#include <err.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/un.h>
#include <unistd.h>

void run_mandoc(int sockfd) {
  char sockfdstr[10];
  if (snprintf(sockfdstr, sizeof(sockfdstr), "%d", sockfd) == -1)
    err(EXIT_FAILURE, "snprintf");
  if (execlp("mandoc", "mandoc", "-Thtml", "-u", sockfdstr, NULL) == -1)
    err(EXIT_FAILURE, "execlp(mandoc)");
}

ssize_t sock_fd_write(int fd, int fd0, int fd1, int fd2) {
  struct msghdr msg;
  struct iovec iov;
  union {
    struct cmsghdr cmsghdr;
    char control[CMSG_SPACE(3 * sizeof(int))];
  } cmsgu;
  struct cmsghdr *cmsg;
  unsigned char dummy[1] = {'\0'};

  iov.iov_base = dummy;
  iov.iov_len = sizeof(dummy);

  msg.msg_name = NULL;
  msg.msg_namelen = 0;
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;

  msg.msg_control = cmsgu.control;
  msg.msg_controllen = sizeof(cmsgu.control);

  cmsg = CMSG_FIRSTHDR(&msg);
  cmsg->cmsg_len = CMSG_LEN(3 * sizeof(int));
  cmsg->cmsg_level = SOL_SOCKET;
  cmsg->cmsg_type = SCM_RIGHTS;

  int *walk = (int *)CMSG_DATA(cmsg);
  *(walk++) = fd0;
  *(walk++) = fd1;
  *(walk++) = fd2;

  return sendmsg(fd, &msg, 0);
}

int process_manpage(struct sockaddr *addr) {
  int fd;

  if ((fd = socket(AF_LOCAL, SOCK_STREAM, 0)) < 0)
    return -1;

  if (connect(fd, addr, sizeof(struct sockaddr_un)) < 0)
    return -1;

  int pstdin[2];
  int pstdout[2];
  int pstderr[2];
  if (pipe(pstdin) == -1 || pipe(pstdout) == -1 || pipe(pstderr) == -1)
    return -1;

  if (sock_fd_write(fd, pstdin[0], pstdout[1], pstderr[1]) < 0)
    return -1;

  close(pstdin[0]);
  close(pstdout[1]);
  close(pstderr[1]);

  const char *man = ".TH example code\n";
  if (write(pstdin[1], man, strlen(man)) < 0)
    return -1;
  close(pstdin[1]);
  char buf[4096];
  ssize_t n = read(pstdout[0], buf, sizeof(buf));
  if (n < 0)
    return -1;
  printf("read: \"%.*s\"\n", (int)n, buf);
  return 0;
}

int setup_socket(struct sockaddr *addr) {
  int sockfd;

  if ((sockfd = socket(AF_LOCAL, SOCK_STREAM, 0)) < 0)
    return -1;

  if (bind(sockfd, addr, sizeof(struct sockaddr_un)) < 0)
    return -1;

  if (listen(sockfd, 1) < 0)
    return -1;

  return sockfd;
}

int main(int argc, char **argv) {
  struct sockaddr_un addr;
  memset(&addr, 0, sizeof(struct sockaddr_un));
  addr.sun_family = AF_LOCAL;

  int sockfd = setup_socket((struct sockaddr *)&addr);
  if (sockfd == -1)
    err(EXIT_FAILURE, "socket setup");

  pid_t pid = fork();
  switch (pid) {
  case -1:
    err(EXIT_FAILURE, "fork");
  case 0:
    run_mandoc(sockfd);
    break;
  default:
    if (process_manpage((struct sockaddr *)&addr) == -1)
      err(EXIT_FAILURE, "process_manpage");
    if (kill(pid, SIGKILL) == -1)
      err(EXIT_FAILURE, "kill(%d)", pid);
    break;
  }
}