HullFootprint: the one footprint cell reading and edit rule (grow, trim, refuse last cell), with tests

Not yet built or run (verify job at the end).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Aetheria.Shared.Tests/HullFootprintTests.cs

@@ -0,0 +1,99 @@
+using System.Linq;
+using CultMath;
+using Xunit;
+
+// HullFootprint: the one rule for editing a hardpoint's schematic footprint (Position + Shape together) and the
+// one reading of its cells in hull space.
+public sealed class HullFootprintTests
+{
+    private static HardpointData Hardpoint(int2 position, params int2[] cellsInShapeSpace)
+    {
+        var max = cellsInShapeSpace.Aggregate(new int2(0, 0), (a, c) => new int2(CultMath.math.max(a.x, c.x), CultMath.math.max(a.y, c.y)));
+        var shape = new Shape(max.x + 1, max.y + 1);
+        foreach (var c in cellsInShapeSpace) shape[c] = true;
+        return new HardpointData { Position = position, Shape = shape };
+    }
+
+    private static int2[] Hull(HardpointData hp) => HullFootprint.Cells(hp).OrderBy(c => c.y).ThenBy(c => c.x).ToArray();
+
+    private static HardpointData Apply(HardpointData hp, int2 hullCell)
+    {
+        Assert.True(HullFootprint.Toggle(hp.Shape, hp.Position, hullCell, out var shape, out var position));
+        return new HardpointData { Position = position, Shape = shape };
+    }
+
+    [Fact]
+    public void CellsAreTheShapeLaidAtPosition()
+    {
+        var hp = Hardpoint(new int2(3, 3), new int2(0, 0), new int2(1, 0));
+        Assert.Equal(new[] { new int2(3, 3), new int2(4, 3) }, Hull(hp));
+    }
+
+    [Fact]
+    public void FootprintGrowsTowardAnyEdgeAndKeepsItsCells()
+    {
+        var hp = Hardpoint(new int2(3, 3), new int2(0, 0));
+
+        var grown = Apply(Apply(hp, new int2(2, 3)), new int2(3, 2));
+
+        Assert.Equal(new int2(2, 2), grown.Position);
+        Assert.Equal(2, grown.Shape.Width);
+        Assert.Equal(2, grown.Shape.Height);
+        Assert.Equal(new[] { new int2(3, 2), new int2(2, 3), new int2(3, 3) }, Hull(grown));
+
+        var right = Apply(Apply(hp, new int2(4, 3)), new int2(3, 4));
+        Assert.Equal(new int2(3, 3), right.Position);
+        Assert.Equal(new[] { new int2(3, 3), new int2(4, 3), new int2(3, 4) }, Hull(right));
+    }
+
+    [Fact]
+    public void FootprintTrimsBackToItsOriginal()
+    {
+        var hp = Hardpoint(new int2(3, 3), new int2(0, 0));
+        var grown = Apply(Apply(hp, new int2(2, 3)), new int2(3, 2));
+
+        var trimmed = Apply(Apply(grown, new int2(2, 3)), new int2(3, 2));
+
+        Assert.Equal(hp.Position, trimmed.Position);
+        Assert.Equal(1, trimmed.Shape.Width);
+        Assert.Equal(1, trimmed.Shape.Height);
+        Assert.Equal(Hull(hp), Hull(trimmed));
+    }
+
+    [Fact]
+    public void FootprintRefusesItsLastCell()
+    {
+        var hp = Hardpoint(new int2(3, 3), new int2(0, 0));
+
+        Assert.False(HullFootprint.Toggle(hp.Shape, hp.Position, new int2(3, 3), out var shape, out var position));
+
+        Assert.Equal(hp.Position, position);
+        Assert.Same(hp.Shape, shape);
+        Assert.Equal(new[] { new int2(3, 3) }, Hull(hp));
+    }
+
+    [Fact]
+    public void FootprintToggleLeavesItsInputUnchanged()
+    {
+        var hp = Hardpoint(new int2(3, 3), new int2(0, 0), new int2(1, 0));
+
+        HullFootprint.Toggle(hp.Shape, hp.Position, new int2(2, 3), out _, out _);
+        HullFootprint.Toggle(hp.Shape, hp.Position, new int2(4, 3), out _, out _);
+
+        Assert.Equal(new int2(3, 3), hp.Position);
+        Assert.Equal(2, hp.Shape.Width);
+        Assert.Equal(1, hp.Shape.Height);
+        Assert.Equal(new[] { new int2(3, 3), new int2(4, 3) }, Hull(hp));
+    }
+
+    [Fact]
+    public void TrimmingAnInteriorBorderMovesPositionWithoutMovingCells()
+    {
+        var hp = Hardpoint(new int2(5, 5), new int2(0, 0), new int2(1, 0), new int2(2, 0));
+
+        var trimmed = Apply(hp, new int2(5, 5));
+
+        Assert.Equal(new int2(6, 5), trimmed.Position);
+        Assert.Equal(new[] { new int2(6, 5), new int2(7, 5) }, Hull(trimmed));
+    }
+}