msh improvements

ParkanPlayground/Msh0x06.cs

@@ -4,42 +4,88 @@ using NResLib;
 namespace ParkanPlayground;
 
 /// <summary>
-/// MSH-компонент 0x06: индексный буфер
+/// MSH-компонент 0x06: индексный буфер.
+/// Используется batch-ами 0x0D через Batch.IndexStart / Batch.IndexCount.
+/// Индексы являются ushort и обычно читаются тройками как triangle indices.
 /// </summary>
 public static class Msh0x06
 {
-    public static List<ushort> ReadComponent(
-        FileStream mshFs, NResArchive archive)
+    public const int ElementSize = 2;
+
+    public static List<ushort> ReadComponent(FileStream mshFs, NResArchive archive)
     {
         var entry = archive.Files.FirstOrDefault(x => x.FileType == "06 00 00 00");
 
         if (entry is null)
         {
-            throw new Exception("Archive doesn't contain file (06)");
+            throw new Exception("Archive doesn't contain index buffer component (0x06)");
         }
 
-        if (entry.ElementSize != 2)
+        if (entry.ElementSize != ElementSize)
         {
             throw new Exception("Index buffer component (0x06) element size is not 2");
         }
 
-        if (entry.FileLength % entry.ElementSize != 0)
+        if (entry.FileLength % ElementSize != 0)
         {
-            throw new Exception("Index buffer component (0x06) payload size is not divisible by element size");
+            throw new Exception("Index buffer component (0x06) payload size is not divisible by 2");
         }
 
         var data = new byte[entry.FileLength];
+
         mshFs.Seek(entry.OffsetInFile, SeekOrigin.Begin);
         mshFs.ReadExactly(data, 0, data.Length);
 
-        var elements = new List<ushort>(entry.FileLength / entry.ElementSize);
-        for (var i = 0; i < entry.FileLength / entry.ElementSize; i++)
+        var span = data.AsSpan();
+        var indices = new List<ushort>(entry.FileLength / ElementSize);
+
+        for (var offset = 0; offset < span.Length; offset += ElementSize)
         {
-            elements.Add(
-                BinaryPrimitives.ReadUInt16LittleEndian(data.AsSpan(i * 2))
-            );
+            indices.Add(BinaryPrimitives.ReadUInt16LittleEndian(span.Slice(offset, ElementSize)));
         }
 
-        return elements;
+        return indices;
     }
-}
+
+    /// <summary>
+    /// Возвращает triangle triplets из диапазона индексов.
+    /// Удобно для обхода batch.IndexStart / batch.IndexCount из 0x0D.
+    /// </summary>
+    public static IEnumerable<TriangleIndices> EnumerateTriangles(
+        IReadOnlyList<ushort> indices,
+        int indexStart,
+        int indexCount)
+    {
+        if (indexStart < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(indexStart));
+        }
+
+        if (indexCount < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(indexCount));
+        }
+
+        if (indexStart + indexCount > indices.Count)
+        {
+            throw new ArgumentException("Index range is outside the 0x06 index buffer");
+        }
+
+        for (var i = 0; i + 2 < indexCount; i += 3)
+        {
+            yield return new TriangleIndices(
+                indices[indexStart + i + 0],
+                indices[indexStart + i + 1],
+                indices[indexStart + i + 2]);
+        }
+    }
+
+    /// <summary>Тройка индексов triangle из MSH 0x06.</summary>
+    /// <param name="A">Первый vertex index внутри batch/base vertex range.</param>
+    /// <param name="B">Второй vertex index внутри batch/base vertex range.</param>
+    /// <param name="C">Третий vertex index внутри batch/base vertex range.</param>
+    public readonly record struct TriangleIndices(
+        ushort A,
+        ushort B,
+        ushort C);
+}