const mineCell = () => ({ isMine: true });
const numberCell = numAdjacentMines => ({ isMine: false, numAdjacentMines });

const genMtx = (w, h) => {
  return Array(h)
    .fill(null)
    .map(() => Array(w).fill(null));
};

function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

const serializeBoard = mtx =>
  mtx
    .map(row =>
      row
        .map(cell => {
          if (cell.isMine) {
            return '*';
          } else {
            return cell.numAdjacentMines;
          }
        })
        .join(''),
    )
    .join('\n');

const genBoard = (w, h, numMines) => {
  const board = genMtx(w, h);

  // 1. populate mines
  for (let index = 0; index < numMines; index++) {
    do {
      const x = getRandomInt(0, w - 1);
      const y = getRandomInt(0, h - 1);
      if (!board[y][x]) {
        board[y][x] = mineCell();
        break;
      }
    } while (true);
  }

  // 2. calculate num adj
  for (let rowIdx = 0; rowIdx < board.length; rowIdx++) {
    const row = board[rowIdx];

    for (let colIdx = 0; colIdx < row.length; colIdx++) {
      let numAdjacentMines = 0;
      const cell = board[rowIdx][colIdx];
      if (cell && cell.isMine) continue;

      // check adj cells
      for (let y = rowIdx - 1; y <= rowIdx + 1; y++) {
        for (let x = colIdx - 1; x <= colIdx + 1; x++) {
          if (board[y]) {
            const row = board[y];
            if (row[x]) {
              if (row[x].isMine) numAdjacentMines++;
            }
          }
        }
      }

      board[rowIdx][colIdx] = numberCell(numAdjacentMines);
    }
  }

  return serializeBoard(board);
};

const b = genBoard(4, 4, 5);
console.log(b);