest>0 → tension → λ sign: dl = (-C - α̃λ)/(wSum+α̃) → negative λ for tension (pulling together). So tension → b.lam negative → force = -b.lam/h² positive for tension. Define b.forceT = -b.lam/h².

XPBD λ units: Δp = λ·∇C/w... force = λ/h² in Newtons when masses kg, lengths m, h s. ✓

Also T2 truckY=1.55 static: with rigid deck it drove across. After force-based breaking it should snap. Also note even if deck holds, 2m segments rigid chain would transmit... fine.

T3 broke 16 but succeeded — with force breaking, re-tune so suspension survives 6t. Suspension: hangers carry deck+tributary truck weight: total deck weight: 15 road beams×2m×40kg/m=1200kg + truck 6000 localized on 2 hangers ~ 30kN each ≪ 220kN ✓. Main cable tension: total load ~ (deck 12kN + truck 59kN) → cable tension ≈ wL²/(8sag')... sag' = th-sag = 7.5m rise: T ≈ (71kN × 30)/(8×7.5×...)≈ 35-90kN ✓ under. Tower compression: ~35kN ✓. Should hold.

Van on arch T4: fine.

Another issue: stress EMA update rate — b.stress += (target - b.stress)*0.06 per frame ✓ fine.

Also should I reduce SUBSTEPS impact on force: force = λ/h² where h = substep dt — with 8 substeps at 60fps h=1/480. If fps drops to 30, h=1/240 → forces computed scale differently but consistently (λ ∝ h²·F). ✓

Budgets: L0 15000, L1 23000, L2 17000, L3 16500.

Also T1 pratt cost ~12360 ≤ 15000 ✓.

Now implement changes:

1. MATS: replace bt/bc with ft/fc.
2. addBeam: beam.bt = mat.ft*(rand), beam.bc = mat.fc*(rand).
3. substep beam loop: track b.lam → after loop compute force & breaking.
4. Remove strain-based break inside loop; set b.forceT = -b.lam/h²; cable slack → forceT=0.
5. stats loop: thr by sign of forceT; sN = |forceT|/thr.
6. MATS.thick etc unchanged.
7. LEVELS budgets.

Note: break check per substep AFTER full G-S pass → forces settled for that substep ✓.

Edge: beams newly created at rest length with λ=0 ✓.

Quake + hauler on pratt → snapping ✓ fun.

One more consideration: the truck wheels pushing deck creates large local contact forces on road beams? Contact correction is positional (not through beam constraints) — deck beams feel it via node displacement in the next constraint pass ✓.

Also the force on beams connected to anchors includes carrying the whole bridge weight: pratt self-weight ~ (deck 3400$→800kg + steel ~1500kg) → modest.

Let me apply the edits to the deployed file directly (parts are stale; from now on edit deployed file only). Also update the harness after edits (re-extract).

Now update the solver loop in p4 section of the deployed file: replace strain-based breaking with force-based, and track b.forceT. Current code:

New: track forceT = -lam/h² (tension positive). Cables slack → forceT=0. Break check after assignment. Also set forceT=0 for skipped beams (dead, cable slack, wSum 0).

Note sign: C>0 (stretched) → dl = (-C - at*lam)/(wSum+at) negative → lam negative → forceT = -lam/h² positive ✓ tension positive.

Compressed (C<0) → lam positive → forceT negative ✓.

Now update the stats loop in stepPhysics that used b.strain:

Change to forceT. Also if (b.mat === 'road' && !b.fixed) unchanged.

Progress: T1 pratt now succeeds with cost 12363 ✓ (fits budget 15000), sag 0.15m, no breaks —

T2 STILL succeeds with 0 broken — bare 20m deck holding a 12t hauler. Why? Let me think: force threshold road 260kN... The deck: 10 road beams 2m, anchored at both ends. Truck 12t → 118 kN total, on 2 wheels → each ~47kN (wm=3840 each... wheel mass 0.32×12000=3840, chassis 4320). Point load mid-span on a "beam chain" spanning 20m: tension in chain = M/w but the chain has no depth! A pure horizontal chain with a point load acts like a cable: T = W·L/(8·sag). As it sags, tension drops. With rigid-ish constraints it sags until T < 260kN: sag needed = W·L/(8·T) = 118×20/(8×260) = 1.13m. So the deck sags ~1.1m+ and holds like a cable net! Physically plausible for a chain — road beams DO carry compression in reality, and as a pure tension chain it behaves like a cable.

But gameplay-wise a bare deck SHOULD fail: real deck beams can't act as tension chain across 20m because joints would be pinned... Actually a chain CAN hold via tension — that's physically correct! The failure mode for a real deck is that short beams cantilever/bend — not modeled in a pin-jointed truss sim.

Standard Poly Bridge approach: roads break because they take compression/tension beyond capacity when the deck bends — in a pin-jointed model, a sagging deck under a wheel creates huge LOCAL tension at the wheel contact... hmm.

Options to make bare deck fail:

1. Lower road tension capacity (260→120kN): bare deck needs T=W·L/(8·sag); if sag can grow unbounded (deck stretching into a V down to water 11m) → T = 118×20/(8×11)=27kN → still holds if it can sag to water! A "chain" will always hold given enough sag unless it touches water (then truck splashes → fail ✓). Sag to water = 11m → truck chassis hits water → fail! So why didn't it splash? sag limited by deck rigidity (road comp 2e-7 is quite stiff → limited stretch). With comp 2e-7, α̃ = α/h² = 2e-7/4.34e-6 = 0.046 → stiff but stretchy under high loads? The stiffness k ≈ 1/α = 5e6 N/m per unit... For a beam, XPBD compliance α = 1/(EA)·rest? For road E·A: comp per constraint = rest/(EA) → with rest 2, EA = rest/comp = 2/2e-7 = 1e7 N (per "strain unit") → k = EA/rest = 5e6 N/m. Chain of 10: effective axial stiffness total ~ k/n = 5e5 N/m. Tension 260kN → stretch per beam = T/k = 0.05m → total 0.5m elongation → sag ≈ sqrt((L+Δ)²-L²)/2-ish... ΔL=0.5m over 20m → sag ≈ √( (20.5/2)² - 10² ) = √(105-100)=2.25m? Enough to geometrically reach ~2.2m sag. T at sag 2.2: 118·20/(8·2.2)=134kN < 260kN ✓ equilibrium found at ~2.2m sag — deck holds, truck rides through a dip. Plausible physics, bad gameplay.

Gameplay fix: make road beams break by COMPRESSION easily and tension moderate, but the key: a chain in pure tension won't break. Real bridges fail via bending. Introduce "angular stress"? Complex.

Alternative pragmatic fix: water kills: if the deck sags enough that the truck chassis (y≈deck+1) — with sag 2.2m the truck rides the dip and climbs out — chassis never near water (-11).

Better fix: wheel grip & climbing: truck needs to climb out of a 2.2m dip over 10m — slope ~25% — my motor applies vx force toward target speed with grip 0.10... it made it (outcome success).

Pragmatic gameplay solution: make road beams WEAK in tension relative to chain loads: if ft_road = 60kN: bare deck at equilibrium needs ≥134kN at 2.2m sag → snaps progressively as it sags ✓. Then pratt bottom chord (road deck in pratt carries chord tension ~150kN for hauler!) would break too... In real pratt, the deck IS the bottom chord. Hmm.

Rethink: in pratt T1, sag 0.15m with 6t truck — bottom chord deck tension: M/w = (59kN×20/4)/4m = 74kN. With hauler: 148kN. If ft_road=200kN: pratt holds hauler (148<200 ✓), bare deck: sag equilibrium at T=200 → sag = 118×20/(8×200)=1.47m, required stretch: sag 1.47 → ΔL ≈ 2×√(10²+1.47²)-20 = 2×10.107-20 = 0.21m → T = k_total×0.21 = 5e5×0.21=107kN < 200kN → equilibrium at sag where T(sag)=118·20/(8·sag) with T = 5e5·ΔL(sag): try sag 3m: ΔL=2√(109)-20=0.88→T=440kN; chain rule T=2360/(8·3)=98kN <440 → equilibrium between: sag 2m: ΔL=2√104-20=0.396→T=198kN; needed=147kN → net pull-in; sag 2.5m: ΔL=0.61→307kN vs needed 118 → pulls back to ~2.0m. Equilibrium 2m, T198kN ≈ ft 200kN — borderline! With ±7% random and dynamic bouncing as truck moves, some decks break, some hold. Meh.

Cleaner: add bending failure via "hinge angle": road beams break if the ANGLE between adjacent road beams at a node exceeds threshold (kinking) — like the deck snapping at a joint. That's physical (deck segments can't hinge 30°). Implement: after constraint pass, for each node, find pairs of alive road beams sharing the node; compute angle between them; if |angle change from original| > ~0.35 rad → break the more stressed one. Need original angles: compute at build/test-start? Original = as-built geometry (straight deck = π). During test, deck sags into V → kink angles grow → break. But a gentle sag of truss deck: bottom chord remains nearly straight under a stiff truss (sag 0.15/20m → angle change tiny ✓). Bare deck at 2m sag: kink at mid ≈ 2×atan(2/10) = 0.4 rad > 0.35 → snaps ✓.

Simpler variant: per-beam "bending stress" = angle difference between this beam and the average direction of neighboring road beams... Let me implement kink check:

• Precompute per node (for nodes with exactly 2 road beams): rest angle between them (as built, typically π).
• Each frame (not substep — cheaper): for each such node pair (b1,b2): current angle θ; kink = |θ - restθ|; if kink > 0.38 → break the beam with higher stress. Throttle: check every frame; also cool-down fine.

Wait — also applies to truss decks: the deck in an arch bridge preset curves?? Arch deck is flat ✓. Suspension deck flat ✓. Presets fine. Player builds with intentionally angled decks (over an arch top)? Rare; the kink threshold 0.38 rad (~22°) allows designed slopes with joints up to 22° — but a designed angled joint would have restθ = as-built angle → kink measures DEVIATION ✓ good.

Also truckWheels crossing a joint adds local kink... rest angles straight; under wheel, local sag kink ~ small for supported deck.

Additionally lower road ft to 240kN & compression fc 200kN (decks buckle). Fine-tune via harness: T2 must FAIL, T1 must PASS with van/truck/hauler, T3 pass (fix 13 broken — investigate which beams broke: probably hangers too weak? hanger cable ft 230kN... or deck kink? currently no kink; 13 broken with force model — which? Add debug: log material of broken beams. Let me add a counter by material in breakBeam? Quick: in driver, after run, can't inspect removed beams... add global array brokenLog push b.mat in breakBeam — add brokenByMat object. I'll patch: in breakBeam add (window.__bbm = window.__bbm || {})[b.mat] = ((window.__bbm||{})[b.mat]||0)+1; — simpler: globalThis.brokenLog array; breakBeam pushes {mat, forceT}. Add to breakBeam: if (globalThis.__brokenLog) globalThis.__brokenLog.push(b.mat + '@' + Math.round(b.forceT/1000) + 'kN'); Then driver sets globalThis.__brokenLog=[] before each test and prints. Minimal invasiveness.

Implement kink check in stepPhysics (per frame, cheap: precompute node→road beams map at test start):

Compute rest angle: vectors from shared node to other endpoints. Store {b1,b2,node,rest}. In stepPhysics: for each pair alive: angle now; if |Δ|>0.38 → break higher-stress beam.

Cost: pairs ≤ roadBeams — fine.

Rebuild kink map at startTest (structure static during test except breaks — breaks remove beams; pairs referencing dead beams skip).

Edge: also kink at nodes with 2 road beams where one is fixed ground? The joint between ground road and player deck at cliff edge: include fixed in map? If deck hinge-rotates at anchor it should break — include pairs where one fixed + one player: break the player one. OK include fixed beams in byNode, but never break fixed (choose the other).

Now T3's 13 broken — need diagnosis; will re-run with brokenLog.

Also T1 hauler+quake test add. Let me implement everything:

1. breakBeam: add logging hook.
2. kink map + check in stepPhysics.
3. Tune: road ft 240kN / fc 200kN; steel ft 330/fc 255; wood 140/110; cable 230.

Then re-run harness with expanded driver: T1 truck; T1b hauler; T2 hauler expect fail; T3; T4; T5 quake on pratt (expect some drama but maybe survive). Update driver.

80% of time used! I must finish quickly. Remaining critical work:

1. Define kinkPairs + buildKinkMap (referenced but not defined!) — CRITICAL, otherwise runtime crash.
2. Re-run harness, tune.
3. Update index.html listing.
4. Final syntax check.

Let me add buildKinkMap function + call in startTest, and declare kinkPairs. Insert near solver globals. I'll add right after breakBeam definition... simplest: add function definition before stepPhysics and call it in startTest. Use replace on deployed file.

My sed hack broke it (startTestCalled undeclared) AND — importantly — T1 now FAILS with 9 broken, sag 10.35! The kink check is too aggressive: the pratt deck under the 6t truck developed >0.38 rad kink somewhere and cascade-failed. Hmm, sag 10.35 = total collapse to water.

Wait — actually maybe the failure isn't kink: 9 broken with sag 10m. Let me revert my sed mess and use a cleaner approach: edit driver.mjs properly with logging, and also print kink info. The __brokenLog already records ' kink' suffix.

Why would pratt deck kink? Kink threshold 0.38 rad deviation from rest. The pratt bottom chord IS the deck; under load it sags slightly but also the truck wheel pushes individual deck beams down between panel points — panel length 2m; wheel on a 2m deck beam supported only at ends → that beam sags locally: wheel load 47kN mid-beam... beam compliance gives local sag δ; kink angle at joints ≈ 2×δ/L. If δ = 0.2m on 2m beam → 0.2 rad OK; if δ=0.4 → 0.4 rad → break. XPBD chain: the deck beam directly under wheel carries the full wheel load with only its axial stiffness → sag until vertical component of tension balances: for mid-point load on a 2m beam: T×2×(δ/1) = W/2 per side... T×(2δ/L)... with T capacity 260kN: W=47kN → δ = W·L/(8T)·? For a point load W mid-span on a cable of length L: T = W·L/(8·δ)... wait same formula: δ = W·L/(8T) = 47000×2/(8×260000) = 4.5e-5 m — tiny! Axial stiffness high. So local kink shouldn't be large...

Unless the kink check has a bug: ox1 = kp.b1.a === n ? kp.b1.b : kp.b1.a ✓. rest angle for straight deck: vectors from node to neighbors point opposite directions: angle between = π ≈ 3.14159. d - rest: when sagging into V at node, angle between vectors shrinks below π → |d - π| grows ✓. Hmm.

Wait — maybe the failure came from force breaking, not kink: with 6t truck the pratt worked before adding kink (T1 success, 0 broken). Now 9 broken. So kink check is the new breaker. Which joints? The deck sits ON TOP: my pratt deck at y=0, nodes at same points — the bottom chord nodes shared. Under truck, bottom chord in tension 74kN — small sag 0.15m over 20m → kink ≈ 2×0.15/10 per joint... 0.03 rad. Not it.

BUG POSSIBILITY: kinkPairs built from beams — includes FIXED ground road beams; ground road at y=0 continues straight into player deck (rest = π) ✓ fine.

Hmm wait, actually the issue could be the truck WHEEL contact: wheels push deck NODES down hard (contact correction pushes nodes). Wheel pushes a single node down between substeps — a node shared by two deck beams gets shoved down 0.1m+ transiently → kink spikes → snap. The wheel contact correction on beam endpoints is aggressive: b.a.x -= nx*corr*b.a.invM*0.5*(1-t) — pushes nodes directly.

Also the drive force accelerates wheel into beam.

Quick fix options:

1. Raise kink threshold to 0.6 rad and apply kink check only if SUSTAINED: track kp.hot time accumulator: if over threshold for >0.4s cumulative → break. Smooths transients.
2. EMA the kink value: kp.kinkE += (dev - kp.kinkE)*0.05; break when kp.kinkE > 0.42.

Option 2 simplest. Bare deck: sustained kink at mid → EMA reaches threshold in ~0.5s ✓ breaks. Transient wheel spikes get smoothed.

Also raise threshold to 0.45.

And revert sed hack in harness: regenerate harness from driver.mjs clean, add brokenLog prints in driver.mjs properly.

Update driver.mjs with logging + more tests. Let me do these edits fast.

92% of time used. I need to wrap up NOW. Remaining must-dos:

1. One final harness run (update driver with brokenLog, quick).
2. Syntax check.
3. Update demos index.html.
4. Screenshot/success.

Skip further tuning iterations — run harness once; if T1 passes and T2 fails, ship it.

T1 success again (kink EMA fixed the pratt). But T2 bare deck STILL succeeds — kink isn't triggering. Why? Bare deck sag equilibrium: T ≈ 198kN < 260kN road ft, sag ~2m. Kink at the middle joints: total sag 2m distributed over 10 joints — per-joint kink ≈ 2×sag×(2/n)... for a V shape concentrated at middle: if sag distributed smoothly like a parabola, per-joint angle = 4×sag/n×(2/2)... parabola sag S over span L with n segments: max slope at ends = 4S/L = 4×2/20 = 0.4 rad; per joint change = slope diff ≈ 8S/L/n = 0.08 rad per joint — below 0.45 threshold! The kink spreads over joints. Of course — a smooth sag has small per-joint angles.

So kink check only catches extreme localized hinges. Bare deck surviving by chain-tension is actually semi-physical; but gameplay-wise we need it to fail. Pragmatic: the deck sags 2m and the truck drives through... is that so bad? In Poly Bridge an unsupported 20m deck absolutely collapses.

Cheapest effective fix: reduce road tension capacity so chain tension exceeds it: bare deck equilibrium T≈198kN (hauler); set road ft=150kN: pratt bottom chord with hauler: 148kN — too close (would break pratt with hauler, OK with truck 74kN). Hmm but breaking pratt with the 12t hauler is actually FINE gameplay — heavy truck needs stronger bridge. But margin too tight (random ±7%).

Alternative: weight the truck more realistically relative to deck: hauler is 12t → the real issue is the deck tension equilibrium at 198kN vs pratt chord at 148kN — barely separated. Because in the truss, the chord takes M/w with w=4m; the chain takes M/w_eff where w_eff = sag = 2m — only 2x difference.

Increase separation: make deck chain sag MORE (softer road compliance): road comp 2e-7 → 8e-7: EA = rest/comp = 2/8e-7 = 2.5e6, k_beam = EA/rest = 1.25e6, chain k = 1.25e5 N/m. Equilibrium: T(sag) = 1.25e5 × ΔL(sag): sag 3m → ΔL 0.88 → T=110kN; needed 98kN → sag 3.2m, T105kN. Pratt chord: 148kN. Set road ft = 120kN: bare deck T 105kN < 120 — STILL holds. Ugh, chain tension decreases with sag; it always wins. The real killer: at sag 3.2m the truck must climb a 3.2m dip out of 10m — 33% grade. My motor is too strong! Real trucks can't climb 33%. Add grade limit: if the local road slope at wheel > ~18°, motor force insufficient (add gravity component along slope). Simple: motor applies vx only; gravity naturally pulls truck back down the slope via wheel contact normal... gravity acts vertically; climbing requires the contact+friction model — my simplified model lacks slope resistance.

Add: motor force proportional but cap climbing: compute slope under wheel (contact normal ny): if ny < 0.8 (slope > ~37°)... at 33% grade ny ≈ 0.95. Limit motor effectiveness by ny: w.vx += (target - w.vx) * 0.10 * clamp((ny-0.75)/0.25, 0, 1)? At ny 0.95 → factor 0.8. Weak.

Better: add rolling resistance against gravity on slopes: when on slope, gravity pulls along slope: apply ax_g = -G_ACC × slope_x to wheel... that's physical: v¨ along surface. My integrate step already applies gravity to wheel vy; the contact constraint projects out normal component leaving tangential → the truck SHOULD feel slope resistance from gravity naturally through the constraint! Then why did it climb 33%? Because motor applies vx directly (horizontal) — on an uphill slope, horizontal force has a component along the slope; gravity along 33% slope = 9.81×0.33 = 3.2 m/s² decel. Motor: vx += (target-vx)×0.1 per FRAME (at 60fps → huge: equivalent to ~ massive accel ≈ (target-vx)×6 per second → at vx 3, (7.4-3)×6 = 26 m/s²) — motor vastly overpowered. Fix: make motor force physical: max tractive force = μ × N ≈ μ × m_wheels... total truck 12000kg: traction F = 0.7×(weight on driven wheels≈half weight) ≈ 0.7×6×9.81×1000 = 41kN → accel on 12t = 3.4 m/s². So set motor accel capacity ~3.5 m/s² for hauler (scaled: van 4.5, truck 4, hauler 3.2), i.e., w.vx += clamp(target-vx, -adt... apply per frame: dv = clamp((target - v)0.5, -Adt, Adt) — accel-limited.

Then climbing: net accel = motor(≤3.2) - 9.81×slope. At slope>0.33 the truck stalls/rolls back → "stuck" or slides into water ✓. Bare deck dip 3.2m/33% → stall → stuck → likely dips further bouncing → break or stuck outcome ✓. Pratt flat deck ✓ unaffected. This is the physical fix.

Implement: replace motor block:

Wait — applying to each wheel doubles force (2 wheels): effective accel 2A. Set A = spec.accel/2. TRUCKS: van accel 4.4, truck 4.0, hauler 3.0 (per-wheel half applied automatically).

With gravity through contacts providing slope resistance — verify in harness T2 → expect stuck/fail. Also van (2t) on bare deck: W=19.6kN, chain T equilibrium ~ soft: needed T = W·L/(8·sag); sag where k×ΔL = T: light van: T ~ 19.6×20/(8×3)=16kN, sag ~2-3m, slope ~30% → van accel 4.4 vs gravity 3.2 → barely climbs ✓ nice — light vehicles can bounce across a sketchy deck, heavy ones can't.

BUT T2 previously succeeded partly because motor infinite. With limited motor + softer road, test again. Also T3's 13 broken — need to know what broke. Add brokenLog print. If hangers/deck break, may need cable/road tweaks. Time is nearly out — I'll do ONE more harness iteration with these changes, then ship regardless (outcomes don't need to be perfect, just plausible and crash-free).

Changes:

1. TRUCKS: add accel field.
2. Motor block with accel limit.
3. road comp 2e-7 → 8e-7.
4. driver: print brokenLog.

Then finalize: index.html listing, success. Also remember T1/T3/T4 all pass → game is playable.

NaN everywhere now! Something broke. truckX NaN immediately. What did I break? The motor block uses dt — but in stepPhysics, dt is the parameter — the motor block is inside stepPhysics ✓ dt defined. Hmm.

NaN truck position → something in the physics... The change list: truck accel specs, road comp, motor block. NaN could come from... t.spec.accel — if replace failed to add accel (assert would have caught motor block but not spec lines — no assert on those!). If spec.accel undefined → A = NaN → w.vx += clamp(NaN) → NaN. Let me check whether the TRUCKS replacement matched — the spec line in file: { name: 'Cargo Van', mass: 2000, speed: 8.5, pay: 2200, col: 0xd8a24a }, — note trailing comma! My python replace omitted the trailing comma → no match → accel never added → NaN. Confirmed.