namespace AcDream.App.Rendering;

/// <summary>
/// Builds the layer-indexed adapter table consumed by
/// <c>terrain_modern.frag</c>. Retail passes <c>TerrainTex::tex_tiling</c>
/// directly to <c>ImgTex::TileCSI</c> / <c>ImgTex::MergeTexture</c>
/// (`TexMerge::CopyAndTile` 0x00503580, `TexMerge::Merge` 0x005038C0).
/// The table is the modern texture-array equivalent of that per-texture
/// argument.
/// </summary>
internal static class TerrainTextureTilingTable
{
    // WorldBuilder's extracted terrain-array path uses the same 36-layer
    // contract. Packed terrain layer indices are validated against this
    // capacity before the shader table is built.
    internal const int LayerCapacity = 36;

    /// <summary>
    /// std140 stride of one <c>float</c> in the shader's <c>uTexTiling</c> array.
    /// std140 pads every array element out to 16 bytes, which is why the block is
    /// 576 bytes rather than the 144 the payload occupies. Packing four values
    /// per <c>vec4</c> would be tighter but would change the shader's use site;
    /// keeping the element type means <c>uTexTiling[int(layer)]</c> reads exactly
    /// as it did when the array was a loose uniform.
    /// </summary>
    internal const int UniformElementStrideBytes = 16;

    /// <summary>
    /// Size of the <c>TerrainTiling</c> uniform buffer
    /// (<c>GpuBindingModel.UniformTerrainTiling</c>, binding 3) that has carried
    /// this table since slice V6f-2. Asserted against the stride above rather
    /// than written as a literal, because the two disagreeing is a mismatch only
    /// a wrongly-tiled terrain texture would reveal.
    /// </summary>
    internal const int UniformBufferBytes = LayerCapacity * UniformElementStrideBytes;

    internal static float[] Build(IEnumerable<(uint Layer, uint RepeatCount)> entries)
    {
        ArgumentNullException.ThrowIfNull(entries);

        var table = new float[LayerCapacity];
        Array.Fill(table, 1f);

        foreach (var (layer, repeatCount) in entries)
        {
            if (layer >= LayerCapacity)
            {
                throw new InvalidOperationException(
                    $"Terrain atlas layer {layer} exceeds the shader capacity of {LayerCapacity} layers.");
            }

            // Preserve the dat value exactly. Retail forwards the unsigned
            // field without normalizing it in both CopyAndTile and Merge.
            table[layer] = repeatCount;
        }

        return table;
    }
}