#include <errno.h>
#include <poll.h>
#include <signal.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

#include "proxy.h"

#define BUFFER_SIZE  (1 << 10)

struct list {
	struct list *next, *prev;
};

struct packet {
	struct list        list;
	unsigned long long recvtime;
	size_t             length;
	char               data[BUFFER_SIZE];
};

static unsigned long long gettime()
{
	struct timespec time;
	clock_gettime(CLOCK_MONOTONIC, &time);
	return time.tv_sec * 1000000000ULL + time.tv_nsec;
}

static int read_packet(int fd, struct list *tail, unsigned long long now)
{
	struct packet *p = malloc(sizeof *p);
	if (!p) return -1;

	ssize_t len;
	while ((len = read(fd, p->data, BUFFER_SIZE)) == -1 && errno == EINTR) ;
	if (len <= 0) {
		free(p);
		return -1;
	}

	p->list.next = tail;
	p->list.prev = tail->prev;
	p->recvtime  = now;
	p->length    = len;
	p->list.next->prev = p->list.prev->next = &p->list;
	return 0;
}

static int write_packet(int fd, struct packet *p)
{
	char  *data = p->data;
	size_t len  = p->length;

	ssize_t r;
	while (len && (r = write(fd, data, len)) != -1 && errno != EINTR) {
		data += r;
		len  -= r;
	}

	if (r == -1) return -1;
	struct list *l = &p->list;
	l->prev->next = l->next;
	l->next->prev = l->prev;
	free(p);
	return 0;
}

static void proxy_one(int readfd, int writefd, unsigned long long delayns)
{
	struct list head = {0}, tail = {0};
	head.next = &tail;
	tail.prev = &head;

	struct pollfd fds = { .fd = readfd, .events = POLLIN };
	int timeout = -1;
	for (;;) {
		while (poll(&fds, 1, timeout) == -1 && errno == EINTR) ;
		if (fds.revents & (POLLERR | POLLHUP)) return;

		unsigned long long now      = gettime();
		unsigned long long sendtime = now - delayns;
		if (fds.revents & POLLIN && read_packet(readfd, &tail, now)) return;

		while (head.next != &tail) {
			struct packet *p = (struct packet *)head.next;
			timeout = (p->recvtime - sendtime) / 1000000;
			if (timeout <= 0) {
				if (write_packet(writefd, p)) return;
			} else break;
		}
		if (head.next == &tail) timeout = -1;
	}
}

static pid_t make_child(int fd0, int fd1, unsigned long long delayns)
{
	pid_t child = fork();
	if (!child) {
		proxy_one(fd0, fd1, delayns);
		exit(0);
	}
	return child;
}

static pid_t wait_one(void)
{
	pid_t p;
	while ((p = wait(NULL)) == -1 && errno == EINTR) ;
	return p;
}

static void kill_wait(pid_t p)
{
	kill(p, SIGINT);
	wait_one();
}

void proxy(int fd0, int fd1, unsigned delayms)
{
	unsigned long long delayns = delayms * 1000000ULL;
	pid_t pid0, pid1;

	if ((pid0 = make_child(fd0, fd1, delayns)) < 0) return;
	if ((pid1 = make_child(fd1, fd0, delayns)) < 0) {
		kill_wait(pid0);
		return;
	}

	pid_t dead = wait_one();
	kill_wait(dead == pid0 ? pid1 : pid0);
}