using System.Numerics;
using AcDream.Core.Terrain;
using DatReaderWriter;
using DatReaderWriter.DBObjs;
using DatReaderWriter.Types;

namespace AcDream.Core.Meshing;

/// <summary>
/// Builds renderable sub-meshes from an EnvCell's room geometry (walls,
/// floors, ceilings). The geometry lives in the linked Environment dat:
/// EnvCell.EnvironmentId → Environment → Cells[CellStructure] → CellStruct.
/// This mirrors GfxObjMesh.Build but reads surfaces from EnvCell.Surfaces
/// (not from the CellStruct itself) and uses the same fan-triangulation
/// and per-surface deduplication pattern.
/// </summary>
public static class CellMesh
{
    /// <summary>
    /// Walk a CellStruct's polygons and produce one <see cref="GfxObjSubMesh"/>
    /// per referenced Surface. Surfaces are resolved from <paramref name="envCell"/>.Surfaces
    /// (OR'd with 0x08000000 to form the full dat id). Polygons are triangulated as fans.
    /// </summary>
    /// <param name="envCell">The EnvCell that owns the surface list.</param>
    /// <param name="cellStruct">The CellStruct containing the polygon + vertex geometry.</param>
    /// <param name="dats">
    /// Optional dat collection used to read Surface.Type flags and set
    /// <see cref="GfxObjSubMesh.Translucency"/>. When null (e.g. offline tests)
    /// all sub-meshes default to <see cref="TranslucencyKind.Opaque"/>.
    /// </param>
    public static IReadOnlyList<GfxObjSubMesh> Build(EnvCell envCell, CellStruct cellStruct, DatCollection? dats = null)
    {
        // Group output vertices and indices per surface dat id.
        var perSurface = new Dictionary<uint, (List<Vertex> Vertices, List<uint> Indices, Dictionary<(int pos, int uv), uint> Dedupe)>();

        foreach (var kvp in cellStruct.Polygons)
        {
            var poly = kvp.Value;

            if (poly.VertexIds.Count < 3)
                continue;  // degenerate polygon

            // Skip if NoPos stippling is set (polygon has no positive surface geometry).
            if (poly.Stippling.HasFlag(DatReaderWriter.Enums.StipplingType.NoPos))
                continue;

            int surfaceIdx = poly.PosSurface;
            if (surfaceIdx < 0 || surfaceIdx >= envCell.Surfaces.Count)
                continue;  // out-of-range surface index

            // Surfaces on EnvCell are unqualified ids; OR with 0x08000000 for the full dat id.
            uint surfaceId = (uint)envCell.Surfaces[surfaceIdx] | 0x08000000u;

            if (!perSurface.TryGetValue(surfaceId, out var bucket))
            {
                bucket = (new List<Vertex>(), new List<uint>(), new Dictionary<(int, int), uint>());
                perSurface[surfaceId] = bucket;
            }

            // Collect output vertex indices for this polygon.
            var polyOut = new List<uint>(poly.VertexIds.Count);
            bool skipPoly = false;

            for (int i = 0; i < poly.VertexIds.Count; i++)
            {
                int posIdx = poly.VertexIds[i];
                int uvIdx = i < poly.PosUVIndices.Count ? poly.PosUVIndices[i] : 0;

                if (!cellStruct.VertexArray.Vertices.TryGetValue((ushort)posIdx, out var sw))
                {
                    skipPoly = true;
                    break;
                }

                var texcoord = uvIdx >= 0 && uvIdx < sw.UVs.Count
                    ? new Vector2(sw.UVs[uvIdx].U, sw.UVs[uvIdx].V)
                    : Vector2.Zero;

                // Use normal from vertex data; fall back to up-vector if missing.
                var normal = sw.Normal != Vector3.Zero ? sw.Normal : Vector3.UnitZ;

                var key = (posIdx, uvIdx);
                if (!bucket.Dedupe.TryGetValue(key, out var outIdx))
                {
                    outIdx = (uint)bucket.Vertices.Count;
                    bucket.Vertices.Add(new Vertex(sw.Origin, normal, texcoord, TerrainLayer: 0));
                    bucket.Dedupe[key] = outIdx;
                }
                polyOut.Add(outIdx);
            }

            if (skipPoly || polyOut.Count < 3)
                continue;

            // Fan triangulation: (v0, v1, v2), (v0, v2, v3), ...
            for (int i = 1; i < polyOut.Count - 1; i++)
            {
                bucket.Indices.Add(polyOut[0]);
                bucket.Indices.Add(polyOut[i]);
                bucket.Indices.Add(polyOut[i + 1]);
            }
        }

        // Emit one sub-mesh per surface.
        var result = new List<GfxObjSubMesh>(perSurface.Count);
        foreach (var kvp in perSurface)
        {
            // Resolve Surface.Type flags when a DatCollection is available so the
            // renderer can split the draw into opaque and translucent passes.
            var translucency = TranslucencyKind.Opaque;
            if (dats is not null)
            {
                var surface = dats.Get<Surface>(kvp.Key);
                if (surface is not null)
                    translucency = TranslucencyKindExtensions.FromSurfaceType(surface.Type);
            }

            result.Add(new GfxObjSubMesh(
                SurfaceId: kvp.Key,
                Vertices: kvp.Value.Vertices.ToArray(),
                Indices: kvp.Value.Indices.ToArray())
            {
                Translucency = translucency,
            });
        }
        return result;
    }
}