10.c

#include <assert.h>
#include <err.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include "inputs/10.h"

static
int compare(const void* p1, const void* p2) {
  long a = * (long*)p1;
  long b = * (long*)p2;

  if (a == b) {
    return 0;
  }

  return (a < b) ? -1 : 1;
}

int day10(void) {
  int64_t adapter_joltages[108];
  size_t adapter_n = 0;
  adapter_joltages[adapter_n++] = 0;  // outlet
  const unsigned char *s = input;
  while (*s != '\0') {
    int64_t n = 0;
    while (*s >= '0' && *s <= '9') {
      n = n * 10 + (*s - '0');
      s++;
    }
    adapter_joltages[adapter_n++] = n;
    if (*s == '\n') s++;
  }

  int64_t charging_outlet_joltage = 0;
  qsort(adapter_joltages, adapter_n, sizeof(int64_t), compare);

  int64_t device_joltage = adapter_joltages[adapter_n - 1] + 3;

  int64_t diff[4] = {[0 ... 3] = 0};
  diff[adapter_joltages[0] - charging_outlet_joltage]++;
  for (size_t i = 1; i < adapter_n; i++) {
    int64_t d = adapter_joltages[i] - adapter_joltages[i - 1];
    diff[d]++;
  }

  // add device difference
  diff[device_joltage - adapter_joltages[adapter_n - 1]]++;
  int answer_pt1 = diff[1] * diff[3];
  printf("%d\n", answer_pt1);
  assert(answer_pt1 == 2400);

  // for each adapter, find the number of available options
  int64_t max_diff = 3;
  int64_t ways[adapter_n];
  ways[0] = 1;
  for (size_t i = 1; i < adapter_n; i++) {
    ways[i] = 0;

    for (int64_t j = i - 1;
         j >= 0 && (adapter_joltages[i] - adapter_joltages[j]) <= max_diff;
         j--) {
      ways[i] += ways[j];
    }
  }
  printf("%ld\n", ways[adapter_n - 1]);
  assert(ways[adapter_n - 1] == 338510590509056);
  return 0;
}