aoc.c

#include <stdio.h>
#define NUMBERS 1000000
#define ROUNDS 10000000
#define NUMBERS_1 NUMBERS + 1
int main(int argc, char const *argv[])
{
    unsigned int suc[NUMBERS_1];
    unsigned int fixed[] = {7, 1, 2, 6, 4, 3, 5, 8, 9};
    unsigned int removed[5];
    unsigned int current = fixed[0];
    unsigned int next;
    for (unsigned int i = 1; i < NUMBERS; i++)
    {
        suc[i] = i + 1;
    }
    for (unsigned int i = 0; i < 8; ++i)
    {
        suc[fixed[i]] = fixed[i + 1];
    }
    suc[fixed[8]] = 10;
    suc[NUMBERS] = fixed[0];
    int rm;
    for (unsigned int i = 0; i < ROUNDS; i++)
    {
        removed[0] = suc[current];
        removed[1] = suc[removed[0]];
        removed[2] = suc[removed[1]];
        removed[3] = suc[removed[2]];
        removed[4] = suc[removed[3]];
        next = current - 1;
        if (!next)
        {
            next = NUMBERS;
        }
        while (next == removed[0] || next == removed[1] || next == removed[2])
        {
            next -= 1;
            if (!next)
            {
                next = NUMBERS;
            }
        }
        suc[current] = removed[3];
        suc[removed[2]] = suc[next];
        suc[next] = removed[0];
        current = suc[current];
    }
    printf("%llu\n", ((unsigned long long)suc[1]) * ((unsigned long long)suc[suc[1]]));
    return 0;
}