using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using AcDream.App.Rendering.Wb;
using Xunit;

namespace AcDream.App.Tests.Rendering.Wb;

/// <summary>
/// Campaign V slice V2 (2026-07-27): CPU-side proof that
/// <see cref="ModernBatchData"/> — EnvCellRenderer's per-batch GPU struct,
/// sharing mesh_modern.vert's BatchData shader-side layout with
/// WbDrawDispatcher — packs at the exact std430 offsets the shader expects
/// after TextureHandle (a raw 64-bit ARB_bindless_texture handle) became
/// TextureTableIndex (a slot into the binding=9 handle table). Mirrors
/// ClipFrameLayoutTests' role for ClipFrame and
/// WbDrawDispatcherIndirectBuilderTests.BatchDataPublic_LayoutMatchesPrivateBatchData
/// for WbDrawDispatcher's own BatchData — a silent layout drift here would
/// desync EnvCellRenderer's uploaded bytes from what mesh_modern.vert reads,
/// with no build error.
///
/// Layout under test (std430, 16 bytes total):
///   offset  0 : uint TextureTableIndex
///   offset  4 : uint Reserved   (pad)
///   offset  8 : uint TextureIndex (layer within the texture array)
///   offset 12 : uint Flags
/// </summary>
public class ModernBatchDataLayoutTests
{
    [Fact]
    public void Size_Is16Bytes_MatchingGpuBatchDataStride()
    {
        Assert.Equal(16, Unsafe.SizeOf<ModernBatchData>());
    }

    [Fact]
    public void FieldOffsets_MatchStd430Layout()
    {
        Assert.Equal(0, (int)Marshal.OffsetOf<ModernBatchData>(nameof(ModernBatchData.TextureTableIndex)));
        Assert.Equal(4, (int)Marshal.OffsetOf<ModernBatchData>(nameof(ModernBatchData.Reserved)));
        Assert.Equal(8, (int)Marshal.OffsetOf<ModernBatchData>(nameof(ModernBatchData.TextureIndex)));
        Assert.Equal(12, (int)Marshal.OffsetOf<ModernBatchData>(nameof(ModernBatchData.Flags)));
    }

    [Fact]
    public void FieldValues_RoundTripThroughTheStruct()
    {
        var data = new ModernBatchData
        {
            TextureTableIndex = 7u,
            TextureIndex = 3u,
        };

        Assert.Equal(7u, data.TextureTableIndex);
        Assert.Equal(0u, data.Reserved);
        Assert.Equal(3u, data.TextureIndex);
        Assert.Equal(0u, data.Flags);
    }
}