advent of code 2020 day 23

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

static int n = 1000000;
static int m = 10000000;

int inArray (int x, int* arr) {
  for (int i = 0; i < 3; i++) {
    if (x == arr[i]) return 1;
  }
  return 0;
}

int findDest (int x, int selected[]) {
  if (x < 0) x = n - 1;
  while (inArray(x, selected)) x--;

  return x;
}

int* part1 (int* nums) {
  int curr = nums[0];
  int selected[3];

  int i = 0;
  while (i < m) {
    selected[0] = nums[curr];
    selected[1] = nums[selected[0]];
    selected[2] = nums[selected[1]];

    int dest = findDest(curr - 1, selected);
    int temp = nums[dest];

    nums[dest] = selected[0];
    nums[curr] = nums[selected[2]];
    nums[selected[2]] = temp;

    curr = nums[curr];

    i++;
  }

  return nums;
}

int main () {
  int i;

  int* nums = calloc (n, sizeof(int));

  nums[0] = 1;
  nums[1] = 2;
  nums[2] = 3;
  nums[3] = 4;
  nums[4] = 8;
  nums[8] = 7;
  nums[7] = 5;
  nums[5] = 9;
  nums[9] = 6;
  nums[6] = 10;
  nums[n - 1] = 0;

  for (i = 10; i < n - 1; i++) {
    nums[i] = i + 1;
  }

  int* seq = part1(nums);

  printf("%d * %d = %ld\n", seq[1], seq[seq[1]], (long)seq[1] * (long)seq[seq[1]]);

  return 0;
}