Advent of Code Day 3 Part 2

day3_part2.c

#include <fcntl.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

static char get_priority(char c)
{
    if (c >= 'a' && c <= 'z') {
        return c - 96;
    } else if (c >= 'A' && c <= 'Z') {
        return c - 38;
    } else {
        fprintf(stderr, "Invalid input\n");
        exit(-1);
    }
}

int main(void)
{
    int fd = open("input", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return -1;
    }
    char buffer[4096];
    ssize_t bytes;
    size_t answer = 0;
    uint64_t bitfield[3];
    int rucksack = 0;
    memset(bitfield, 0, 3 * sizeof(uint64_t));
    while((bytes = read(fd, buffer, 4096)) > 0) {
        for (ssize_t i = 0; i < bytes; i++) {
            if (buffer[i] == '\n') {
                rucksack++;
                if (rucksack > 2) {
                    rucksack = 0;
                    uint64_t priority = bitfield[0] & bitfield[1] & bitfield[2];
                    for (int j = 1; j < 53; j++) {
                        uint64_t test = (uint64_t)1 << j;
                        if (test & priority) {
                            answer += j;
                            break;
                        }
                    }
                    memset(bitfield, 0, 3 * sizeof(uint64_t));
                }
            } else {
                bitfield[rucksack] |= (uint64_t)1 << get_priority(buffer[i]);
            }
        }
    }
    printf("Answer: %ld\n", answer);
    return 0;
}