using System.Collections.Immutable;

namespace AcDream.App.Rendering.Gpu;

/// <summary>Vertex attribute component layout. Only the formats acdream's meshes actually use.</summary>
internal enum GpuVertexFormat
{
    Float1,
    Float2,
    Float3,
    Float4,

    /// <summary>Four unsigned bytes scaled to [0,1] floats — a shader <c>vec4</c> input.</summary>
    UByte4Normalized,

    /// <summary>
    /// Four unsigned bytes delivered as INTEGERS — a shader <c>uvec4</c> input.
    ///
    /// Distinct from <see cref="UByte4Normalized"/> in kind, not just in scaling:
    /// GL requires <c>glVertexAttribIPointer</c> for an integer shader input and
    /// leaves the value undefined if it arrives through the float path, and Vulkan
    /// needs the format named as <c>R8G8B8A8_UINT</c> rather than <c>_UNORM</c>.
    ///
    /// Added at slice V4d, which found `terrain_modern.vert` declares locations 2–5
    /// as <c>uvec4</c> and feeds them with <c>glVertexAttribIPointer</c>. Those
    /// packed bytes carry terrain-type, road and split-direction codes that drive
    /// every blend decision, so normalising them would not be an approximation —
    /// it would be garbage.
    /// </summary>
    UByte4UInt,
}

/// <summary>One vertex attribute, matching a <c>layout(location = N) in</c> declaration.</summary>
internal readonly record struct GpuVertexAttribute(
    uint Location,
    GpuVertexFormat Format,
    uint OffsetBytes);

/// <summary>Interleaved vertex layout for a single bound vertex buffer.</summary>
internal sealed record GpuVertexLayout(uint StrideBytes, ImmutableArray<GpuVertexAttribute> Attributes)
{
    /// <summary>
    /// The world mesh vertex shared by <c>mesh_modern</c>, EnvCells, and terrain:
    /// position, normal, texcoord — 32 bytes, matching the format
    /// <c>ObjectMeshManager</c> packs into <c>GlobalMeshBuffer</c>.
    /// </summary>
    public static GpuVertexLayout WorldMesh { get; } = new(
        StrideBytes: 32,
        [
            new GpuVertexAttribute(0, GpuVertexFormat.Float3, 0),
            new GpuVertexAttribute(1, GpuVertexFormat.Float3, 12),
            new GpuVertexAttribute(2, GpuVertexFormat.Float2, 24),
        ]);

    /// <summary>Empty layout for pipelines whose vertices come entirely from storage buffers.</summary>
    public static GpuVertexLayout None { get; } = new(0, []);
}

/// <summary>
/// Names one GLSL shader pair. The backend resolves it: the GL backend loads
/// <c>Rendering/Shaders/{Name}.vert</c> and <c>.frag</c> and compiles at startup;
/// the Vulkan backend loads the committed <c>Rendering/Shaders/spv/{Name}.vert.spv</c>
/// and <c>.frag.spv</c> produced by <c>tools/compile-shaders.ps1</c>. One source
/// of truth (the GLSL), two consumption paths.
/// </summary>
internal readonly record struct GpuShaderSet(string Name);

/// <summary>Depth-buffer behaviour baked into a pipeline.</summary>
/// <param name="Test">Whether depth testing is enabled at all.</param>
/// <param name="Write">Default depth-write state; overridable per draw via dynamic state.</param>
/// <param name="Compare">Comparison function when testing is enabled.</param>
internal readonly record struct GpuDepthState(bool Test, bool Write, GpuCompareOp Compare)
{
    /// <summary>Standard opaque geometry: test and write, nearer wins.</summary>
    public static GpuDepthState OpaqueDefault { get; } = new(Test: true, Write: true, GpuCompareOp.LessOrEqual);

    /// <summary>Translucent geometry: test against existing depth but do not occlude later draws.</summary>
    public static GpuDepthState TranslucentDefault { get; } = new(Test: true, Write: false, GpuCompareOp.LessOrEqual);

    /// <summary>Sky and 2-D overlays: depth is irrelevant.</summary>
    public static GpuDepthState Disabled { get; } = new(Test: false, Write: false, GpuCompareOp.Always);
}

/// <summary>
/// Everything a draw needs beyond its buffers: the shader pair and all fixed
/// state. Vulkan bakes this into one <c>VkPipeline</c> at startup, which is why
/// runtime shader compilation and driver state revalidation both disappear.
///
/// Equality is NOT part of this contract — pipelines are created explicitly and
/// held by their renderer. <see cref="Name"/> is the identity used by debug
/// tooling and by the backend's own pipeline cache key.
/// </summary>
internal sealed record GpuPipelineDescription
{
    /// <summary>Stable identifier, e.g. <c>"mesh-opaque"</c>. Surfaced to RenderDoc and validation layers.</summary>
    public required string Name { get; init; }

    /// <summary>The GLSL pair this pipeline draws with.</summary>
    public required GpuShaderSet Shaders { get; init; }

    /// <summary>Vertex input layout; <see cref="GpuVertexLayout.None"/> for buffer-fed geometry.</summary>
    public required GpuVertexLayout VertexLayout { get; init; }

    public GpuPrimitiveTopology Topology { get; init; } = GpuPrimitiveTopology.TriangleList;

    public GpuBlendMode Blend { get; init; } = GpuBlendMode.None;

    public GpuDepthState Depth { get; init; } = GpuDepthState.OpaqueDefault;

    /// <summary>Default cull mode; overridable per draw via <see cref="IGpuPassEncoder.SetCullMode"/>.</summary>
    public GpuCullMode Cull { get; init; } = GpuCullMode.Back;

    /// <summary>
    /// Winding treated as front-facing, expressed in GL convention. The Vulkan
    /// backend inverts it internally to compensate for its negative viewport
    /// height; no renderer performs that flip itself.
    /// </summary>
    public GpuFrontFace FrontFace { get; init; } = GpuFrontFace.CounterClockwise;

    /// <summary>
    /// Alpha-to-coverage for foliage. Only meaningful when the pass is
    /// multisampled; the backend ignores it at one sample.
    /// </summary>
    public bool AlphaToCoverage { get; init; }

    /// <summary>Whether the pipeline writes colour at all. False for depth/stencil-only prepasses.</summary>
    public bool ColorWrite { get; init; } = true;

    /// <summary>
    /// Format of the colour attachment this pipeline renders into.
    ///
    /// Vulkan's dynamic rendering bakes the attachment format into the pipeline:
    /// <c>VkPipelineRenderingCreateInfo</c> has to name it at creation, and a
    /// pipeline whose declared format differs from the attachment it is used
    /// with is undefined. GL has no equivalent — a framebuffer carries its own
    /// attachment formats and a program is bound to whatever is attached — so
    /// the GL backend ignores this field entirely.
    ///
    /// The default is the offscreen render-target format, which the Vulkan
    /// backend maps to the swapchain's <c>B8G8R8A8_UNORM</c>
    /// (<c>VulkanTextureFormatMapping.CanonicalColorAttachmentFormat</c>) so
    /// that backbuffer and offscreen pipelines really do agree. That mapping is
    /// what made this field necessary and is why the default preserves it: slice
    /// V6c had to hard-code one format for every pipeline because the contract
    /// could not express the question, and recorded the gap rather than hiding
    /// it. Naming the format here is the reviewed fix, in the same shape as
    /// <see cref="GpuBlendMode.InverseAlpha"/> (V4c) and
    /// <see cref="GpuVertexFormat.UByte4UInt"/> (V4d).
    /// </summary>
    public GpuTextureFormat ColorFormat { get; init; } = GpuTextureFormat.Rgba8UnormRenderTarget;

    /// <summary>Sample count of the passes this pipeline is used in. Must match the pass.</summary>
    public int SampleCount { get; init; } = 1;
}