// !> A pipe between two programs [fv]
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <wait.h>

#include <stdio.h>

#include <assert.h>

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

  /* Open files after program starts:
   * 0: stdin
   * 1: stdout
   * 2: stderr
   */

  int pipe_fd[2];

  // create a pipe:
  // pipe_fd[0] is the _read_ end
  // pipe_fd[1] is the _write_ end 
  assert(pipe(pipe_fd) != -1);
  /* Open files after pipe call:
   * 0: stdin
   * 1: stdout
   * 2: stderr
   * 3: pipe read end (pipe_fd[0])
   * 4: pipe write end (pipe_fd[1])
   */

  int write_fd = pipe_fd[1];
  int read_fd = pipe_fd[0];


  printf("read: %d, write: %d\n", read_fd, write_fd);

  pid_t reading_child = fork();
  assert(reading_child != -1);

  if (reading_child == 0) {
    /* Open files in reading_child:
     * 0: stdin
     * 1: stdout
     * 2: stderr
     * 3: pipe read end (pipe_fd[0] / read_fd)
     * 4: pipe write end (pipe_fd[1] / write_fd)
     */

    // both parent and child share both ends, so we'll just close the end we 
    // don't use
    close(pipe_fd[1]);
    /* Open files after closing write end:
     * 0: stdin
     * 1: stdout
     * 2: stderr
     * 3: pipe read end (pipe_fd[0] / read_fd)
     * 4: -
     */

    // close stdin
    close(0);
    /* Open files after closing stdin:
     * 0: -
     * 1: stdout
     * 2: stderr
     * 3: pipe read end (pipe_fd[0] / read_fd)
     * 4: -
     */


    // replace fd 0 with pipe_fd[0] / read_fd
    assert(dup(read_fd) == 0);
    /* Open files after dup:
     * 0: pipe read end (pipe_fd[0] / read_fd)
     * 1: stdout
     * 2: stderr
     * 3: pipe read end (pipe_fd[0] / read_fd)
     * 4: -
     */
    printf("About to run sort...\n");
    assert(-1 != execlp("sort", "sort", NULL));
    // should never get here
    return 1;
  }

  int writing_child = fork();
  assert(writing_child != -1);

  if (writing_child == 0) {
    /* Open files in writing_child:
     * 0: stdin
     * 1: stdout
     * 2: stderr
     * 3: pipe read end (pipe_fd[0] / read_fd)
     * 4: pipe write end (pipe_fd[1] / write_fd)
     */

    close(read_fd);    
    /* Open files after closing read end:
     * 0: stdin
     * 1: stdout
     * 2: stderr
     * 3: - 
     * 4: pipe write end (pipe_fd[1] / write_fd)
     */

    // close stdout
    close(1);
    /* Open files after closing stdout:
     * 0: stdin
     * 1: -
     * 2: stderr
     * 3: - 
     * 4: pipe write end (pipe_fd[1] / write_fd)
     */


    // replace stsdout with write end of pipe
    assert(dup(write_fd) == 1);
    /* Open files after dup:
     * 0: stdin
     * 1: pipe write end (pipe_fd[1] / write_fd)
     * 2: stderr
     * 3: - 
     * 4: pipe write end (pipe_fd[1] / write_fd)
     */

    // run: shuf -i 1-9
    assert(-1 != execlp("shuf", "shuf", "-i", "1-9", NULL));
    // should never get here
    close(pipe_fd[1]);
    close(1);
    return 1;
  }

  /* Open files in parent:
   * 0: stdin
   * 1: stdout
   * 2: stderr
   * 3: pipe read end (pipe_fd[0] / read_fd)
   * 4: pipe write end (pipe_fd[1] / write_fd)
   */
  close(write_fd);
  close(read_fd);
  /* Open files in parent after closing both ends of the pipe:
   * 0: stdin
   * 1: stdout
   * 2: stderr
   * 3: - 
   * 4: - 
   */

  // Wait for both children to finish
  wait(NULL);
  wait(NULL);

  return 0;
}