import * as THREE from 'three';
import type { TerrainGrid } from '../core/ground-model';

/**
 * Terrain grid → mesh in scene coordinates. The scene origin is the pier centre (pivot), with y up:
 * x = X − pivot.e, y = EL, z = −(Y − pivot.n). This is the straight-frame matrix of the pier scene (stretchMatrix, k = 0),
 * so the terrain lines up with the footings and copings. A cell is drawn only when its four corners have data.
 */
export function terrainGeometry(grid: TerrainGrid, pivot: { e: number; n: number }): THREE.BufferGeometry {
  const { origin, step, nx, ny, z } = grid;
  const positions: number[] = [];
  const index: number[] = [];
  const vertexOf = new Int32Array(nx * ny).fill(-1);
  for (let j = 0; j < ny; j++) {
    for (let i = 0; i < nx; i++) {
      const h = z[j * nx + i];
      if (h === null) continue;
      vertexOf[j * nx + i] = positions.length / 3;
      positions.push(origin[0] + i * step - pivot.e, h, -(origin[1] + j * step - pivot.n));
    }
  }
  for (let j = 0; j < ny - 1; j++) {
    for (let i = 0; i < nx - 1; i++) {
      const a = vertexOf[j * nx + i];
      const b = vertexOf[j * nx + i + 1];
      const c = vertexOf[(j + 1) * nx + i];
      const d = vertexOf[(j + 1) * nx + i + 1];
      if (a < 0 || b < 0 || c < 0 || d < 0) continue;
      index.push(a, c, b, b, c, d);
    }
  }
  const geometry = new THREE.BufferGeometry();
  geometry.setAttribute('position', new THREE.Float32BufferAttribute(positions, 3));
  geometry.setIndex(index);
  geometry.computeVertexNormals();
  return geometry;
}