advent_of_code/day_8/2.c

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

#define GRID_SIZE 50

typedef struct point {
  int x, y;
} point_t;

typedef struct lots_of_points {
  point_t points[4];
  unsigned char index;
} lots_of_points_t;

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

  if (fp == NULL) {
    perror("input");
    return 1;
  }

  char antinodes[GRID_SIZE][GRID_SIZE] = {0};

  lots_of_points_t data['z' + 1] = {0};

  for (unsigned int i = 0; i < GRID_SIZE; i++) {
    unsigned char buffer[GRID_SIZE] = {0};
    size_t read = fread(&buffer, sizeof(char), GRID_SIZE, fp);

    if (read == 0) {
      fprintf(stderr, "oopsy daisy\n");
      fclose(fp);
      return 1;
    }

    // move past the newline
    fseek(fp, 1, SEEK_CUR);

    for (unsigned int j = 0; j < GRID_SIZE; j++) {
      const unsigned char freq = buffer[j];
      if (freq != '.') {
        const point_t point = {(int)j, (int)i};
        data[freq].points[data[freq].index] = point;
        data[freq].index++;
      }
    }
  }

  unsigned int sum = 0;
  for (unsigned int i = 0; i < 'z' + 1; i++) {
    if (data[i].index == 0) {
      continue;
    }

    for (unsigned int j = 0; j < data[i].index; j++) {
      const point_t point = data[i].points[j];
      for (unsigned int k = 0; k < data[i].index; k++) {
        if (j == k) {
          continue;
        }

        const point_t other_point = data[i].points[k];

        for (int l = 0;; l++) {
          const int dist_x = point.x - other_point.x;
          const int dist_y = point.y - other_point.y;

          const point_t antinode = {point.x + dist_x * l, point.y + dist_y * l};

          if (antinode.x >= GRID_SIZE || antinode.x < 0 ||
              antinode.y >= GRID_SIZE || antinode.y < 0) {
            break;
          }

          if (antinodes[antinode.y][antinode.x] == 1) {
            continue;
          }

          antinodes[antinode.y][antinode.x] = 1;

          sum++;
        }
      }
    }
  }

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

  fclose(fp);

  return 0;
}