using DatReaderWriter.Enums;

namespace AcDream.Core.Meshing;

/// <summary>
/// Categorizes how a sub-mesh should be composited into the frame. Determined
/// from the Surface.Type flags on the AC dat surface that owns the sub-mesh.
/// </summary>
public enum TranslucencyKind
{
    /// <summary>Standard opaque. Depth write + test, no blend.</summary>
    Opaque = 0,

    /// <summary>
    /// Alpha-keyed (clip-map). Treated as opaque for sorting; the fragment
    /// shader discards low-alpha fragments. Matches the current rendering of
    /// doors, windows, and vegetation.
    /// </summary>
    ClipMap = 1,

    /// <summary>
    /// Standard alpha blend: src*a + dst*(1-a).
    /// Depth-write off, depth-test on. Used for semi-transparent glass,
    /// water decals, and flame alpha surfaces.
    /// </summary>
    AlphaBlend = 2,

    /// <summary>
    /// Additive blend: src*a + dst. Depth-write off, depth-test on.
    /// Used for portal swirls, magical glows, and particle effects.
    /// </summary>
    Additive = 3,

    /// <summary>
    /// Inverted alpha blend: src*(1-a) + dst*a. Rare but present in
    /// the AC dat files.
    /// </summary>
    InvAlpha = 4,
}

public static class TranslucencyKindExtensions
{
    // Priority order (highest wins):
    //   1. Additive   — SurfaceType.Additive (0x10000)
    //   2. InvAlpha   — SurfaceType.InvAlpha (0x200)
    //   3. AlphaBlend — SurfaceType.Alpha (0x100) OR SurfaceType.Translucent (0x10)
    //   4. ClipMap    — SurfaceType.Base1ClipMap (0x04)
    //   5. Opaque     — everything else
    //
    // Note: ACViewer groups Base1ClipMap with the alpha-draw bucket (AlphaSurfaceTypes),
    // but acdream keeps its existing alpha-discard approach for clip-map surfaces
    // (they render opaque with per-fragment discard) and introduces a separate
    // translucent pass only for the genuinely blended surface types.

    /// <summary>
    /// Maps a <see cref="SurfaceType"/> flags value to the correct
    /// <see cref="TranslucencyKind"/> for the two-pass render split.
    /// </summary>
    public static TranslucencyKind FromSurfaceType(SurfaceType type)
    {
        if ((type & SurfaceType.Additive) != 0)
            return TranslucencyKind.Additive;

        if ((type & SurfaceType.InvAlpha) != 0)
            return TranslucencyKind.InvAlpha;

        if ((type & (SurfaceType.Alpha | SurfaceType.Translucent)) != 0)
            return TranslucencyKind.AlphaBlend;

        if ((type & SurfaceType.Base1ClipMap) != 0)
            return TranslucencyKind.ClipMap;

        return TranslucencyKind.Opaque;
    }
}