advent-of-code/2022/6.c

#include <stddef.h>
#include <stdio.h>

// Part 1 - finds and returns the start-of-packet marker, or 0 if none exists
size_t get_pkt_mark(FILE *file) {

  size_t pos = 0;
  int c = 0, c_arr[4] = {0};

  while ((c = fgetc(file)) != EOF) {

    if (pos < 4) {
      c_arr[pos] = c;
      pos++;
      continue;
    }

    int invalid_marker = 0;
    size_t len = sizeof c_arr / sizeof *c_arr;
    for (size_t i = 0; i < len - 1; i++) {
      for (size_t j = i + 1; j < len; j++) {
        if (c_arr[i] == c_arr[j]) {
          invalid_marker = 1;
        }
      }
    }

    if (!invalid_marker) {
      return pos;
    }

    c_arr[0] = c_arr[1];
    c_arr[1] = c_arr[2];
    c_arr[2] = c_arr[3];
    c_arr[3] = c;
    pos++;
  }
  return 0;
}

size_t get_msg_mark(FILE *file) {

  size_t pos = 0;
  int c = 0, c_arr[14] = {0};

  while ((c = fgetc(file)) != EOF) {

    if (pos < 14) {
      c_arr[pos] = c;
      pos++;
      continue;
    }

    int invalid_marker = 0;
    size_t len = sizeof c_arr / sizeof *c_arr;
    for (size_t i = 0; i < len - 1; i++) {
      for (size_t j = i + 1; j < len; j++) {
        if (c_arr[i] == c_arr[j]) {
          invalid_marker = 1;
        }
      }
    }

    if (!invalid_marker) {
      return pos;
    }

    for (size_t i = 0; i < len - 1; i++) {
      c_arr[i] = c_arr[i + 1];
    }
    c_arr[len - 1] = c;
    pos++;
  }
  return 0;
}

int main(int argc, char *argv[]) {

  if (argc != 2) {
    fprintf(stderr, "USAGE: %s input_file\n", argv[0]);
    return 1;
  }

  // Part 1 - find the start-of-packet marker
  FILE *file = fopen(argv[1], "r");
  if (!file) {
    perror("fopen part1");
    return 1;
  }

  size_t pkt_mark = get_pkt_mark(file);
  if (!pkt_mark) {
    fprintf(stderr, "Start-of-Packet marker not found\n");
  } else {
    printf("Part 1: Start-of-Packet marker at position %lu\n", pkt_mark);
  }

  fclose(file);

  // Part 2 - find the start-of-message marker
  file = fopen(argv[1], "r");
  if (!file) {
    perror("fopen part2");
    return 1;
  }

  size_t msg_mark = get_msg_mark(file);
  if (!msg_mark) {
    fprintf(stderr, "Start-of-Message marker not found\n");
  } else {
    printf("Part 2: Start-of-Message marker at position %lu\n", msg_mark);
  }

  fclose(file);
  return 0;
}