19.py

"""--- Day 19: An Elephant Named Joseph ---"""

def solve_1(elves):
    circle = list(map(lambda x: [1, 1+x], range(elves)))

    while len(circle) > 1:
        for i in range(0, len(circle)):
            p = circle[i]
            if p[0] > 0:
                # get second elf
                p2 = circle[(i+1) % len(circle)]
                # steal presents
                p2[0] = 0
        # only need to update once we've been around once
        circle = [e for e in circle if e[0] > 0]
    return circle[0][1]

def solve_2(elves):
    circle = list(map(lambda x: 1+x, range(elves)))
    # start at first elf
    i = 0
    while len(circle) > 1:
        # find elf opposite
        opposite = (i + int(len(circle) / 2)) % len(circle)
        # remove that elf
        circle.pop(opposite)
        # determine next elf in circle
        if i == len(circle):
            i = 0         # last elf, next is first
        elif i < opposite:
            i += 1        # we removed elf ahead of us
        else:
            continue      # we removed an elf before us
    return circle[0]

with open("input/19.txt", "r") as f:
    text = int(f.readline())
    print(solve_1(text))
    print(solve_2(text))