function find(col1, col2, from) {
  for (y = from; y >= 2; y--) {
    if ([1,y] != col1) {
      return 0;
    }
    if ([2,y] == col2) {
      return [3,y];
    }
  }
  return 0;
}

next = 1;
for (y = 2; y <= Bottom; y++) {
  if ([1,y] != [1,y-1]) {
    next = 1;
  }
  found = find([1,y], [2,y], y - 1);
  if (found == 0) {
    [3,y] = next;
    next++;
  } else {
    [3,y] = found;
  }
}