aoc 2020 day 5

2020_day5_part1.c

#include <stdio.h>
int main(void)
{
    FILE *f = fopen("input.txt", "r");
    char line[BUFSIZ];
    int highest = 0, y, x, i;
    while (fgets(line, BUFSIZ, f))
    {
        for (y = x = i = 0; i < 10; i++)
        {
            if (line[i] == 'B') y += 64 >> i;
            if (line[i] == 'R') x += 4 >> (i - 7);
        }
        if (y * 8 + x > highest) highest = y * 8 + x;
    }
    printf("%d\n", highest);
}

2020_day5_part2.c

#include <stdio.h>
int main(void)
{
    FILE *f = fopen("input.txt", "r");
    char line[BUFSIZ], seats[128] = {0};
    int H = sizeof seats, W = 8, y, x, i, row, me;
    while (fgets(line, BUFSIZ, f))
    {
        for (y = x = i = 0; i < 10; i++)
        {
            if (line[i] == 'B') y += H >> (i + 1);
            if (line[i] == 'R') x += W >> (i - 6);
        }
        seats[y] |= 1 << x;
    }
    for (y = 0; y < H; y++)
    {
        for (x = row = 0; x < W; x++)
            if (seats[y] & 1 << x) row++; else me = x;
        if (row == 7) printf("%d\n", y * 8 + me);
    }
}

Alternative input reading loop:

    while (fgets(line, BUFSIZ, f))
    {
        y = x = i = 0;
        while (i <  7) if (line[i++] == 'B') y += H >> i;
        while (i < 10) if (line[i++] == 'R') x += W >> (i - 7);
        seats[y] |= 1 << x;
    }

Also, you can replace the += addition with a |= bitwise OR. They're equivalent, in this case.