aoc2023/day6/p1.c

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


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

int64_t parse_number(char *end);


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 *times = (uint64_t *) calloc(2, sizeof(uint64_t));
    uint64_t *distances = (uint64_t *) calloc(2, sizeof(uint64_t));

    ssize_t times_length = 2, distances_length = 2;

    for (ssize_t i = time_line_length-2, j = 0;;) {
        while (!ISDIGIT(time_line[i]))
            if (time_line[--i] == ':')
                goto tl_end;

        if (j == times_length)
            times = (uint64_t *) realloc(times, ++times_length * sizeof(uint64_t));

        times[j++] = parse_number(time_line+i);

        while(ISDIGIT(time_line[i])) --i;
    }
    tl_end:

    for (ssize_t i = distance_line_length-2, j = 0;;) {
        while (!ISDIGIT(distance_line[i]))
            if (distance_line[--i] == ':')
                goto dl_end;

        if (j == distances_length)
            distances = (uint64_t *) realloc(distances, ++distances_length * sizeof(uint64_t));

        distances[j++] = parse_number(distance_line+i);

        while(ISDIGIT(distance_line[i])) --i;
    }
    dl_end:

    uint64_t total_ways = 1;

    for (ssize_t i = 0; i < times_length; ++i) {
        uint64_t tmid = times[i] / 2;
        uint64_t ways = 0;

        for (uint64_t ht = tmid; ht <= times[i]; ++ht)
            if (ht * (times[i] - ht) > distances[i])
                ++ways;
            else
                break;

        for (uint64_t ht = tmid-1; ht != 0; --ht)
            if (ht * (times[i] - ht) > distances[i])
                ++ways;
            else
                break;

        total_ways *= ways;
    }

    free(times);
    free(distances);

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

    return 0;
}

int64_t parse_number(char *end) {
    int64_t num = 0;

    for (size_t m = 1;; m *= 10) {
        if (*end>>4 != 0x3)
            break;
        num += (*end&0xf) * m;
        --end;
    }

    return num;
}