Advent of Code Day 25

day25.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>

static long read_digit(char c)
{
    switch (c) {
        case '-':
            return -1;
        case '=':
            return -2;
        case '0':
            return 0;
        case '1':
            return 1;
        case '2':
            return 2;
        default:
            fprintf(stderr, "Invalid input digit: %c\n", c);
            exit(-1);
    }
}

int main(void)
{
    struct timespec start, end;
    clock_gettime(CLOCK_MONOTONIC, &start);
    FILE *file = fopen("input", "r");
    if (file == NULL) {
        perror("fopen");
        return -1;
    }
    char buffer[32];
    long sum = 0;
    while (fgets(buffer, 32, file) != NULL) {
        long entry = 0;
        long mul = 1;
        char *ptr = buffer;
        while (*ptr != '\n') {
            ptr++;
        }
        ptr--;
        while (ptr != buffer) {
            entry = entry + (mul * read_digit(*ptr));
            ptr--;
            mul *= 5;
        }
        entry = entry + (mul * read_digit(*ptr));
        sum += entry;
    }
    fclose(file);
    char answer[32];
    int digits = 1;
    long max = 2;
    long mul = 1;
    while (sum > max) {
        mul *= 5;
        max = max + (mul * 2);
        digits += 1;
    }
    for (int i = 0; i < digits; i++) {
        max = max - (mul * 2);
        long cur = 0;
        if (sum > max) {
            while (sum > max) {
                cur += 1;
                sum -= mul;
            }
        } else {
            while (labs(sum) > max) {
                cur -= 1;
                sum += mul;
            }
        }
        mul /= 5;
        switch(cur) {
            case -2:
                answer[i] = '=';
                break;
            case -1:
                answer[i] = '-';
                break;
            case 0:
                answer[i] = '0';
                break;
            case 1:
                answer[i] = '1';
                break;
            case 2:
                answer[i] = '2';
                break;
            default:
                fprintf(stderr, "Invalid output digit: %ld\n", cur);
                return -1;
        }
    }
    answer[digits] = '\0';
    puts(answer);
    clock_gettime(CLOCK_MONOTONIC, &end);
    int64_t nanoseconds = ((end.tv_sec - start.tv_sec) * 1000000000) + (end.tv_nsec - start.tv_nsec);
    double milliseconds = nanoseconds / 1000000.0;
    printf("Time: %fms\n", milliseconds);
    return 0;
}