feat(core): add SurfaceDecoder for A8R8G8B8 and BCn formats

tests/AcDream.Core.Tests/Textures/SurfaceDecoderTests.cs

@@ -0,0 +1,89 @@
+using AcDream.Core.Textures;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Enums;
+
+namespace AcDream.Core.Tests.Textures;
+
+public class SurfaceDecoderTests
+{
+    [Fact]
+    public void Decode_A8R8G8B8_ConvertsToRgba8()
+    {
+        // Source format is B, G, R, A in memory (little-endian ARGB).
+        // One 2x2 image: red, green, blue, white pixels.
+        var src = new byte[]
+        {
+            0x00, 0x00, 0xFF, 0xFF,  // red   (B=0, G=0, R=255, A=255)
+            0x00, 0xFF, 0x00, 0xFF,  // green
+            0xFF, 0x00, 0x00, 0xFF,  // blue
+            0xFF, 0xFF, 0xFF, 0xFF,  // white
+        };
+        var rs = new RenderSurface
+        {
+            Width = 2,
+            Height = 2,
+            Format = PixelFormat.PFID_A8R8G8B8,
+            SourceData = src,
+        };
+
+        var decoded = SurfaceDecoder.DecodeRenderSurface(rs);
+
+        Assert.Equal(2, decoded.Width);
+        Assert.Equal(2, decoded.Height);
+        Assert.Equal(16, decoded.Rgba8.Length);  // 2*2*4
+        // red pixel, in RGBA: 255, 0, 0, 255
+        Assert.Equal(0xFF, decoded.Rgba8[0]);
+        Assert.Equal(0x00, decoded.Rgba8[1]);
+        Assert.Equal(0x00, decoded.Rgba8[2]);
+        Assert.Equal(0xFF, decoded.Rgba8[3]);
+    }
+
+    [Fact]
+    public void Decode_UnsupportedFormat_ReturnsMagenta()
+    {
+        var rs = new RenderSurface
+        {
+            Width = 4,
+            Height = 4,
+            Format = PixelFormat.PFID_INDEX16,  // not implemented path
+            SourceData = new byte[32],
+        };
+
+        var decoded = SurfaceDecoder.DecodeRenderSurface(rs);
+
+        Assert.Same(DecodedTexture.Magenta, decoded);
+    }
+
+    [Fact]
+    public void Decode_NullSourceData_ReturnsMagenta()
+    {
+        var rs = new RenderSurface
+        {
+            Width = 4,
+            Height = 4,
+            Format = PixelFormat.PFID_A8R8G8B8,
+            SourceData = null!,
+        };
+
+        var decoded = SurfaceDecoder.DecodeRenderSurface(rs);
+
+        Assert.Same(DecodedTexture.Magenta, decoded);
+    }
+
+    [Fact]
+    public void Decode_TruncatedA8R8G8B8_ReturnsMagenta()
+    {
+        // Buffer too small for width*height*4.
+        var rs = new RenderSurface
+        {
+            Width = 2,
+            Height = 2,
+            Format = PixelFormat.PFID_A8R8G8B8,
+            SourceData = new byte[8],  // should be 16
+        };
+
+        var decoded = SurfaceDecoder.DecodeRenderSurface(rs);
+
+        Assert.Same(DecodedTexture.Magenta, decoded);
+    }
+}