indices.py

def indices(m, n):
  """Index generator to visit a 2-dimensional matrix in increasing i + j.

  :param m: Number of rows.
  :param n: Number of columns.

  """
  for i in range(m):
    for j in range(min(n, i + 1)):
      yield (i - j, j)
  for j in range(1, n):
    for i in range(m - 1, max(-1, m + j - n - 1), -1):
      yield (i, m - 1 - i + j)