Given a 2D matrix (flattened to 1D) and an ID, get the 8 surrounding neighbours

neighbours.ts

    public getNeighbours(idx: number): number[] {
    const neighbours: number[] = []
    const size = this.rows * this.cols
    const w = this.cols
    const h = this.rows
    const debug = {
      cell: idx,
      n: -1,
      w: -1,
      e: -1,
      s: -1,
    }
    if ((idx - w) >= 0) {
      neighbours.push(idx - w) // north
      debug.n = idx - w
    }
    if ((idx % w) !== 0) {
      neighbours.push(idx - 1) // west
      debug.w = idx - 1
    }
    if (((idx + 1) % w) !== 0) {
      neighbours.push(idx + 1) // east
      debug.e = idx + 1
    }
    if (idx + w < size) {
      neighbours.push(idx + w) // south
      debug.s = idx + w
    }

    if (((idx - w - 1) >= 0) && (idx % w !== 0)) {
      neighbours.push(idx - w - 1)  // northwest
    }

    if (((idx - w + 1) >= 0) && ((idx + 1) % w !== 0)) {
      neighbours.push(idx - w + 1)  // northeast
    }

    if (((idx + w - 1) < size) && (idx % w !== 0)) {
      neighbours.push(idx + w - 1)  // southwest
    }

    if (((idx + w + 1) < size) && ((idx + 1) % w !== 0)) {
      neighbours.push(idx + w + 1)  // southeast
    }
    return neighbours
  }