#include <ctype.h>
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/vt.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <unistd.h>

// check if argument is tty[0-9]
static bool is_vt_basename(const char *s) {
    return !strncmp(s, "tty", 3) && isdigit(s[3]);
}

// return the number of the active virtual terminal or -1 on error
static int get_active_vt() {
    int dfd = open("/dev", O_RDONLY | O_DIRECTORY);
    if (dfd < 0) {
        perror("open /dev");
        return 1;
    }

    DIR *d = fdopendir(dfd);
    if (!d) {
        perror("fdopendir");
        close(dfd);
        return 1;
    }

    int active_vt = -1;
    struct dirent *ent;

    while ((ent = readdir(d)) != NULL) {
        if (!is_vt_basename(ent->d_name))
            continue;

        int fd = openat(dfd, ent->d_name, O_RDONLY | O_NONBLOCK);
        if (fd < 0)
            continue;

        struct vt_stat state;
        if (ioctl(fd, VT_GETSTATE, &state) == 0) {
            active_vt = state.v_active;
            close(fd);
            break;
        }

        close(fd);
    }

    closedir(d);

    return active_vt;
}

int main(void) {
    int active_vt = get_active_vt();

    if (active_vt < 0) {
        fprintf(stderr, "Could not determine active VT\n");
        return 1;
    }

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