spiral

function spiralOrderTwo(n) {
  // create nxn grid set to 0.
  const grid = new Array(n).fill().map(_ => new Array(n).fill(0));

  // clockwise directions
  const directions = [
    [1,  0], // down
    [0, -1], // left
    [-1, 0], // up
    [0,  1], // right
  ];
  // determines which direction to go.
  let dirIdx = 0;

  const totalDirectionChanges = 2;
  let stepsNumber = 1;
  let directionChangesCount = 0;

  let [r, c] = [Math.floor(n / 2), Math.floor(n / 2)];
  let value = 1;
  while (value < n * n - 1) {
    if (directionChangesCount < totalDirectionChanges) {
      // for every direction change, do stepsNumber of steps.
      for (let i = 0; i < stepsNumber; i++) {
        [r, c] = [r + directions[dirIdx][0], c + directions[dirIdx][1]];

        // exit loop when we go beyond the grid's bottom right cell.
        if (r > n - 1 || c > n - 1) {
          break;
        }

        grid[r][c] = value;
        value += 1;
      }
    
      directionChangesCount += 1;

      // change directions.
      dirIdx = (dirIdx + 1) % directions.length;
    } else {
      // number of steps increases after every 2 direction changes.
      stepsNumber += 1;
      // reset count of direction changes.
      directionChangesCount = 0;
    }
  }

  return grid;
}

const res = spiralOrderTwo(3);
console.log(res.map(row => row.join("    ")).join("\n"));

































function runTests() {
    const tests = [
        // Example from book
        [5, [
            [16, 17, 18, 19, 20],
            [15, 4, 5, 6, 21],
            [14, 3, 0, 7, 22],
            [13, 2, 1, 8, 23],
            [12, 11, 10, 9, 24]
        ]],
        // Edge case - 1x1
        [1, [[0]]],
        // Edge case - 3x3
        [3, [
            [4, 5, 6],
            [3, 0, 7],
            [2, 1, 8]
        ]],
    ];

    console.log("RUN");
    for (const [n, want] of tests) {
        const got = spiralOrderTwo(n);
        console.assert(JSON.stringify(got) === JSON.stringify(want),
            `\nspiral(${n}): got: ${JSON.stringify(got)}, want: ${JSON.stringify(want)}\n`);
    }
}

runTests();
// runTests();