#define _POSIX_C_SOURCE 200809l
#define _DEFAULT_SOURCE

#include <linux/input.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/epoll.h>
#include <sys/ioctl.h>

#include <stdio.h>
#include <stdlib.h>
#include <err.h>
#include <string.h>
#include <dirent.h>
#include <fcntl.h>
#include <unistd.h>

#define test_bit(yalv, abs_b) ((((char *)abs_b)[yalv/8] & (1<<yalv%8)) > 0)

#define EVDEV_ROOT "/dev/input/"

int main(int argc, char **argv)
{
	(void)argc;
	(void)argv;

	DIR *dir = opendir(EVDEV_ROOT);
	if (!dir)
		err(1, "opendir:");
	struct dirent *dp;
	struct stat st = {0};
	int stylus = -1;

	while ((dp = readdir(dir)) != NULL) {
		if (fstatat(dirfd(dir), dp->d_name, &st, 0) == -1)
			err(1, "fstatat:");
		if ((st.st_mode & S_IFMT) != S_IFCHR)
			continue;

		// blocking io
		int tmp_fd = openat(dirfd(dir), dp->d_name, O_RDWR);
		if (tmp_fd < 0)
			err(1, "openat:");
		union {
			char b[EV_MAX];
			char k[KEY_MAX];
		} vec;
		if (ioctl(tmp_fd, EVIOCGBIT(0, EV_MAX), vec.b) < 0)
			err(1, "ioctl:");
		if (!test_bit(EV_KEY, vec.b) || !test_bit(EV_ABS, vec.b))
			continue;
		if (ioctl(tmp_fd, EVIOCGBIT(EV_KEY, KEY_MAX), vec.k) < 0)
			err(1, "ioctl:");

		if (test_bit(BTN_STYLUS, vec.k) && test_bit(BTN_TOOL_RUBBER, vec.k)) {
			stylus = tmp_fd;
			break;
		}
		close(tmp_fd);
	}
	closedir(dir);
	if (stylus == -1)
		err(1, "no stylus input found");

	int poll_set = epoll_create(1);
	if (poll_set < 0)
		err(1, "epoll_create:");
	struct epoll_event poll_ev = { .events = EPOLLIN };
	if (epoll_ctl(poll_set, EPOLL_CTL_ADD, stylus, &poll_ev) < 0)
		err(1, "epoll_ctl:");

	struct input_event in_ev, out_ev;
	while (epoll_wait(poll_set, &poll_ev, 1, -1) != -1) {
		switch (poll_ev.events) {
		case EPOLLIN:
			if (read(stylus, &in_ev, sizeof(struct input_event)) < 0)
				err(1, "read:");
			if (in_ev.code != BTN_STYLUS)
				break;
			out_ev = in_ev;
			out_ev.code = BTN_TOOL_RUBBER;
			if (write(stylus, &out_ev, sizeof(struct input_event)) < 0)
				err(1, "write:");
			break;			
		default:
			break;
		}
	}

	return 0;
}