AoC-2022 Day 3b -- single pass approach

#include <stdio.h>

const char* input = R"(vJrwpWtwJgWrhcsFMMfFFhFp
jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL
PmmdzqPrVvPwwTWBwg
wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn
ttgJtRGJQctTZtZT
CrZsJsPPZsGzwwsLwLmpwMDw
)";

int getPriority(char c)
{
    if (c > 'Z')
        return c - 'a' + 1;
    return c - 'A' + 27;
}

int processSack(int from, int count, int tripletPos)
{
    static char basket[52] = { 0 };

    if (0 == tripletPos)
        memset(basket, 0, sizeof basket);

    for (int i = 0; i < count; i++)
    {
        char c = input[from + i];
        int priority = getPriority(c);
        basket[priority - 1] |= (1 << tripletPos);
        if (0b0111 == basket[priority - 1])
            return priority;
    }

    return 0;
}

int main()
{
    int sum = 0;

    int from = 0;
    int count = 0;
    int tripletPos = 0;

    while (input[from + count])
    {
        count++;
        char c = input[from + count];
        if ('\n' == c)
        {
            if (count % 2)
                return -1; // odd count of items => input error

            sum += processSack(from, count, tripletPos);
            from += count + 1;
            count = 0;
            tripletPos++;
            tripletPos = tripletPos % 3;
            continue;
        }

        if ((c >= 'a') && (c <= 'z'))
            continue;

        if ((c >= 'A') && (c <= 'Z'))
            continue;

        return -1; // invalid character => input error
    }

    printf("%d\n", sum);
    return 0;
}