Advent of Code Day 13 Part 1

part1.c

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

typedef struct Packet
{
    int *data;
    size_t size;
    size_t capacity;
    size_t num_lists;
} Packet;

static bool is_num(char c)
{
    return c >= '0' && c <= '9';
}

static void add_packet(Packet *packet, int i)
{
    if (packet->size >= packet->capacity) {
        packet->capacity *= 2;
        packet->data = realloc(packet->data, packet->capacity * sizeof(int));
        if (packet->data == NULL) {
            fprintf(stderr, "realloc failed\n");
            exit(-1);
        }
    }
    packet->data[packet->size] = i;
    packet->size += 1;
}

static void parse_packet(FILE *file, Packet *packet)
{
    char buffer[512];
    if (fgets(buffer, 512, file) == NULL) {
        fprintf(stderr, "Unexpected end of file\n");
        exit(-1);
    }
    packet->size = 0;
    packet->num_lists = 0;
    char *ptr = buffer;
    while (*ptr != '\n') {
        if (is_num(*ptr)) {
            add_packet(packet, strtol(ptr, &ptr, 10));
        } else if (*ptr == '[') {
            packet->num_lists += 1;
            ptr += 1;
        } else {
            ptr += 1;
        }
    }
}

static bool compare_packets(Packet *p1, Packet *p2)
{
    size_t smallest;
    if (p1->size == 0 && p2->size == 0) {
        return p1->num_lists <= p2->num_lists;
    }
    if (p1->size < p2->size) {
        smallest = p1->size;
    } else {
        smallest = p2->size;
    }
    for (size_t i = 0; i < smallest; i++) {
        if (p1->data[i] < p2->data[i]) {
            return true;
        } else if (p1->data[i] > p2->data[i]) {
            return false;
        }
    }
    return p1->size <= p2->size;
}

int main(void)
{
    FILE *file = fopen("input", "r");
    if (file == NULL) {
        perror("fopen");
        return -1;
    }
    Packet p1;
    Packet p2;
    p1.capacity = 16;
    p1.size = 0;
    p1.data = malloc(p1.capacity * sizeof(int));
    if (p1.data == NULL) {
        fprintf(stderr, "malloc failed\n");
        return -1;
    }
    p2.capacity = 16;
    p2.size = 0;
    p2.data = malloc(p2.capacity * sizeof(int));
    if (p2.data == NULL) {
        fprintf(stderr, "malloc failed\n");
        return -1;
    }
    int pair = 1;
    int answer = 0;
    do {
        parse_packet(file, &p1);
        parse_packet(file, &p2);
        if (compare_packets(&p1, &p2)) {
            printf("Pair %d: Correct\n", pair);
            answer += pair;
        } else {
            printf("Pair: %d: Incorrect\n", pair);
        }
        pair += 1;
    } while (getc(file) != EOF);
    printf("Answer: %d\n", answer);
    return 0;
}