Rosetta Stone

function solve(arr) {
  const rosetta = {
    0: ' ', 1: 'A', 2: 'B', 3: 'C', 4: 'D', 5: 'E', 6: 'F', 7: 'G', 8: 'H', 9: 'I', 10: 'J', 11: 'K', 12: 'L', 13: 'M',
    14: 'N', 15: 'O', 16: 'P', 17: 'Q', 18: 'R', 19: 'S', 20: 'T', 21: 'U', 22: 'V', 23: 'W', 24: 'X', 25: 'Y', 26: 'Z',
  };

  const matrixSize = Number(arr.shift());
  const template = createMatrix(matrixSize);
  let matrix = [...arr].map(x => x.split(' ').map(Number));
  let i = 0;
  let j = 0;

  while (i < matrix.length && j < matrix.length) {
    for (let x = 0; x < template.length; x++) {
      for (let y = 0; y < template[x].length; y++) {
        const templateCell = template[x][y];
        if (doesExist(i + x) && doesExist(j + y)) {
          let matrixCell = rotate(matrix[i + x][j + y] + templateCell);
          const letter = rosetta[matrixCell];
          matrix[i + x][j + y] = letter;
        }
      }
    }
    shift();
  }

  return matrix.map(x => x.join('')).join('');
  // return matrix;

  function doesExist(x) {return x >= 0 && x < matrix.length}

  function createMatrix(x) {
    let m = [];
    for (let i = 0; i < x; i++) {m.push(arr.shift().split(' ').map(Number));}
    return m;
  }

  function rotate(h) {
    if (h >= 27) {
      let rotations = Math.floor(h / 27);
      return h - rotations * 27;
    }
    return h;
  }

  function shift() {
    if (i == matrix.length - 1) {
      i  = 0;
      j += template[0].length;
    } else {
      i += template.length;
      if(i >= matrix.length) {
        i = 0;
        j += template[0].length;
      }
    }
  }
}