tiling verification

# checks for each line that the level is an odd number between 1 and 2k-1,
# places a blocking configuration in the n x n x n grid at the corresponding level,
# and checks that no blocking configurations placed in this process ever overlap

def verify(xlist, ylist, zlist, k):
    filled = [[[0 for z in range(k)] for y in range(k)] for x in range(k)]
    def is_filled(a, b, c):
        return filled[(a-1)/2][(b-1)/2][(c-1)/2]
    def fill(a, b, c):
        filled[(a-1)/2][(b-1)/2][(c-1)/2] = 1

    #format check
    if len(xlist) != 2*k - 1 or len(ylist) != 2*k - 1 or len(zlist) != 2*k - 1:
        return 0

    for a in range(1, 2*k):
        #format check
        if len(xlist[a-1]) != 2*k - 1 or len(ylist[a-1]) != 2*k - 1 or len(zlist[a-1]) != 2*k - 1:
            return 0

        for b in range(1, 2*k):
            u, v, w = xlist[a-1][b-1], ylist[a-1][b-1], zlist[a-1][b-1]

            # format check
            for coord in [u, v, w]:
                if coord%2 == 0 or coord < 1 or coord > 2*k - 1:
                    return 0

            # case: both odd
            if a%2 == 1 and b%2 == 1:
                if is_filled(u, a, b):
                    return 0
                fill(u, a, b)
                if is_filled(a, v, b):
                    return 0
                fill(a, v, b)
                if is_filled(a, b, w):
                    return 0
                fill(a, b, w)

            # case: a odd
            if a%2 == 1 and b%2 == 0:
                if is_filled(u, a, b-1) or is_filled(u, a, b+1):
                    return 0
                fill(u, a, b-1)
                fill(u, a, b+1)
                if is_filled(a, v, b-1) or is_filled(a, v, b+1):
                    return 0
                fill(a, v, b-1)
                fill(a, v, b+1)
                if is_filled(a, b-1, w) or is_filled(a, b+1, w):
                    return 0
                fill(a, b-1, w)
                fill(a, b+1, w)

            # case: b odd
            if a%2 == 0 and b%2 == 1:
                if is_filled(u, a-1, b) or is_filled(u, a+1, b):
                    return 0
                fill(u, a-1, b)
                fill(u, a+1, b)
                if is_filled(a-1, v, b) or is_filled(a+1, v, b):
                    return 0
                fill(a-1, v, b)
                fill(a+1, v, b)
                if is_filled(a-1, b, w) or is_filled(a+1, b, w):
                    return 0
                fill(a-1, b, w)
                fill(a+1, b, w)

            # case: both even
            if a%2 == 0 and b%2 == 0:
                if is_filled(u, a-1, b-1) or is_filled(u, a-1, b+1) or is_filled(u, a+1, b-1) or is_filled(u, a+1, b+1):
                    return 0
                fill(u, a-1, b-1)
                fill(u, a-1, b+1)
                fill(u, a+1, b-1)
                fill(u, a+1, b+1)
                if is_filled(a-1, v, b-1) or is_filled(a-1, v, b+1) or is_filled(a+1, v, b-1) or is_filled(a+1, v, b+1):
                    return 0
                fill(a-1, v, b-1)
                fill(a-1, v, b+1)
                fill(a+1, v, b-1)
                fill(a+1, v, b+1)
                if is_filled(a-1, b-1, w) or is_filled(a-1, b+1, w) or is_filled(a+1, b-1, w) or is_filled(a+1, b+1, w):
                    return 0
                fill(a-1, b-1, w)
                fill(a-1, b+1, w)
                fill(a+1, b-1, w)
                fill(a+1, b+1, w)

    return 1