/**
 * @param {number[][]} matrix
 * @return {number[]}
 */
const spiralOrder = matrix => {
  const result = [];

  const boundary = {
    left: 0,
    top: 0,
    bottom: matrix.length - 1,
    right: (matrix[0] || 0).length - 1,
  };

  let direction = 'right';
  let cursor = [0, 0];

  const walk = direction => {
    switch (direction) {
      case 'right':
        cursor[1] += 1;
        break;
      case 'bottom':
        cursor[0] += 1;
        break;
      case 'left':
        cursor[1] -= 1;
        break;
      case 'top':
        cursor[0] -= 1;
        break;
    }
  };

  const overBoundary = () =>
    cursor[1] > boundary.right ||
    cursor[0] > boundary.bottom ||
    cursor[1] < boundary.left ||
    cursor[0] < boundary.top;

  while (!overBoundary()) {
    result.push(matrix[cursor[0]][cursor[1]]);

    switch (direction) {
      case 'right':
        if (cursor[1] + 1 > boundary.right) {
          boundary.top += 1;
          direction = 'bottom';
        }
        break;
      case 'bottom':
        if (cursor[0] + 1 > boundary.bottom) {
          boundary.right -= 1;
          direction = 'left';
        }
        break;
      case 'left':
        if (cursor[1] - 1 < boundary.left) {
          boundary.bottom -= 1;
          direction = 'top';
        }
        break;
      case 'top':
        if (cursor[0] - 1 < boundary.top) {
          boundary.left += 1;
          direction = 'right';
        }
        break;
    }

    walk(direction);
  }

  return result;
};