aoc2023/day6/p2.c

#define  _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <stdbool.h>


#define ISDIGIT(c) (c >= 0x30 && c <= 0x39)

int main(int argc, char **argv) {
    FILE *input;
    size_t line_buf_length = 0;

    if (argc == 1 || argv[1][0] == '-')
		input = stdin;
	else if ((input = fopen(argv[1], "r")) == NULL) {
		fprintf(stderr, "Cannot open file %s\n", argv[1]);
		return -1;
	}

    char *time_line = NULL, *distance_line = NULL;
    ssize_t time_line_length = 0, distance_line_length = 0;

    if ((time_line_length = getline(&time_line, &line_buf_length, input)) == -1) {
        fprintf(stderr, "No time line in a file %s.", argv[1]);
        return -1;
    }

    if ((distance_line_length = getline(&distance_line, &line_buf_length, input)) == -1) {
        fprintf(stderr, "No distance line in a file %s.", argv[1]);
        return -1;
    }

    uint64_t time = 0, distance = 0;

    for (ssize_t i = time_line_length-2, m = 1;; --i) {
        while (!ISDIGIT(time_line[i])) {
            if (time_line[--i] == ':')
                goto tl_end;
        }

        while (ISDIGIT(time_line[i])) {
            time += (time_line[i--]&0xf) * m;
            m *= 10;
        }
    }
    tl_end:

    for (ssize_t i = distance_line_length-2, m = 1;; --i) {
        while (!ISDIGIT(distance_line[i])) {
            if (distance_line[--i] == ':')
                goto dl_end;
        }

        while (ISDIGIT(distance_line[i])) {
            distance += (distance_line[i--]&0xf) * m;
            m *= 10;
        }
    }
    dl_end:

    uint64_t total_ways = 0;

    uint64_t tmid = time / 2;

    for (uint64_t ht = tmid; ht <= time; ++ht) {
        if (ht * (time - ht) > distance)
            ++total_ways;
        else
            break;
    }

    for (uint64_t ht = tmid-1; ht != 0; --ht) {
        if (ht * (time - ht) > distance)
            ++total_ways;
        else
            break;
    }

    printf("%lu\n", total_ways);

    return 0;
}