advent-of-code/2022/4.c

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

struct Assignment {
  char *range;
  int start;
  int end;
};

// Split each line into 2 strings representing the assignment range for each elf
void parse_assignments(char *assignments, struct Assignment *elf1,
                       struct Assignment *elf2) {

  elf1->range = strtok(assignments, ",");
  elf2->range = strtok(NULL, ",");
}

// Split each assignment range into a start and end
void parse_sections(struct Assignment *elf) {
  elf->start = atoi(strtok(elf->range, "-,"));
  elf->end = atoi(strtok(NULL, "-,"));
}

// Count the number of assignments that fully contain the other
void count_subsets(int *count, struct Assignment *elf1,
                   struct Assignment *elf2) {

  if ((elf1->start >= elf2->start && elf1->end <= elf2->end) ||
      (elf2->start >= elf1->start && elf2->end <= elf1->end)) {
    *count += 1;
  }
}

// Count the number of assignments with any overlapping sections
void count_overlaps(int *count, struct Assignment *elf1,
                    struct Assignment *elf2) {

  if ((elf1->start <= elf2->end && elf1->end >= elf2->start) ||
      (elf2->start <= elf1->end && elf2->end >= elf1->start)) {
    *count += 1;
  }
}

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

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

  FILE *file = fopen(argv[1], "r");
  if (!file) {
    perror("fopen");
    return 1;
  }

  int part1_count = 0;
  int part2_count = 0;
  char assignments[16] = {0};

  while (fgets(assignments, sizeof assignments / sizeof *assignments, file)) {

    if (strlen(assignments) == 1) {
      break;
    }

    struct Assignment elf1;
    struct Assignment elf2;

    assignments[strlen(assignments) - 1] = '\0';

    // split input into starting and ending points
    parse_assignments(assignments, &elf1, &elf2);
    parse_sections(&elf1);
    parse_sections(&elf2);

    // Part 1 - count sections that fully are fully contained by another
    count_subsets(&part1_count, &elf1, &elf2);

    // Part 2 - count all overlapping sections
    count_overlaps(&part2_count, &elf1, &elf2);
  }

  printf("Part 1 - %d ranges fully contain another\n", part1_count);
  printf("part 2 - %d assignments overlap\n", part2_count);

  fclose(file);
  return 0;
}