day15.c

#define CYCLES 30000000
#include <stdlib.h>
#include <stdio.h>

static int memory[CYCLES] = {[19] =  1, [0] =  2, [5] = 3, [1] = 4, [10] = 5};

int main () {
  int i = 7;
  int prev = 13;
  int k;
  while (i <= CYCLES) {
    k = memory[prev];
    memory[prev] = i - 1;
    prev = 0;
    if (k != 0) {
      prev = i - k - 1;
    }
    ++i;
  }
  printf("%i\n", prev);
}