#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <poll.h>
#include <sched.h>
#include <time.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>

#define GPIO "512"
#define MAX_BUF 64

void set_realtime_priority(int priority) {
    struct sched_param sched;
    sched.sched_priority = priority;
    if (sched_setscheduler(0, SCHED_FIFO, &sched) != 0) {
        perror("sched_setscheduler");
        exit(EXIT_FAILURE);
    }
}

void set_cpu_affinity(int core_id) {
    cpu_set_t cpuset;
    CPU_ZERO(&cpuset);
    CPU_SET(core_id, &cpuset);
    if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0) {
        perror("sched_setaffinity");
        exit(EXIT_FAILURE);
    }
}

void export_gpio() {
    char path[MAX_BUF];
    int fd = open("/sys/class/gpio/export", O_WRONLY);
    if (fd < 0 && errno != EBUSY) {
        perror("export");
        exit(EXIT_FAILURE);
    }
    if (fd >= 0) {
        write(fd, GPIO, strlen(GPIO));
        close(fd);
    }

    snprintf(path, sizeof(path), "/sys/class/gpio/gpio%s/direction", GPIO);
    fd = open(path, O_WRONLY);
    write(fd, "in", 2);
    close(fd);

    snprintf(path, sizeof(path), "/sys/class/gpio/gpio%s/edge", GPIO);
    fd = open(path, O_WRONLY);
    write(fd, "rising", 6);
    close(fd);
}

int main(int argc, char *argv[]) {
    char path[MAX_BUF];
    struct pollfd pfd;
    struct timespec ts;

    int core = 1;         // Run on CPU core 1
    int priority = 90;    // Real-time priority

    set_cpu_affinity(core);
    set_realtime_priority(priority);
    export_gpio();

    snprintf(path, sizeof(path), "/sys/class/gpio/gpio%s/value", GPIO);
    pfd.fd = open(path, O_RDONLY);
    if (pfd.fd < 0) {
        perror("gpio value open");
        exit(EXIT_FAILURE);
    }

    pfd.events = POLLPRI | POLLERR;
    lseek(pfd.fd, 0, SEEK_SET);
    char buf[8];
    read(pfd.fd, buf, sizeof(buf));  // Clear initial state

    while (1) {
        poll(&pfd, 1, -1);
        lseek(pfd.fd, 0, SEEK_SET);
        read(pfd.fd, buf, sizeof(buf));  // Clear interrupt

        if (clock_gettime(CLOCK_REALTIME, &ts) == 0) {
            printf("%ld.%09ld\n", ts.tv_sec, ts.tv_nsec);
            fflush(stdout);
        } else {
            perror("clock_gettime");
        }
    }

    close(pfd.fd);
    return 0;
}