aoc 2020 day 3

part1.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main(void)
{
    FILE *f = fopen("input.txt", "r");
    char *buf = malloc(1 << 26);
    int width = 0, height = 0, x, y, hits;
    while (fgets(buf + width * height, BUFSIZ, f))
    {
        width = (int)strlen(buf + width * height) - 1;
        height++;
    }
    for (x = 0, y = 1, hits = 0; y < height; y++)
    {
        x = (x + 3) % width;
        if (buf[y * width + x] == '#') hits++;
    }
    printf("%d\n", hits);
}

part2.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main(void)
{
    FILE *f = fopen("input.txt", "r");
    char *buf = malloc(1 << 26);
    int width = 0, height = 0, dx[] = {1, 3, 5, 7, 1}, dy[] = {1, 1, 1, 1, 2};
    long long total = 1;
    while (fgets(buf + width * height, BUFSIZ, f))
    {
        width = (int)strlen(buf + width * height) - 1;
        height++;
    }
    for (int i = 0, x, y, hits; i < 5; i++)
    {
        for (x = 0, y = dy[i], hits = 0; y < height; y += dy[i])
        {
            x = (x + dx[i]) % width;
            if (buf[y * width + x] == '#') hits++;
        }
        total *= hits;
    }
    printf("%lld\n", total);
}