using System.Numerics;
using DatReaderWriter.DBObjs;

namespace AcDream.Core.Terrain;

public sealed record LandblockMeshData(Vertex[] Vertices, uint[] Indices);

public static class LandblockMesh
{
    private const int VerticesPerSide = 9;
    private const int CellsPerSide = VerticesPerSide - 1;
    private const float CellSize = 24.0f;
    // Phase 2b: tile terrain textures ~4x per landblock instead of stretching
    // a single texture across the whole 192-unit patch.
    private const float TexCoordDivisor = CellsPerSide / 4.0f;

    public static LandblockMeshData Build(
        LandBlock block,
        float[] heightTable,
        IReadOnlyDictionary<uint, uint> terrainTypeToLayer)
    {
        ArgumentNullException.ThrowIfNull(heightTable);
        ArgumentNullException.ThrowIfNull(terrainTypeToLayer);
        if (heightTable.Length < 256)
            throw new ArgumentException("heightTable must have 256 entries", nameof(heightTable));

        var vertices = new Vertex[VerticesPerSide * VerticesPerSide];
        for (int y = 0; y < VerticesPerSide; y++)
        {
            for (int x = 0; x < VerticesPerSide; x++)
            {
                int vi = y * VerticesPerSide + x;
                int hi = x * VerticesPerSide + y;

                float height = heightTable[block.Height[hi]];

                // TerrainInfo is bit-packed: bits 0-1 Road, bits 2-6 Type (5-bit
                // TerrainTextureType enum), bits 11-15 Scenery. The atlas keys on
                // Type only, matching Region.TerrainInfo.LandSurfaces.TexMerge.TerrainDesc
                // which lists SurfaceTexture ids per TerrainTextureType.
                uint terrainType = (uint)block.Terrain[hi].Type;
                if (!terrainTypeToLayer.TryGetValue(terrainType, out var layer))
                    layer = 0;

                vertices[vi] = new Vertex(
                    Position: new Vector3(x * CellSize, y * CellSize, height),
                    Normal: Vector3.UnitZ,
                    TexCoord: new Vector2(x / TexCoordDivisor, y / TexCoordDivisor),
                    TerrainLayer: layer);
            }
        }

        var indices = new uint[CellsPerSide * CellsPerSide * 6];
        int idx = 0;
        for (int y = 0; y < CellsPerSide; y++)
        {
            for (int x = 0; x < CellsPerSide; x++)
            {
                uint a = (uint)(y * VerticesPerSide + x);
                uint b = (uint)(y * VerticesPerSide + x + 1);
                uint c = (uint)((y + 1) * VerticesPerSide + x);
                uint d = (uint)((y + 1) * VerticesPerSide + x + 1);
                indices[idx++] = a; indices[idx++] = b; indices[idx++] = d;
                indices[idx++] = a; indices[idx++] = d; indices[idx++] = c;
            }
        }

        return new LandblockMeshData(vertices, indices);
    }
}