Day 19 - Advent of Code 2016

""" day_19_01.py """

# usage: python3 day_19_01.py 3014387

import sys


def next_elf(x):
    """ next elf with gifts """
    while circle[x := (x + 1) % elves] == 0:
        pass
    return x


elves = int(sys.argv[1])
circle = [1 for i in range(1, elves + 1)]

i = 0
while circle[i] != elves:
    j = next_elf(i)
    circle[i] += circle[j]
    circle[j] = 0
    i = next_elf(j)

print(i + 1)

/* day_19_02.c */

// usage: gcc -o day_19_02 day_19_02.c
//        ./day_19_02 3014387

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv[]) {

    int elves = atoi(argv[1]);
    int remaining = elves;

    int** circle = (int**) malloc(elves * sizeof(int*));
    for (int i = 0; i < elves; i++) {
        circle[i] = (int*) malloc(3 * sizeof(int));
    }

    for (int i = 0; i < elves; i++) {
        circle[i][0] = 1;
        circle[i][1] = (i - 1) % elves;
        circle[i][2] = (i + 1) % elves;
    }

    int elf = 0, other_elf;
    while (circle[elf][0] != elves) {
        other_elf = elf;
        for (int i = 0; i < remaining / 2; i++) {
            other_elf = circle[other_elf][2];
        }

        circle[elf][0] += circle[other_elf][0];
        circle[circle[other_elf][1]][2] = circle[other_elf][2];
        circle[circle[other_elf][2]][1] = circle[other_elf][1];
        remaining -= 1;
        elf = circle[elf][2];
    }

    printf("%d\n", elf + 1);

    for (int i = 0; i < elves; i++) {
        free(circle[i]);
    }

    free(circle);

    return 0;
}