Advent of Code Day 2 Part 2

day2.c

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

typedef enum ParseState
{
    OPPONENT,
    SPACE,
    YOU,
    NEWLINE
} ParseState;

int main(void)
{
    int fd = open("input", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return -1;
    }
    char buffer[4096];
    ssize_t bytes;
    size_t score = 0;
    ParseState state = OPPONENT;
    char you;
    char opponent;
    while ((bytes = read(fd, buffer, 4096)) > 0) {
        for (ssize_t i = 0; i < bytes; i++) {
            switch (state) {
                case OPPONENT:
                    opponent = buffer[i];
                    state = SPACE;
                    break;
                case SPACE:
                    state = YOU;
                    break;
                case YOU:
                    you = buffer[i];
                    state = NEWLINE;
                    break;
                case NEWLINE:
                    switch (you) {
                        case 'X':
                            switch (opponent) {
                                case 'A':
                                    score += 3;
                                    break;
                                case 'B':
                                    score += 1;
                                    break;
                                case 'C':
                                    score += 2;
                                    break;
                                default:
                                    fprintf(stderr, "Invalid input\n");
                                    return -1;
                            }
                            break;
                        case 'Y':
                            score += 3;
                            switch (opponent) {
                                case 'A':
                                    score += 1;
                                    break;
                                case 'B':
                                    score += 2;
                                    break;
                                case 'C':
                                    score += 3;
                                    break;
                                default:
                                    fprintf(stderr, "Invalid input\n");
                                    return -1;
                            }
                            break;
                        case 'Z':
                            score += 6;
                            switch (opponent) {
                                case 'A':
                                    score += 2;
                                    break;
                                case 'B':
                                    score += 3;
                                    break;
                                case 'C':
                                    score += 1;
                                    break;
                                default:
                                    fprintf(stderr, "Invalid input\n");
                                    return -1;
                            }
                            break;
                        default:
                            fprintf(stderr, "Invalid input\n");
                            return -1;
                    }
                    state = OPPONENT;
                    break;
            }
        }
    }
    close(fd);
    printf("Score: %lu\n", score);
    return 0;
}