#define _GNU_SOURCE

#include <limits.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <errno.h>
#include <error.h>
#include <fcntl.h>
#include <poll.h>
#include <sysexits.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/statfs.h>
#include <sys/types.h>


static inline size_t splice_len(off_t len) {
	return (size_t) (len > INT_MAX ? INT_MAX : len);
}

int main(int argc, char *argv[]) {
	bool defer = false, keep = false;
	if (argc > 1 && strcmp(argv[1], "--defer") == 0) {
		defer = true;
		argv[1] = argv[0];
		--argc, ++argv;
	}
	if (argc > 1 && strcmp(argv[1], "--keep") == 0) {
		keep = true;
		argv[1] = argv[0];
		--argc, ++argv;
	}
	if (argc > 2 || keep && argc < 2) {
		fprintf(stderr, "usage: %s [--defer] { [<tmpdir>] | --keep <file> }\n", argv[0]);
		return EX_USAGE;
	}
	const char *path;
	int fd;
	if (keep) {
		if ((fd = open(argv[1], O_RDWR | O_CREAT | O_EXCL, 0666)) < 0) {
			error(EX_NOINPUT, errno, "%s", argv[1]);
		}
	}
	else {
		if (argc > 1) {
			path = argv[1];
		}
		else if (!(path = getenv("TMPDIR"))) {
			path = "/tmp";
		}
		if ((fd = open(path, O_RDWR | O_TMPFILE | O_EXCL, 0000)) < 0) {
			error(EX_NOINPUT, errno, "%s", path);
		}
	}

	bool eof = false, punch = !keep;
	off_t n_in = 0, n_out = 0, max = 0, mask = 0;
	for (;;) {
		off_t n_buf = n_in - n_out;
		if (n_buf == max) {
			struct statfs f;
			if (fstatfs(fd, &f) < 0) {
				error(EX_NOINPUT, errno, "%s", path);
			}
			max = f.f_bavail / 2 * f.f_bsize;
			mask = ~((off_t) f.f_bsize - 1);
		}
		struct pollfd pfds[2] = { };
		nfds_t nfds = 0;
		if (!eof && n_buf < max) {
			pfds[nfds].fd = STDIN_FILENO;
			pfds[nfds].events = POLLIN;
			++nfds;
		}
		if (n_buf > 0 && (!defer || eof)) {
			pfds[nfds].fd = STDOUT_FILENO;
			pfds[nfds].events = POLLOUT;
			++nfds;
		}
		if (nfds == 0) {
			if (eof) {
				break;
			}
			sleep(1);
			continue;
		}
		if (poll(pfds, nfds, -1) > 0) {
			for (nfds_t i = 0; i < nfds; ++i) {
				if (pfds[i].revents) {
					if (pfds[i].fd == STDIN_FILENO) {
						ssize_t n = splice(STDIN_FILENO, NULL, fd, &n_in, splice_len(max - n_buf), SPLICE_F_MOVE | SPLICE_F_NONBLOCK | SPLICE_F_MORE);
						if (n <= 0) {
							if (n == 0) {
								eof = true;
							}
							else if (errno != EAGAIN) {
								error(EX_IOERR, errno, "%s: splice", "<stdin>");
							}
						}
						else {
							n_buf += n;
						}
					}
					else if (pfds[i].fd == STDOUT_FILENO) {
						ssize_t n = splice(fd, NULL, STDOUT_FILENO, NULL, splice_len(n_buf), SPLICE_F_MOVE | SPLICE_F_NONBLOCK | SPLICE_F_MORE);
						if (n <= 0) {
							if (n < 0 && errno != EAGAIN) {
								error(EX_IOERR, errno, "%s: splice", "<stdout>");
							}
						}
						else if (((n_out += n) & mask) && punch && fallocate(fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, 0, n_out & mask) < 0) {
							if (errno != EOPNOTSUPP && errno != ENOSYS) {
								error(EX_IOERR, errno, "%s: fallocate", path);
							}
							fprintf(stderr, "WARNING: temp file in %s does not support FALLOC_FL_PUNCH_HOLE\n", path);
							punch = false;
						}
					}
				}
			}
		}
		else if (errno != EINTR) {
			error(EX_OSERR, errno, "poll");
		}
	}
	return EX_OK;
}