using System.Numerics;

namespace AcDream.Core.Terrain;

/// <summary>
/// Per-vertex terrain geometry + blend data. Terrain uses a cell-based
/// layout: 64 cells per landblock, 6 vertices per cell (two triangles),
/// 384 vertices per landblock total. All 6 vertices in a cell carry
/// identical <c>Data0..3</c> (the cell's blend recipe from
/// <see cref="TerrainBlending.FillCellData"/>); the vertex shader derives
/// which of the 4 cell corners a given vertex represents from
/// <c>gl_VertexID % 6</c> plus the split direction bit.
///
/// Normal is stored per vertex via Phase 3b's central-difference scheme on
/// the 9×9 heightmap — this lets the fragment shader interpolate a smooth
/// normal across triangles (softer than WorldBuilder's <c>dFdx</c>/<c>dFdy</c>
/// flat-shaded approach). UVs are derived from the corner index in the
/// vertex shader — not stored here.
///
/// Size: 12 (position) + 12 (normal) + 4*4 (Data0..3) = 40 bytes.
/// </summary>
public readonly record struct TerrainVertex(
    Vector3 Position,
    Vector3 Normal,
    uint Data0,
    uint Data1,
    uint Data2,
    uint Data3);