#define _GNU_SOURCE

#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define FUSE_USE_VERSION 317

#include <fuse3/fuse.h>

static char *progname;
static int fd1, fd2;

static int get_fd() {
	pid_t pid = fork();
	if (pid == -1) {
		perror("fork");
		exit(1);
	}
	if (pid == 0) {
		execl(progname, progname, (char *) NULL);

		exit(2);
	}

	int wstatus;
	if (waitpid(pid, &wstatus, 0) == -1) {
		perror("waitpid");
		exit(1);
	}

	int code = WEXITSTATUS(wstatus);
	if (code == 0) return fd1;
	if (code == 1) return fd2;

	return -1;
}

static int condviewfs_open(const char *name, struct fuse_file_info *fi) {
	int dirfd = get_fd();
	if (dirfd == -1)
		return -ENOLINK;

	int fd = openat(dirfd, &name[1], fi->flags);
	if (fd < 0)
		return -errno;

	fi->fh = (uint64_t)fd;
	return 0;
}

static int condviewfs_close(const char *name, struct fuse_file_info *fi) {
	if (close((int)fi->fh))
		return -errno;

	return 0;
}

static int condviewfs_getattr(const char *name,
                        struct stat *stbuf,
                        struct fuse_file_info *fi) {
	int dirfd = get_fd();
	if (dirfd == -1)
		return -ENOLINK;

	int ret;

	if (fi)
		ret = fstat((int)fi->fh, stbuf);
	else
		ret = fstatat(dirfd,
	                  &name[1],
	                  stbuf,
	                  AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);

	if (ret < 0)
		return -errno;

	return 0;
}

static int condviewfs_access(const char *name, int mask) {
	int dirfd = get_fd();
	if (dirfd == -1)
		return -ENOLINK;

	if ((name[0] != '/') || (name[1] != '\0'))
		++name;

	if (faccessat(dirfd, name, mask, AT_SYMLINK_NOFOLLOW) < 0)
		return -errno;

	return 0;
}

static int condviewfs_read(const char *path,
                     char *buf,
                     size_t size,
                     off_t off,
                     struct fuse_file_info *fi) {
	ssize_t out;

	out = pread((int)fi->fh, buf, size > INT_MAX ? INT_MAX : size, off);
	if (out < 0)
		return -errno;

	return (int)out;
}

static int condviewfs_opendir(const char *name, struct fuse_file_info *fi) {
	DIR *dirp;
	int dirfd = get_fd();
	if (dirfd == -1)
		return -ENOLINK;

	int fd, err;

	if ((name[0] == '/') && (name[1] == '\0'))
		fd = dup(dirfd);
	else
		fd = openat(dirfd, &name[1], O_DIRECTORY);

	if (fd < 0)
		return -errno;

	dirp = fdopendir(fd);
	if (!dirp) {
		err = -errno;
		close(fd);
		return err;
	}

	fi->fh = (uint64_t)dirp;
	return 0;
}

static int condviewfs_closedir(const char *name, struct fuse_file_info *fi) {
	if (closedir((DIR *)(uintptr_t)fi->fh) < 0)
		return -errno;

	return 0;
}

static int condviewfs_readdir(const char *path,
                        void *buf,
                        fuse_fill_dir_t filler,
                        off_t offset,
                        struct fuse_file_info *fi,
                        enum fuse_readdir_flags flags) {
	int dirfd = get_fd();
	if (dirfd == -1)
		return -ENOLINK;

	DIR *dirp = (DIR *)(uintptr_t)fi->fh;

	if (offset == 0)
		rewinddir(dirp);

	struct stat stbuf;
	struct dirent *dent;
	while (1) {
		errno = 0;
		dent = readdir(dirp);
		if (!dent) {
			if (errno == 0)
				break;
			return -errno;
		}

		if (fstatat(dirfd,
		            dent->d_name,
		            &stbuf,
		            AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW) < 0)
			return -errno;

		if (filler(buf, dent->d_name, &stbuf, 0, FUSE_FILL_DIR_DEFAULTS) == 1) {
			return -errno;
		}
	}

	return 0;
}

static int condviewfs_readlink(const char *name, char *buf, size_t size) {
	int dirfd = get_fd();
	if (dirfd == -1)
		return -ENOLINK;

	ssize_t len;

	len = readlinkat(dirfd,
	                 &name[1],
	                 buf,
	                 size - 1);
	if (len < 0)
		return -errno;

	buf[len] = '\0';
	return 0;
}

static struct fuse_operations condviewfs_oper = {
	.open		= condviewfs_open,
	.release	= condviewfs_close,

	.read		= condviewfs_read,

	.getattr	= condviewfs_getattr,
	.access		= condviewfs_access,

	.opendir	= condviewfs_opendir,
	.releasedir	= condviewfs_closedir,
	.readdir	= condviewfs_readdir,

	.readlink	= condviewfs_readlink,
};

int main(int argc, char *argv[]) {
	char *fuse_argv[5];

	if (argc != 5) {
		fprintf(stderr, "Usage: %s target dir1 dir2 prog\n", argv[0]);
		exit(-1);
	}

	fd1 = open(argv[2], O_DIRECTORY);
	if (fd1 < 0) {
		perror("open 1");
		exit(-1);
	}

	fd2 = open(argv[3], O_DIRECTORY);
	if (fd2 < 0) {
		perror("open 2");
		exit(-1);
	}

	progname = argv[4];

	fuse_argv[0] = argv[0];
	fuse_argv[1] = argv[1];
	fuse_argv[2] = "-oallow_other,default_permissions";
	fuse_argv[3] = "-f";
	fuse_argv[4] = NULL;
	return fuse_main(4, fuse_argv, &condviewfs_oper, NULL);
}