Spiral Iterator

spiral.py

# pylint: disable=invalid-name
"Spiral iterator."
import sys

if sys.version_info[0] >= 3:
    xrange = range

def revrange(b, a=0, step=1):
    "Reverse iterator."
    return xrange(b-step, a-step, -step)

def spiral(rows, columns):
    "Spiral iterator, yields indices."
    if not columns or not rows:
        return
    # Go through the first row left to right.
    for x in xrange(columns):
        yield 0, x
    # then last colum top to bottom.
    for y in xrange(1, rows-1):
        yield y, columns-1
    # then last row right to left (if not already visited).
    if rows != 1:
        for x in revrange(columns):
            yield rows-1, x
    # then first column bottom to top (if not already visited).
    if columns != 1:
        for y in revrange(rows-1, 1):
            yield y, 0
    # then peel.
    for y, x in spiral(rows-2, columns-2):
        yield y+1, x+1