/** Compare only visible mesh neighborhoods; unchanged retained sections keep their buffers. */
export function changedLightSections(previous, next, sections) {
  if (!previous) return [...sections];
  const index = (field, x, y, z) => {
    x -= field.min[0]; y -= field.min[1]; z -= field.min[2];
    if (x < 0 || y < 0 || z < 0 || x >= field.size[0] || y >= field.size[1] || z >= field.size[2]) return -1;
    return x + field.size[0] * (y + field.size[1] * z);
  };
  const changed = [];
  for (const id of sections) {
    const base = id.split(",").map(v => Number(v) * 16);
    let dirty = false;
    outer: for (let z = base[2] - 1; z <= base[2] + 16; z++)
      for (let y = base[1] - 1; y <= base[1] + 16; y++)
        for (let x = base[0] - 1; x <= base[0] + 16; x++) {
          const a = index(previous, x, y, z), b = index(next, x, y, z);
          if (a < 0 && b < 0) continue;
          if (a < 0 || b < 0 || previous.skyLevels[a] !== next.skyLevels[b] ||
              previous.data[a * 4] !== next.data[b * 4] ||
              previous.data[a * 4 + 1] !== next.data[b * 4 + 1] ||
              previous.data[a * 4 + 2] !== next.data[b * 4 + 2]) {
            dirty = true; break outer;
          }
        }
    if (dirty) changed.push(id);
  }
  return changed;
}