using System.Numerics;
using System.Runtime.InteropServices;

namespace AcDream.App.Rendering.Gpu;

/// <summary>
/// The single push-constant block shared by every acdream pipeline — 96 of the
/// 128 bytes Vulkan guarantees, leaving 32 bytes of headroom for later slices.
///
/// One shared block (rather than a per-shader block) is what lets every world
/// pipeline share ONE pipeline layout, so switching pipelines mid-pass does not
/// invalidate bound descriptor sets or push constants. Shaders declare only the
/// fields they read; unused fields cost nothing.
///
/// The GL backend maps each field to the correspondingly named uniform and skips
/// any the program does not declare (location -1). The Vulkan backend writes the
/// struct verbatim with one <c>vkCmdPushConstants</c>.
///
/// Layout is asserted by <c>GpuContractTests</c>; changing it is a contract change
/// that must land together with the matching edit to the shared GLSL preamble.
/// </summary>
[StructLayout(LayoutKind.Sequential, Pack = 4)]
internal struct GpuPushConstants
{
    /// <summary>
    /// GLSL <c>uViewProjection</c>. acdream's cameras build this with
    /// <c>Matrix4x4.CreatePerspectiveFieldOfView</c>, whose NDC z range is [0,1] —
    /// already Vulkan's convention (see <c>PortalProjection.cs</c>). No projection
    /// rework is needed for the migration, and Vulkan gains the half of the depth
    /// range GL was discarding.
    /// </summary>
    public Matrix4x4 ViewProjection;

    /// <summary>
    /// GLSL <c>uDrawIDOffset</c>. Issue #52: the draw index resets to 0 at the
    /// start of each multi-draw-indirect call, so a pass that begins partway into
    /// the batch array must offset its lookup. Vulkan's <c>gl_DrawID</c> resets
    /// identically per <c>vkCmdDrawIndexedIndirect</c>, so the pattern carries over
    /// unchanged.
    /// </summary>
    public int DrawIdOffset;

    /// <summary>GLSL <c>uLightingMode</c>: 0 = object (plain Lambert + sun), 1 = EnvCell (half-Lambert wrap, no sun).</summary>
    public int LightingMode;

    /// <summary>GLSL <c>uRenderPass</c>: 0 = opaque, 1 = translucent.</summary>
    public int RenderPass;

    /// <summary>GLSL <c>uLightDebug</c>: #176 stripe-hunt isolation modes; 0 = off.</summary>
    public int LightDebug;

    /// <summary>
    /// GLSL <c>uTextureIndexA</c>. Primary texture-table slot for pipelines whose
    /// texture is per-pass rather than per-batch — currently the terrain atlas.
    /// </summary>
    public uint TextureIndexA;

    /// <summary>GLSL <c>uTextureIndexB</c>. Secondary per-pass slot — currently the terrain alpha-mask array.</summary>
    public uint TextureIndexB;

    /// <summary>
    /// GLSL <c>uParamA</c>. Spare scalar, claimed at slice V6e by
    /// <c>particle_mesh</c> as the array layer its per-pass texture is sampled
    /// from — a value the shader converted to float anyway.
    /// </summary>
    public float ParamA;

    /// <summary>GLSL <c>uParamB</c>. Spare scalar; unclaimed at V0.</summary>
    public float ParamB;

    /// <summary>Neutral defaults: identity transform, opaque object lighting, no debug mode.</summary>
    public static GpuPushConstants Default => new()
    {
        ViewProjection = Matrix4x4.Identity,
        DrawIdOffset = 0,
        LightingMode = 0,
        RenderPass = 0,
        LightDebug = 0,
        TextureIndexA = 0,
        TextureIndexB = 0,
        ParamA = 0f,
        ParamB = 0f,
    };
}