advent_of_code/day_1/2.c

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

unsigned int hash_int(unsigned int key) {
  /* Robert Jenkins' 32 bit Mix Function */
  key += (key << 12);
  key ^= (key >> 22);
  key += (key << 4);
  key ^= (key >> 9);
  key += (key << 10);
  key ^= (key >> 2);
  key += (key << 7);
  key ^= (key >> 12);

  /* Knuth's Multiplicative Method */
  key = (key >> 3) * 2654435761;

  return key;
}

#define TABLE_SIZE 16384
#define BUFFER_SIZE 1024

int main(void) {
  FILE *fp = fopen("input", "r");

  size_t lineCount = 0;

  char buffer[BUFFER_SIZE] = {0};

  // get number of lines
  size_t read = 0;

  while ((read = fread(&buffer, sizeof(char), BUFFER_SIZE, fp))) {
    for (size_t i = 0; i < read; i++) {
      if (buffer[i] == '\n') {
        lineCount++;
      }
    }
  }

  rewind(fp);

  // get data from file
  unsigned int *left = malloc((lineCount * 2 + 1) * sizeof(int));
  // haha pointer arithmetic go brrr
  unsigned int *right = left + lineCount;

  for (size_t i = 0; i < lineCount; i++) {
    unsigned int leftNum, rightNum;
    int numsGot = fscanf(fp, "%u   %u\n", &leftNum, &rightNum);

    if (numsGot == 2) {
      left[i] = leftNum;
      right[i] = rightNum;
    }
  }

  unsigned int hash_map[TABLE_SIZE] = {0};

  unsigned sum = 0;
  for (size_t i = 0; i < lineCount; i++) {
    const unsigned int key = hash_int(left[i]) % TABLE_SIZE;

    if (hash_map[key] == 0) {
      unsigned int similarity_score = 0;
      for (size_t j = 0; j < lineCount; j++) {
        if (left[i] == right[j]) {
          similarity_score++;
        }
      }

      similarity_score *= left[i];

      hash_map[key] = similarity_score;
    }

    sum += hash_map[key];
  }

  printf("%d\n", sum);

  free(left);
  fclose(fp);

  return 0;
}