using System.Numerics;
using AcDream.App.Rendering.Wb;
using AcDream.Core.Meshing;
using Xunit;

namespace AcDream.Core.Tests.Rendering.Wb;

/// <summary>
/// Pure CPU test of <see cref="WbDrawDispatcher.BuildIndirectArrays"/>.
/// Verifies that a synthetic group set lays out into the indirect buffer
/// + parallel batch data with opaque section first, transparent second,
/// per-group fields propagated correctly.
/// </summary>
public sealed class WbDrawDispatcherIndirectBuilderTests
{
    [Fact]
    public void TwoOpaqueGroupsAndOneTransparent_LaysOutContiguouslyOpaqueFirst()
    {
        // Arrange — three groups: 2 opaque (12+1 instances) + 1 transparent (12 instances)
        var groups = new List<WbDrawDispatcher.IndirectGroupInput>
        {
            new(IndexCount: 100, FirstIndex: 0,   BaseVertex: 0,   InstanceCount: 12, FirstInstance: 0,  TextureHandle: 0xAA, TextureLayer: 0, Translucency: TranslucencyKind.Opaque),
            new(IndexCount: 200, FirstIndex: 100, BaseVertex: 0,   InstanceCount: 12, FirstInstance: 12, TextureHandle: 0xBB, TextureLayer: 0, Translucency: TranslucencyKind.AlphaBlend),
            new(IndexCount: 50,  FirstIndex: 300, BaseVertex: 100, InstanceCount: 1,  FirstInstance: 24, TextureHandle: 0xCC, TextureLayer: 0, Translucency: TranslucencyKind.Opaque),
        };

        var indirect = new DrawElementsIndirectCommand[16];
        var batch = new WbDrawDispatcher.BatchDataPublic[16];

        // Act
        var result = WbDrawDispatcher.BuildIndirectArrays(groups, indirect, batch);

        // Assert layout
        Assert.Equal(2, result.OpaqueCount);
        Assert.Equal(1, result.TransparentCount);
        Assert.Equal(2 * 20, result.TransparentByteOffset);  // sizeof(DEIC) = 20

        // Opaque section, in input order (Task 10 callers sort)
        Assert.Equal(100u, indirect[0].Count);
        Assert.Equal(0u,   indirect[0].FirstIndex);
        Assert.Equal(0,    indirect[0].BaseVertex);
        Assert.Equal(12u,  indirect[0].InstanceCount);
        Assert.Equal(0u,   indirect[0].BaseInstance);

        Assert.Equal(50u,  indirect[1].Count);
        Assert.Equal(300u, indirect[1].FirstIndex);
        Assert.Equal(100,  indirect[1].BaseVertex);
        Assert.Equal(1u,   indirect[1].InstanceCount);
        Assert.Equal(24u,  indirect[1].BaseInstance);

        // Transparent section
        Assert.Equal(200u, indirect[2].Count);
        Assert.Equal(100u, indirect[2].FirstIndex);
        Assert.Equal(12u,  indirect[2].InstanceCount);
        Assert.Equal(12u,  indirect[2].BaseInstance);

        // BatchData parallel — same indices as indirect
        Assert.Equal(0xAAul, batch[0].TextureHandle);
        Assert.Equal(0xCCul, batch[1].TextureHandle);
        Assert.Equal(0xBBul, batch[2].TextureHandle);
    }

    [Fact]
    public void EmptyGroupList_ProducesZeroCounts()
    {
        var groups = new List<WbDrawDispatcher.IndirectGroupInput>();
        var indirect = new DrawElementsIndirectCommand[0];
        var batch = new WbDrawDispatcher.BatchDataPublic[0];

        var result = WbDrawDispatcher.BuildIndirectArrays(groups, indirect, batch);

        Assert.Equal(0, result.OpaqueCount);
        Assert.Equal(0, result.TransparentCount);
        Assert.Equal(0, result.TransparentByteOffset);
    }

    [Fact]
    public void ClipMapTreatedAsOpaque()
    {
        // ClipMap surfaces (alpha-cutout) belong with the opaque pass
        // because the discard handles transparency, not blending.
        var groups = new List<WbDrawDispatcher.IndirectGroupInput>
        {
            new(IndexCount: 10, FirstIndex: 0, BaseVertex: 0, InstanceCount: 1, FirstInstance: 0, TextureHandle: 0x1, TextureLayer: 0, Translucency: TranslucencyKind.ClipMap),
        };
        var indirect = new DrawElementsIndirectCommand[4];
        var batch = new WbDrawDispatcher.BatchDataPublic[4];

        var result = WbDrawDispatcher.BuildIndirectArrays(groups, indirect, batch);

        Assert.Equal(1, result.OpaqueCount);
        Assert.Equal(0, result.TransparentCount);
    }

    [Fact]
    public void BatchDataPublic_LayoutMatchesPrivateBatchData()
    {
        // Task 10 will use MemoryMarshal.Cast<BatchData, BatchDataPublic> to
        // expose the dispatcher's per-frame BatchData[] scratch to BuildIndirectArrays
        // without copying. The cast is only safe if the structs have identical
        // layout (size, field offsets). Both use [StructLayout(Sequential, Pack=8)].
        Assert.Equal(16, System.Runtime.CompilerServices.Unsafe.SizeOf<WbDrawDispatcher.BatchDataPublic>());
        Assert.Equal(0,  (int)System.Runtime.InteropServices.Marshal.OffsetOf<WbDrawDispatcher.BatchDataPublic>(nameof(WbDrawDispatcher.BatchDataPublic.TextureHandle)));
        Assert.Equal(8,  (int)System.Runtime.InteropServices.Marshal.OffsetOf<WbDrawDispatcher.BatchDataPublic>(nameof(WbDrawDispatcher.BatchDataPublic.TextureLayer)));
        Assert.Equal(12, (int)System.Runtime.InteropServices.Marshal.OffsetOf<WbDrawDispatcher.BatchDataPublic>(nameof(WbDrawDispatcher.BatchDataPublic.Flags)));
    }

    [Fact]
    public void DrawCommandStride_MatchesStructSize()
    {
        Assert.Equal(WbDrawDispatcher.DrawCommandStride, System.Runtime.CompilerServices.Unsafe.SizeOf<DrawElementsIndirectCommand>());
    }
}