feat(geom): signed clearance kernel between polygons
Omnidirectional minimum distance with separating direction (positive) and penetration depth with minimum-translation direction (negative), for the PlateView spacing expander. Overlap verdict defers to Collision.HasOverlap so kernels never disagree. Basis for the fixed-s separation solver.
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using System;
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using System.Collections.Generic;
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using OpenNest.Geometry;
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namespace OpenNest.Tests.Geometry;
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public class ClearanceTests
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{
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private const double Tol = 1e-9;
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private static Polygon Square(double x, double y, double w, double h)
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{
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var p = new Polygon
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{
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Vertices = new List<Vector>
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{
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new Vector(x, y),
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new Vector(x + w, y),
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new Vector(x + w, y + h),
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new Vector(x, y + h),
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}
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};
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p.Close();
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p.UpdateBounds();
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return p;
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}
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private static Polygon Triangle(params double[] xy)
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{
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var p = new Polygon();
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for (var i = 0; i + 1 < xy.Length; i += 2)
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p.Vertices.Add(new Vector(xy[i], xy[i + 1]));
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p.Close();
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p.UpdateBounds();
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return p;
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}
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// ---- Separation ----
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[Fact]
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public void Between_SeparatedHorizontally_DistanceAndDirection()
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{
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var a = Square(0, 0, 1, 1);
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var b = Square(3, 0, 1, 1);
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var r = Clearance.Between(a, b);
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Assert.Equal(2.0, r.Distance, 6);
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// Pushing a away from b means moving left.
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Assert.Equal(-1.0, r.Direction.X, 6);
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Assert.Equal(0.0, r.Direction.Y, 6);
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}
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[Fact]
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public void Between_SeparatedDiagonally_CornerDistance()
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{
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var a = Square(0, 0, 1, 1);
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var b = Square(2, 2, 1, 1);
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var r = Clearance.Between(a, b);
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Assert.Equal(System.Math.Sqrt(2.0), r.Distance, 6);
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Assert.Equal(-1 / System.Math.Sqrt(2), r.Direction.X, 6);
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Assert.Equal(-1 / System.Math.Sqrt(2), r.Direction.Y, 6);
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}
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[Fact]
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public void Between_Touching_ZeroDistance()
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{
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var a = Square(0, 0, 1, 1);
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var b = Square(1, 0, 2, 1);
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var r = Clearance.Between(a, b);
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Assert.True(System.Math.Abs(r.Distance) < 1e-6, $"expected ~0, got {r.Distance}");
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var mag = System.Math.Sqrt(
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r.Direction.X * r.Direction.X + r.Direction.Y * r.Direction.Y
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);
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Assert.Equal(1.0, mag, 6);
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}
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[Fact]
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public void Between_VertexToEdge_DistanceIsPerpendicular()
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{
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// Triangle above a wide square; the base sits 3 above the square's top edge.
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var a = Triangle(1, 3, 3, 3, 2, 4);
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var b = Square(0, -4, 10, 4); // top edge at y = 0
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var r = Clearance.Between(a, b);
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Assert.Equal(3.0, r.Distance, 6); // base y=3 to y=0
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Assert.Equal(0.0, r.Direction.X, 6);
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Assert.Equal(1.0, r.Direction.Y, 6);
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}
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[Fact]
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public void Between_ParallelStaggeredEdges_MinimumAcrossAllPairs()
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{
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// Two L-ish shapes (as simple polys) offset so the true minimum is
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// between mid-edges, not vertices.
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var a = Square(0, 0, 4, 1);
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var b = Square(1, 2, 1, 3);
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var r = Clearance.Between(a, b);
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Assert.Equal(1.0, r.Distance, 6);
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Assert.Equal(-1.0, r.Direction.Y, 6);
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}
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// ---- Penetration ----
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[Fact]
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public void Between_OverlappingSquares_MinimumTranslationAxis()
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{
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// Overlap 0.5 in X, 1.0 in Y -> cheapest exit is X.
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var a = Square(0, 0, 1, 1);
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var b = Square(0.5, 0, 1.5, 1);
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var r = Clearance.Between(a, b);
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Assert.Equal(-0.5, r.Distance, 6);
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Assert.Equal(-1.0, r.Direction.X, 6); // push a left, out of b
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Assert.Equal(0.0, r.Direction.Y, 6);
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}
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[Fact]
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public void Between_OverlappingVerticallyCheaper_ExitsInY()
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{
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// Overlap 0.8 in X, 0.2 in Y -> cheapest exit is Y.
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var a = Square(0, 0, 1, 1);
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var b = Square(0.2, 0.8, 1.2, 1.8);
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var r = Clearance.Between(a, b);
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Assert.Equal(-0.2, r.Distance, 6);
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Assert.Equal(0.0, r.Direction.X, 6);
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Assert.Equal(-1.0, r.Direction.Y, 6);
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}
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[Fact]
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public void Between_ContainedSquare_ExitsThroughNearestWall()
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{
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// Inner square near the left wall: the translation that ENDS the overlap
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// carries its right edge (x=1.2) past the outer's left edge (x=0).
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var outer = Square(0, 0, 10, 10);
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var inner = Square(0.2, 4, 1, 1);
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var r = Clearance.Between(inner, outer);
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Assert.Equal(-1.2, r.Distance, 6);
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Assert.Equal(-1.0, r.Direction.X, 6);
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}
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[Fact]
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public void Between_ConcentricSquares_DepthIsExitTranslation()
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{
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var outer = Square(0, 0, 10, 10);
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var inner = Square(2, 2, 4, 4); // spans [2,6]; leftmost exit carries 6 to 0
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var r = Clearance.Between(inner, outer);
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Assert.Equal(-6.0, r.Distance, 6);
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Assert.Equal(0.0, r.Direction.X, 6);
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Assert.Equal(-1.0, r.Direction.Y, 6);
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}
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[Fact]
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public void Between_TrianglesPenetrating_ReportsNegativeDepth()
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{
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var a = Triangle(0, 0, 4, 0, 2, 3);
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var b = Triangle(1, 0, 5, 0, 3, 3);
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var r = Clearance.Between(a, b);
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Assert.True(r.Distance < 0, $"expected penetration, got {r.Distance}");
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}
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// ---- Direction is actionable: moving a by -Distance * dir clears contact ----
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[Fact]
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public void Between_PenetrationApplyingDirection_EndsContact()
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{
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var a = Square(0, 0, 1, 1);
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var b = Square(0.3, 0, 1.6, 2);
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var r = Clearance.Between(a, b);
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var moved = (Polygon)a.Clone();
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moved.Offset(r.Direction * (-r.Distance + 0.001));
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moved.UpdateBounds();
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Assert.False(Collision.HasOverlap(moved, b));
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}
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[Fact]
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public void Between_SeparationApplyingDirection_NeverReducesDistance()
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{
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var a = Square(0, 0, 1, 1);
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var b = Square(4, 1, 2, 2);
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var r = Clearance.Between(a, b);
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Assert.True(r.Distance > 0);
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// A tiny step along the reported direction must not move closer.
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var moved = (Polygon)a.Clone();
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moved.Offset(r.Direction * (r.Distance / 2));
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moved.UpdateBounds();
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var r2 = Clearance.Between(moved, b);
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Assert.True(
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r2.Distance >= r.Distance - Tol,
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$"moving along dir reduced clearance {r.Distance} -> {r2.Distance}"
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);
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}
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// ---- Determinism ----
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[Fact]
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public void Between_RepeatedCalls_IdenticalResult()
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{
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var a = Square(0, 0, 1, 1);
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var b = Square(0.5, 0.25, 2, 1.5);
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var r1 = Clearance.Between(a, b);
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var r2 = Clearance.Between(a, b);
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Assert.Equal(r1.Distance, r2.Distance);
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Assert.Equal(r1.Direction.X, r2.Direction.X);
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Assert.Equal(r1.Direction.Y, r2.Direction.Y);
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}
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[Fact]
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public void Between_SymmetricSwap_MirrorsDirection()
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{
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var a = Square(0, 0, 1, 1);
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var b = Square(0.5, 0, 1.5, 1);
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var ab = Clearance.Between(a, b);
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var ba = Clearance.Between(b, a);
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Assert.Equal(ab.Distance, ba.Distance, 6);
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Assert.Equal(-ab.Direction.X, ba.Direction.X, 6);
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Assert.Equal(-ab.Direction.Y, ba.Direction.Y, 6);
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}
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// ---- Agreement with the Collision oracle ----
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[Fact]
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public void Between_SignMatchesCollisionVerdict()
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{
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var polygons = new List<Polygon>
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{
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Square(0, 0, 1, 1),
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Square(1, 0, 2, 1),
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Square(0.5, 0, 1.5, 1),
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Square(0.25, 0.25, 0.75, 0.75),
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Square(5, 5, 6, 6),
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Triangle(0, 0, 2, 0, 1, 2),
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Triangle(0.5, -1, 2.5, -1, 1.5, 1),
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};
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for (var i = 0; i < polygons.Count; i++)
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{
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for (var j = i + 1; j < polygons.Count; j++)
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{
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var overlaps = Collision.HasOverlap(polygons[i], polygons[j]);
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var r = Clearance.Between(polygons[i], polygons[j]);
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if (overlaps)
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{
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Assert.True(
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r.Distance <= Tol,
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$"pair {i},{j}: Collision overlaps but clearance {r.Distance}"
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);
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}
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else
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{
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Assert.True(
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r.Distance >= -Tol,
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$"pair {i},{j}: Collision clear but clearance {r.Distance}"
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);
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}
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}
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}
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}
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}
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