spiral.py

# Given a position (x,y), returns next position in a grid spiral.
def next(pos):
        x, y = pos

        top     = False
        right   = False
        bottom  = False
        left    = False
        horizontal      = abs(x) <= abs(y)
        vertical        = abs(y) <= abs(x)

        if y < 0 and horizontal:
                top     = True
        if y > 0 and horizontal:
                bottom  = True
        if x < 0 and vertical:
                left    = True
        if x > 0 and vertical:
                right   = True

        if top:
                x+=1
        elif left:
                y-=1
        elif bottom:
                x-=1
        elif right:
                y+=1

        return (x, y)

if __name__ == '__main__':
        pos = (0, 0)
        assertvalues = [
                (1,0),
                (1,1),
                (0,1),
                (-1,1),
                (-1,0),
                (-1,-1),
                (0,-1),
                (1,-1),
                (2,-1),
                (2,0),
                (2,1),
                (2,2),
                (1,2),
                (0,2),
                (-1,2),
                (-2, 2),
                (-2,1),
                (-2,0),
                (-2,-1),
                (-2,-2),
                (-1, -2),
                (0, -2),
                (1, -2),
                (2, -2),
                (3, -2)
        ]
        values = [pos]
        for i in range(len(assertvalues)):
                pos = next(pos)
                values.append(pos)

        for assertvalue, value in zip(assertvalues, values):
                print str(value).ljust(7), "\t", str(assertvalue).ljust(7), "\t", str(assertvalue==value)