feat(physics): add wall barrier

ROADMAP.md

@@ -122,7 +122,7 @@
   - [x] Stiffness design principle for frozen barrier stiffness
   - [x] Contact barrier with extended direction handling
   - [x] Pin constraint barrier using cubic barrier formulation
-  - [ ] Wall constraint barrier for plane collisions
+  - [x] Wall constraint barrier for plane collisions
   - [ ] Triangle strain-limiting barrier driven by deformation singular values
 - **Integrator and solver**
   - [ ] Inexact Newton integrator with beta accumulation

docs/index.d.ts

@@ -121,6 +121,7 @@ export const examples: {
     readonly computeFrozenStiffness: 'examples/foldStiffness.ts';
     readonly createContactBarrier: 'examples/foldContactBarrier.ts';
     readonly createPinBarrier: 'examples/foldPinBarrier.ts';
+    readonly createWallBarrier: 'examples/foldWallBarrier.ts';
   };
   readonly performance: {
     readonly debounce: 'examples/requestDedup.ts';
@@ -3361,6 +3362,20 @@ export interface PinBarrierOptions {
 }
 export function createPinBarrier(options?: PinBarrierOptions): FoldConstraint;
 
+/**
+ * Wall constraint barrier for plane contacts.
+ * Use for: enforcing collision against static planes with Fold guarantees.
+ * Import: physics/fold/wallBarrier.ts
+ */
+export interface WallBarrierOptions {
+  id?: string;
+  stiffnessOverride?: number;
+  maxGap?: number;
+  normal?: Vector3D;
+  planePoint?: Vector3D;
+}
+export function createWallBarrier(options?: WallBarrierOptions): FoldConstraint;
+
 export type FoldConstraintType =
   | 'cubic-barrier'
   | 'contact-barrier'

examples/foldWallBarrier.ts

@@ -0,0 +1,27 @@
+import { createWallBarrier } from '../src/index.js';
+
+const barrier = createWallBarrier({
+  planePoint: { x: 0, y: 0, z: 0 },
+  normal: { x: 0, y: 1, z: 0 },
+});
+
+const evaluation = barrier.evaluate(
+  {
+    gap: -0.03,
+    maxGap: 0,
+    stiffness: 0,
+    direction: { x: 0, y: 1, z: 0 },
+    effectiveMass: 0.25,
+    metadata: {
+      position: { x: 0, y: -0.03, z: 0 },
+      hessian: [
+        [3, 0, 0],
+        [0, 3, 0],
+        [0, 0, 3],
+      ],
+    },
+  },
+  { deltaTime: 1 / 90 }
+);
+
+console.log('wall energy', evaluation.energy);

src/index.ts

@@ -119,6 +119,7 @@ export const examples = {
     computeFrozenStiffness: 'examples/foldStiffness.ts',
     createContactBarrier: 'examples/foldContactBarrier.ts',
     createPinBarrier: 'examples/foldPinBarrier.ts',
+    createWallBarrier: 'examples/foldWallBarrier.ts',
   },
   performance: {
     debounce: 'examples/requestDedup.ts',
@@ -1197,6 +1198,7 @@ export {
   computeFrozenStiffness,
   createContactBarrier,
   createPinBarrier,
+  createWallBarrier,
 } from './physics/fold/index.js';
 
 export type {
@@ -1214,6 +1216,7 @@ export type {
   StiffnessDesignOptions,
   ContactBarrierOptions,
   PinBarrierOptions,
+  WallBarrierOptions,
 } from './physics/fold/index.js';
 
 // ============================================================================

src/physics/fold/index.ts

@@ -3,3 +3,4 @@ export * from './cubicBarrier.js';
 export * from './stiffness.js';
 export * from './contactBarrier.js';
 export * from './pinBarrier.js';
+export * from './wallBarrier.js';

src/physics/fold/wallBarrier.ts

@@ -0,0 +1,112 @@
+import type { Vector3D, Matrix3x3 } from '../../types.js';
+import type {
+  FoldComputationContext,
+  FoldConstraint,
+  FoldConstraintEvaluation,
+  FoldConstraintState,
+} from './types.js';
+import { createCubicBarrier } from './cubicBarrier.js';
+import { computeFrozenStiffness } from './stiffness.js';
+
+export interface WallBarrierOptions {
+  id?: string;
+  stiffnessOverride?: number;
+  maxGap?: number;
+  normal?: Vector3D;
+  planePoint?: Vector3D;
+}
+
+const ZERO_GRADIENT: Vector3D = { x: 0, y: 0, z: 0 };
+const ZERO_HESSIAN: Matrix3x3 = [
+  [0, 0, 0],
+  [0, 0, 0],
+  [0, 0, 0],
+];
+
+export function createWallBarrier(options: WallBarrierOptions = {}): FoldConstraint {
+  const baseBarrier = createCubicBarrier({
+    id: options.id,
+    maxGap: options.maxGap,
+    direction: options.normal,
+  });
+
+  return {
+    type: 'wall-barrier',
+    id: options.id,
+    enabled: true,
+    evaluate(state: FoldConstraintState, context: FoldComputationContext): FoldConstraintEvaluation {
+      const wallNormal = normalise(options.normal ?? state.direction);
+      if (!wallNormal) {
+        return { energy: 0, gradient: ZERO_GRADIENT, hessian: ZERO_HESSIAN };
+      }
+
+      const adjustedGap = adjustGap(state, wallNormal, options.planePoint);
+
+      const stiffness = options.stiffnessOverride ??
+        computeFrozenStiffness(
+          {
+            gap: adjustedGap,
+            effectiveMass: state.effectiveMass ?? 0,
+            direction: wallNormal,
+            hessian: (state.metadata?.hessian as Matrix3x3 | undefined) ?? ZERO_HESSIAN,
+          },
+          { min: 0 }
+        );
+
+      if (stiffness <= 0) {
+        return { energy: 0, gradient: ZERO_GRADIENT, hessian: ZERO_HESSIAN };
+      }
+
+      const evaluation = baseBarrier.evaluate(
+        {
+          ...state,
+          gap: adjustedGap,
+          direction: wallNormal,
+          stiffness,
+          maxGap: options.maxGap ?? state.maxGap,
+        },
+        context
+      );
+
+      return evaluation;
+    },
+  };
+}
+
+function adjustGap(
+  state: FoldConstraintState,
+  normal: Vector3D,
+  planePoint: Vector3D | undefined
+): number {
+  if (!planePoint) {
+    return state.gap;
+  }
+
+  const position = state.metadata?.position as Vector3D | undefined;
+  if (!position) {
+    return state.gap;
+  }
+
+  const offsetVector = {
+    x: position.x - planePoint.x,
+    y: position.y - planePoint.y,
+    z: position.z - planePoint.z,
+  };
+  const signedDistance = dot(offsetVector, normal);
+  return Math.min(state.gap, signedDistance);
+}
+
+function normalise(vector: Vector3D | undefined): Vector3D | null {
+  if (!vector) return null;
+  const length = Math.hypot(vector.x, vector.y, vector.z);
+  if (length === 0) return null;
+  return {
+    x: vector.x / length,
+    y: vector.y / length,
+    z: vector.z / length,
+  };
+}
+
+function dot(a: Vector3D, b: Vector3D): number {
+  return a.x * b.x + a.y * b.y + a.z * b.z;
+}

tests/index.test.ts

@@ -62,6 +62,7 @@ describe('package entry point', () => {
     expect(examples.physics.computeFrozenStiffness).toBe('examples/foldStiffness.ts');
     expect(examples.physics.createContactBarrier).toBe('examples/foldContactBarrier.ts');
     expect(examples.physics.createPinBarrier).toBe('examples/foldPinBarrier.ts');
+    expect(examples.physics.createWallBarrier).toBe('examples/foldWallBarrier.ts');
   });
 
   it('provides strong typing for example categories and names', () => {
@@ -207,6 +208,7 @@ describe('package entry point', () => {
       | 'computeFrozenStiffness'
       | 'createContactBarrier'
       | 'createPinBarrier'
+      | 'createWallBarrier'
     >();
 
     expectTypeOf<ExampleName<'ai'>>().toEqualTypeOf<

tests/wallBarrier.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest';
+
+import { createWallBarrier } from '../src/physics/fold/wallBarrier.js';
+
+describe('wall barrier', () => {
+  it('returns zero when outside penetration region', () => {
+    const barrier = createWallBarrier({
+      normal: { x: 0, y: 1, z: 0 },
+      planePoint: { x: 0, y: 0, z: 0 },
+    });
+
+    const evaluation = barrier.evaluate(
+      {
+        gap: 0.1,
+        maxGap: 0,
+        stiffness: 0,
+        direction: { x: 0, y: 1, z: 0 },
+        effectiveMass: 0.2,
+        metadata: { position: { x: 0, y: 0.5, z: 0 } },
+      },
+      { deltaTime: 1 }
+    );
+
+    expect(evaluation.energy).toBe(0);
+  });
+
+  it('computes energy when penetrating the wall', () => {
+    const barrier = createWallBarrier({
+      normal: { x: 0, y: 1, z: 0 },
+      planePoint: { x: 0, y: 0, z: 0 },
+    });
+
+    const evaluation = barrier.evaluate(
+      {
+        gap: -0.05,
+        maxGap: 0,
+        stiffness: 0,
+        direction: { x: 0, y: 1, z: 0 },
+        effectiveMass: 0.3,
+        metadata: {
+          position: { x: 0, y: -0.05, z: 0 },
+          hessian: [
+            [5, 0, 0],
+            [0, 5, 0],
+            [0, 0, 5],
+          ],
+        },
+      },
+      { deltaTime: 1 / 60 }
+    );
+
+    expect(evaluation.energy).toBeGreaterThan(0);
+    expect(evaluation.gradient.y).toBeGreaterThan(0);
+  });
+});