acdream/src/AcDream.App/Rendering/Gpu/GpuBindingModel.cs

namespace AcDream.App.Rendering.Gpu;

/// <summary>
/// Campaign V (V0): the PINNED binding model shared by the CPU renderers, the
/// GLSL sources, and both RHI backends. Every number here appears in a shader
/// file; changing one is a contract change that must land in the same commit as
/// the matching shader edit.
///
/// The model is deliberately Vulkan-shaped and dual-legal:
///
///   set 0  — storage buffers, bindings 0..9. GLSL omits the set qualifier, and
///            GL_KHR_vulkan_glsl defines an omitted set as set 0, so one source
///            file compiles for both backends.
///   set 1  — uniform buffers. Vulkan has ONE binding namespace per set, while
///            GL keeps GL_SHADER_STORAGE_BUFFER and GL_UNIFORM_BUFFER tables
///            separate. Today mesh_modern.vert exploits that: the BatchBuffer
///            SSBO and the SceneLighting UBO both sit at binding=1. Moving UBOs
///            to their own set preserves both numbers and removes the collision.
///   set 2  — the global sampled-texture table that replaces ARB_bindless_texture.
///            Vulkan binds it as one variable-count, partially-bound,
///            update-after-bind descriptor array. GL used to emulate it with a
///            storage buffer of uvec2 handles at set 0 binding 9
///            (<c>StorageTextureTable</c>); Campaign V slice V11 deleted that
///            binding along with the rest of the raw-GL arm, so set 2 is the
///            only texture table left.
///
/// A batch no longer carries a 64-bit bindless handle; it carries a
/// <see cref="GpuTextureSlot"/> index into the table. That single change is what
/// makes the CPU-side data model backend-neutral (Campaign V slice V2), and it
/// lands on GL — pixel-gated — long before any Vulkan code exists.
/// </summary>
internal static class GpuBindingModel
{
    // ---- set 0: storage buffers (identical numbering to today's SSBO bindings) ----

    /// <summary>Per-instance transforms. std430 <c>InstanceData { mat4 transform; }</c>.</summary>
    public const uint StorageInstances = 0;

    /// <summary>Per-draw batch metadata. std430 <c>BatchData</c> (see <see cref="GpuBatchDataStrideBytes"/>).</summary>
    public const uint StorageBatches = 1;

    /// <summary>Phase U.3 shared per-frame clip regions (<c>CellClip</c>, 144 B/slot). Slot 0 = no-clip.</summary>
    public const uint StorageClipRegions = 2;

    /// <summary>Phase U.3 per-instance clip-slot index, parallel to <see cref="StorageInstances"/>.</summary>
    public const uint StorageClipSlots = 3;

    /// <summary>A7 Fix B global point/spot light array.</summary>
    public const uint StorageGlobalLights = 4;

    /// <summary>A7 Fix B per-instance light set: 8 indices into the global light array, -1 = unused.</summary>
    public const uint StorageInstanceLightSets = 5;

    /// <summary>#142 per-instance indoor flag (1 = parented to an EnvCell, skip the sun).</summary>
    public const uint StorageInstanceIndoor = 6;

    /// <summary>#188 per-instance opacity multiplier for TransparentPartHook fades.</summary>
    public const uint StorageInstanceAlpha = 7;

    /// <summary>Retail SmartBox selection lighting: one vec2 (luminosity, diffuse) per instance.</summary>
    public const uint StorageInstanceSelectionLighting = 8;

    // Campaign V slice V11 deleted StorageTextureTable (binding 9): the GL-only
    // emulation of the Vulkan texture table via a storage buffer of uvec2
    // bindless handles indexed by GpuTextureSlot.Index. The Vulkan backend
    // always bound TextureTableSet instead and never used this binding — every
    // Vulkan descriptor set layout declared it anyway (seeded with a dummy
    // buffer, like every other unused-by-a-given-shader binding), purely
    // because it counted toward StorageBindingCount.

    /// <summary>One past the highest storage binding — the count the backend must support.</summary>
    public const uint StorageBindingCount = 9;

    // ---- set 1: uniform buffers ----

    /// <summary>
    /// SceneLighting std140 block. Keeps binding=1 so the existing shader source
    /// and <c>SceneLightingUboBinding</c> layout are untouched; the set index is
    /// what disambiguates it from <see cref="StorageBatches"/> under Vulkan.
    /// </summary>
    public const uint UniformSceneLighting = 1;

    /// <summary>
    /// Terrain per-layer texture tiling factors — 36 floats.
    ///
    /// Added at slice V4d. These live in a <c>uniform float[36]</c> today, which
    /// is 144 bytes: too large for the 96-byte push-constant block (and for
    /// Vulkan's guaranteed 128-byte ceiling), and there is no RHI verb for setting
    /// a uniform array. A small uniform buffer is the Vulkan-legal home. Binding 2
    /// is taken by the terrain clip block, so this is 3.
    /// </summary>
    public const uint UniformTerrainTiling = 3;

    /// <summary>
    /// Sky per-draw parameters — the sky/model transforms, the keyframe's
    /// ambient/sun colours and sun direction, the UV scroll, and the
    /// per-surface emissive/diffuse/opacity/fog scalars.
    ///
    /// Added at slice V6e for the same reason as
    /// <see cref="UniformTerrainTiling"/>: Vulkan GLSL has no default uniform
    /// block, so a loose <c>uniform mat4 uSkyView;</c> is unspellable, and this
    /// set is 256 bytes in std140 — three matrices alone are twice the entire
    /// 96-byte push-constant block. A uniform buffer is the only legal home.
    /// Bindings 1, 2 and 3 are taken by SceneLighting, the terrain clip block
    /// and terrain tiling, so this is 4.
    /// </summary>
    public const uint UniformSkyParams = 4;

    /// <summary>Set index carrying every uniform buffer.</summary>
    public const uint UniformSet = 1;

    // ---- set 2: the global texture table ----

    /// <summary>Set index of the sampled-texture descriptor array.</summary>
    public const uint TextureTableSet = 2;

    /// <summary>Binding of the descriptor array within <see cref="TextureTableSet"/>.</summary>
    public const uint TextureTableBinding = 0;

    /// <summary>
    /// Upper bound on simultaneously registered textures. Vulkan requires
    /// <c>maxDescriptorSetUpdateAfterBindSampledImages</c> to reach this; the
    /// capability probe asserts it rather than discovering it at draw time.
    /// </summary>
    public const uint TextureTableCapacity = 16384;

    // ---- push constants ----

    /// <summary>
    /// Bytes actually written by <see cref="GpuPushConstants"/>. Vulkan guarantees
    /// at least <see cref="MaxPushConstantBytes"/>, so 32 bytes of headroom remain
    /// for later slices; any growth updates this constant and the shader block in
    /// the same commit.
    /// </summary>
    public const int PushConstantBytes = 96;

    /// <summary>The Vulkan-guaranteed minimum push-constant budget. A hard ceiling for us.</summary>
    public const int MaxPushConstantBytes = 128;

    // ---- shared layout facts the CPU writers and the shaders must agree on ----

    /// <summary>
    /// std430 stride of <c>BatchData</c>. The bindless <c>uvec2 textureHandle</c>
    /// becomes <c>uint textureIndex</c> plus one pad word at slice V2, so the
    /// stride is unchanged and every existing CPU writer keeps its offsets.
    /// </summary>
    public const int GpuBatchDataStrideBytes = 16;

    /// <summary>Clip planes per <c>CellClip</c> slot; also the required <c>gl_ClipDistance</c> size.</summary>
    public const int ClipPlanesPerSlot = 8;

    /// <summary>std430 stride of one <c>CellClip</c> slot: 16 B header + 8 × vec4.</summary>
    public const int ClipRegionStrideBytes = 16 + (ClipPlanesPerSlot * 16);

    /// <summary>Lights selected per object by retail's <c>minimize_object_lighting</c>.</summary>
    public const int MaxLightsPerObject = 8;
}