feat: generate triangle colors utility

Runtime/Triangulator.cs

@@ -763,12 +763,9 @@ public static class Utilities
         /// <param name="allocator">The allocator to use for temporary data.</param>
         public static void GenerateHalfedges(Span<int> halfedges, ReadOnlySpan<int> triangles, Allocator allocator)
         {
-            CheckAndThrowIfLengthNotEqual(halfedges, triangles);
+            ThrowCheckGenerateHalfedges(halfedges, triangles);
 
-            for (int i = 0; i < halfedges.Length; i++)
-            {
-                halfedges[i] = -1;
-            }
+            halfedges.Fill(-1);
 
             using var tmp = new NativeHashMap<int2, int>(triangles.Length, allocator);
             for (int he = 0; he < halfedges.Length; he++)
@@ -790,6 +787,56 @@ public static void GenerateHalfedges(Span<int> halfedges, ReadOnlySpan<int> tria
             }
         }
 
+        /// <summary>
+        /// Generates triangle <paramref name="colors"/> using the provided <paramref name="halfedges"/>.
+        /// Triangles that share a common edge are assigned the same color index.
+        /// The resulting <paramref name="colors"/> contains values in the range <tt>[0, <paramref name="colorsCount"/>)</tt>.
+        /// Check the documentation for further details.
+        /// </summary>
+        /// <param name="colors">A buffer that will be populated with triangle colors. Its length must be three times smaller than <paramref name="halfedges"/>.</param>
+        /// <param name="halfedges">The halfedge data used for generating colors.</param>
+        /// <param name="colorsCount">The total number of unique colors assigned.</param>
+        /// <param name="allocator">The allocator to use for temporary data.</param>
+        /// <seealso cref="GenerateHalfedges(Span{int}, ReadOnlySpan{int}, Allocator)"/>
+        public static void GenerateTriangleColors(Span<int> colors, ReadOnlySpan<int> halfedges, out int colorsCount, Allocator allocator)
+        {
+            ThrowCheckGenerateTriangleColors(colors, halfedges);
+
+            colorsCount = 0;
+            colors.Fill(-1);
+
+            using var heQueue = new NativeQueueList<int>(allocator);
+            for (int t = 0; t < colors.Length; t++)
+            {
+                if (colors[t] == -1)
+                {
+                    heQueue.Enqueue(3 * t + 0);
+                    heQueue.Enqueue(3 * t + 1);
+                    heQueue.Enqueue(3 * t + 2);
+                    colors[t] = colorsCount;
+                    BFS(colorsCount++, colors, heQueue, halfedges);
+                }
+            }
+
+            static void BFS(int color, Span<int> colors, NativeQueueList<int> heQueue, ReadOnlySpan<int> halfedges)
+            {
+                while (heQueue.TryDequeue(out var he))
+                {
+                    var ohe = halfedges[he];
+                    var t = ohe / 3;
+                    if (ohe == -1 || colors[t] != -1)
+                    {
+                        continue;
+                    }
+
+                    heQueue.Enqueue(3 * t + 0);
+                    heQueue.Enqueue(3 * t + 1);
+                    heQueue.Enqueue(3 * t + 2);
+                    colors[t] = color;
+                }
+            }
+        }
+
         /// <summary>
         /// Inserts a sub-mesh, defined by (<paramref name="subpositions"/>, <paramref name="subtriangles"/>), into the main mesh
         /// represented by (<paramref name="positions"/>, <paramref name="triangles"/>).
@@ -837,7 +884,7 @@ public static unsafe void InsertSubMesh<T>(NativeList<T> positions, NativeList<i
         public static int NextHalfedge(int he) => he % 3 == 2 ? he - 2 : he + 1;
 
         [System.Diagnostics.Conditional("ENABLE_UNITY_COLLECTIONS_CHECKS")]
-        private static void CheckAndThrowIfLengthNotEqual(ReadOnlySpan<int> halfedges, ReadOnlySpan<int> triangles)
+        private static void ThrowCheckGenerateHalfedges(ReadOnlySpan<int> halfedges, ReadOnlySpan<int> triangles)
         {
             if (halfedges.Length != triangles.Length)
             {
@@ -846,6 +893,17 @@ private static void CheckAndThrowIfLengthNotEqual(ReadOnlySpan<int> halfedges, R
                 );
             }
         }
+
+        [System.Diagnostics.Conditional("ENABLE_UNITY_COLLECTIONS_CHECKS")]
+        private static void ThrowCheckGenerateTriangleColors(ReadOnlySpan<int> colors, ReadOnlySpan<int> halfedges)
+        {
+            if (3 * colors.Length != halfedges.Length)
+            {
+                throw new ArgumentException(
+                    $"The provided colors[{colors.Length}] must be one-third of the length of halfedges [{halfedges.Length}]."
+                );
+            }
+        }
     }
 }
 

Tests/UtilitiesTests.cs

@@ -47,10 +47,9 @@ public int[] GenerateHalfedgesTest(int[] triangles)
         }
 
         [Test]
-        public void GenerateHalfedgesThrowIfDifferentLengthTest()
-        {
-            Assert.Throws<ArgumentException>(() => Utilities.GenerateHalfedges(new int[4], new int[7], Allocator.Persistent));
-        }
+        public void GenerateHalfedgesThrowTest() => Assert.Throws<ArgumentException>(() =>
+            Utilities.GenerateHalfedges(halfedges: new int[4], triangles: new int[7], Allocator.Persistent)
+        );
 
         private static readonly TestCaseData[] nextHalfedgeTestData =
         {
@@ -116,5 +115,68 @@ public int[] InsertSubMeshTest((float2[] p, int[] t) mesh, (float2[] p, int[] t)
             Assert.That(positions.AsArray(), Is.EqualTo(mesh.p.Concat(subMesh.p)));
             return triangles.AsReadOnly().ToArray();
         }
+
+        private static readonly TestCaseData[] generateTriangleColorsTestData =
+        {
+            new(new int[]{ }, 0)
+            {
+                ExpectedResult = new int[]{ },
+                TestName = "Test case 0 - 0 triangles 0 colors (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ -1, -1, -1 }, 1)
+            {
+                ExpectedResult = new int[]{ 0 },
+                TestName = "Test case 1 - 1 triangle 1 color (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ -1, -1, -1, -1, -1, -1 }, 2)
+            {
+                ExpectedResult = new int[]{ 0, 1 },
+                TestName = "Test case 2a - 2 triangles 2 colors (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ 3, -1, -1, 0, -1, -1 }, 1)
+            {
+                ExpectedResult = new int[]{ 0, 0 },
+                TestName = "Test case 2b - 2 triangles 1 colors (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ -1, -1, -1, -1, -1, -1, -1, -1, -1 }, 3)
+            {
+                ExpectedResult = new int[]{ 0, 1, 2 },
+                TestName = "Test case 3a - 3 triangles 3 colors (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ 3, -1, -1, 0, -1, -1, -1, -1, -1 }, 2)
+            {
+                ExpectedResult = new int[]{ 0, 0, 1 },
+                TestName = "Test case 3b - 3 triangles 2 colors (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ 6, -1, -1, -1, -1, -1, 0, -1, -1 }, 2)
+            {
+                ExpectedResult = new int[]{ 0, 1, 0 },
+                TestName = "Test case 3c - 3 triangles 2 colors (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ -1, -1, -1, 6, -1, -1, 3, -1, -1 }, 2)
+            {
+                ExpectedResult = new int[]{ 0, 1, 1 },
+                TestName = "Test case 3d - 3 triangles 2 colors (GenerateTriangleColorsTest)"
+            },
+            new(new int[]{ 3, -1, -1, 0, 6, -1, 4, -1, -1 }, 1)
+            {
+                ExpectedResult = new int[]{ 0, 0, 0 },
+                TestName = "Test case 3e - 3 triangles 1 color (GenerateTriangleColorsTest)"
+            },
+        };
+
+        [Test, TestCaseSource(nameof(generateTriangleColorsTestData))]
+        public int[] GenerateTriangleColorsTest(int[] halfedges, int expectedCount)
+        {
+            var colors = new int[halfedges.Length / 3];
+            Utilities.GenerateTriangleColors(colors, halfedges, out var count, Allocator.Persistent);
+            Assert.That(count, Is.EqualTo(expectedCount));
+            return colors;
+        }
+
+        [Test]
+        public void GenerateTriangleColorsThrowTest() => Assert.Throws<ArgumentException>(() =>
+            Utilities.GenerateTriangleColors(colors: new int[1], halfedges: new int[30], out _, Allocator.Persistent)
+        );
     }
 }