using AcDream.Core.Meshing;

namespace AcDream.App.Rendering.Wb;

/// <summary>
/// Campaign VM VM6: the two <c>BatchData.flags</c> bits that mark a world
/// draw batch as swaying foliage — bit 0 remains #226's built-mesh marker
/// (see <c>mesh_detail.frag</c>'s <c>vBatchFlags &amp; 1u</c> check) and is
/// never touched here. <c>mesh_atmospheric.vert</c> and the four
/// <c>directional_shadow_world_*.vert</c> caster shaders read these bits
/// through the shared <c>foliage_wind.glsl</c> include
/// (<c>acdreamFoliageDisplace</c>'s <c>batchFlags &amp; 0x6u</c> gate); the
/// retail <c>mesh_modern</c> pipelines never read them, so setting them does
/// not change pack-off output.
/// </summary>
internal static class FoliageWindClassification
{
    /// <summary>
    /// Bit 1: the subset is an alpha-cutout/ClipMap material AND its owning
    /// entity is procedural scenery (see <see cref="Classify"/>). Leaves,
    /// fronds, bushes, grass tufts.
    /// </summary>
    internal const uint CutoutFoliageFlag = 0x2u;

    /// <summary>
    /// Bit 2: the subset is opaque AND its owning entity is procedural
    /// scenery AND that same entity owns at least one cutout subset — a
    /// tree trunk or branch, not a rock (rocks have no cutout subset).
    /// </summary>
    internal const uint TrunkFlag = 0x4u;

    private const uint SceneryEntityIdBit = 0x8000_0000u;

    /// <summary>
    /// Pure classification: no allocation, no scan. <paramref name="meshHasCutoutSubset"/>
    /// is expected to be computed once per GfxObj/Setup mesh and cached with
    /// the mesh record (<see cref="AcDream.App.Rendering.Wb.ObjectRenderData.HasCutoutSubset"/>),
    /// not recomputed per call.
    /// </summary>
    internal static uint Classify(
        uint entityId,
        bool isExcluded,
        TranslucencyKind translucency,
        bool meshHasCutoutSubset)
    {
        if (!IsProceduralScenery(entityId) || isExcluded)
            return 0u;
        if (translucency == TranslucencyKind.ClipMap)
            return CutoutFoliageFlag;
        if (translucency == TranslucencyKind.Opaque && meshHasCutoutSubset)
            return TrunkFlag;
        return 0u;
    }

    /// <summary>
    /// Bit 31 of the entity id — see <c>ProceduralSceneryIdAllocator</c>'s
    /// <c>0x8XXYYIII</c> namespace. No other consumer decodes more than this
    /// one bit.
    /// </summary>
    internal static bool IsProceduralScenery(uint entityId) =>
        (entityId & SceneryEntityIdBit) != 0u;
}