Unix File Reading using Chunking

char_at.c

#define CHUNK_SIZE 256

char char_at(int fd, long long at, enum Direction D) {
    static int FD;
    static long long left, right;
    static char CHUNK[CHUNK_SIZE];

    // If outside the file, stop this function.
    long long min = lseek(fd, 0, SEEK_SET), max = lseek(fd, 0, SEEK_END);
    if (at < min || at >= max) return -1;

    // If new file is present, do some resets.
    if (FD != fd) goto CHUNK_SETTING;

    if (at >= left && at < right) {
        return CHUNK[at - left];
    } else {
        CHUNK_SETTING:
        lseek(fd, at, SEEK_SET);
        switch (D) {
            case AHEAD:
                left = at;
                right = left + read(fd, CHUNK, CHUNK_SIZE);
                return CHUNK[0];
            case BEHIND:
                // If reading behind from current point would go outside the file, simply read from start.
                if (at + 1 < CHUNK_SIZE) {
                    lseek(fd, 0, SEEK_SET);
                    read(fd, CHUNK, CHUNK_SIZE);
                    left = 0;
                    right = CHUNK_SIZE;
                    return CHUNK[at];
                } else {
                    lseek(fd, -(CHUNK_SIZE - 1), SEEK_CUR);
                    read(fd, CHUNK, CHUNK_SIZE);
                    left = at - (CHUNK_SIZE - 1);
                    right = left + CHUNK_SIZE;
                    return CHUNK[CHUNK_SIZE - 1];
                }
            default:
                return -1;
        }
    }
}