spyral.py

#!/usr/bin/env python3

check_results = {
    5: """
1 2 3 4 5
16 17 18 19 6
15 24 25 20 7
14 23 22 21 8
13 12 11 10 9""",
    7: """
1 2 3 4 5 6 7
24 25 26 27 28 29 8
23 40 41 42 43 30 9
22 39 48 49 44 31 10
21 38 47 46 45 32 11
20 37 36 35 34 33 12
19 18 17 16 15 14 13 """,
    3: """
1 2 3
8 9 4
7 6 5 """
}


def numerator(n):
    threshold = n
    turn = 0
    second_pass = True
    step = 1
    yield 0
    s_i, s_j = [0, 0]
    for x in range(n**2 - 1):
        s_i, s_j = {
            0: [s_i, s_j + 1],      # right
            1: [s_i + 1, s_j],      # down
            2: [s_i, s_j - 1],      # left
            3: [s_i - 1, s_j]       # up
        }[turn]
        yield (s_i * n) + s_j
        step += 1
        if step == threshold:
            turn = (turn + 1) % 4
            if second_pass:
                threshold -= 1
            second_pass = not second_pass
            step = 0


def generate(n):
    num = 1
    l = [None] * (n**2)
    for idx in numerator(n):
        l[idx] = str(num)
        num += 1
    return '\n'.join(' '.join(l[x:x+n]) for x in range(0, len(l), n))


def check(n):
    result = generate(n, l)
    print("DONE" if result == check_results[n].strip() else "FAIL:\n%s\n" % result)


if __name__ == "__main__":
    check(5)
    check(3)
    check(7)