using System.Reflection;
using System.Runtime.InteropServices;
using AcDream.App.Rendering;

namespace AcDream.App.Tests.Rendering;

public sealed class ParticleBindlessInstanceTests
{
    [Fact]
    public void BillboardGpuInstance_MatchesVertexAttributeAbi()
    {
        // Campaign V slice V2c (2026-07-27): TextureHandleLow/High (the split
        // halves of a raw 64-bit ARB_bindless_texture handle, 8 bytes) became
        // one TextureIndex (a binding=9 handle-table slot, 4 bytes), so the
        // struct shrank by 4 bytes; TextureIndex keeps TextureHandleLow's
        // former offset (64 — right after the four vec4 fields).
        Assert.Equal(68, Marshal.SizeOf<ParticleRenderer.BillboardGpuInstance>());
        Assert.Equal(
            new IntPtr(64),
            Marshal.OffsetOf<ParticleRenderer.BillboardGpuInstance>(
                nameof(ParticleRenderer.BillboardGpuInstance.TextureIndex)));
    }

    [Fact]
    public void BillboardShaders_ConsumeOneBindlessTextureHandlePerInstance()
    {
        string shadersDirectory = Path.Combine(
            AppContext.BaseDirectory,
            "Rendering",
            "Shaders");
        string vertex = File.ReadAllText(Path.Combine(shadersDirectory, "particle.vert"));
        string fragment = File.ReadAllText(Path.Combine(shadersDirectory, "particle.frag"));

        // Campaign V slice V2c: the per-instance attribute carries a binding=9
        // table slot, not the raw handle.
        //
        // Campaign V slice V6e: the SLOT is what crosses the stage boundary now,
        // and the fragment stage does the lookup — a varying cannot carry a
        // Vulkan descriptor. The untextured particle is spelled by the reserved
        // index rather than by a null handle, because Vulkan's descriptor array
        // cannot be asked whether an element was ever written.
        Assert.Contains("layout(location = 6) in uint aTextureIndex;", vertex);
        Assert.Contains("flat out uint vTextureIndex;", vertex);
        Assert.Contains("vTextureIndex = aTextureIndex;", vertex);
        Assert.Contains("#extension GL_ARB_bindless_texture : require", fragment);
        Assert.Contains("flat in uint vTextureIndex;", fragment);
        Assert.Contains("ACDREAM_SAMPLE_ARRAY(vTextureIndex, vec3(vTex, 0.0))", fragment);
        Assert.Contains("vTextureIndex != ACDREAM_TEXTURE_NONE", fragment);
        Assert.DoesNotContain("uniform sampler2D uParticleTexture", fragment);
    }

    /// <summary>
    /// Campaign V slice V6e: "this particle has no texture" is a reserved index
    /// rather than a null handle, and that value is written in two places — the
    /// CPU that produces it and the Vulkan preamble that tests it in the shader.
    /// Before Campaign V slice V11 deleted the GL arm there was a third copy in
    /// common.glsl, which this test also cross-checked; two copies of a magic
    /// number is still a drift waiting to happen, and its failure mode is
    /// silent: a particle would sample slot 0xFFFFFFFF instead of drawing the
    /// procedural blob.
    /// </summary>
    [Fact]
    public void TheReservedNoTextureSlotAgreesBetweenCpuAndTheVulkanPreamble()
    {
        const string literal = "0xFFFFFFFF";

        object? cpuValue = typeof(ParticleRenderer)
            .GetField("NoTextureSlot", BindingFlags.NonPublic | BindingFlags.Static)
            ?.GetRawConstantValue();
        Assert.Equal(0xFFFFFFFFu, Assert.IsType<uint>(cpuValue));

        // The Vulkan half is injected by the offline compiler at compile time,
        // not carried in a shared GLSL source, so it is a separate declaration
        // that has to say the same thing.
        string preamble = File.ReadAllText(Path.Combine(
            RepositoryRoot(), "tools", "ShaderCompiler", "VulkanGlslPreamble.cs"));
        Assert.Contains($"#define ACDREAM_TEXTURE_NONE {literal}u", preamble);
    }

    private static string RepositoryRoot()
    {
        var directory = new DirectoryInfo(AppContext.BaseDirectory);
        while (directory is not null && !File.Exists(Path.Combine(directory.FullName, "AcDream.slnx")))
            directory = directory.Parent;
        return directory?.FullName
            ?? throw new InvalidOperationException(
                "Could not locate the repository root from the test binary.");
    }
}