#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <string.h>
#include <sys/time.h>

int main(int argc, char* argv[]);
void print_time();

void print_time() {
  struct timeval tv;
  struct timezone tz;
  gettimeofday(&tv, &tz);
  printf("%d\n", tv.tv_usec);
}

int main(int argc, char* argv[]) {
  const char* verbose_flag = "-v";
  int verbose = 0;
  int arg_str_index = 0;

  if (argc == 1) {
    verbose = 0;
    arg_str_index = 0;
  }
  else if (argc == 2) {
    if (strcmp(argv[1], verbose_flag) == 0) {
      verbose = 1;
      arg_str_index = 0;
    }
    else {
      verbose = 0;
      arg_str_index = 1;
    }
  }
  else if (argc == 3 && (strcmp(argv[1], verbose_flag) == 0)) {
    verbose = 1;
    arg_str_index = 2;
  }
  else {
    fprintf(stderr, " Usage: %s [%s] [msg_word]\n", argv[0], verbose_flag);
    exit(EXIT_FAILURE);
  }

  // setup one-way pipe as a file descriptor:
  // index 0 is read, index 1 is write
  enum { READ_END, WRITE_END };
  int fd[2];
  int p = pipe(fd);
  if (p == -1) {
    perror("Pipe not created.");
    exit(EXIT_FAILURE);
  }

  // create a child process
  // fork returns twice:
  //   1. in the parent (return value the child's PID)
  //   2. in the child  (return value 0)
  pid_t fork_ret = fork();
  if (fork_ret == -1) {
    perror("Couldn't fork.");
    exit(EXIT_FAILURE);
  }
  else if (fork_ret == 0) { /* THE CHILD */
    if (verbose) { printf(" This is the child.\n"); }
    close(fd[WRITE_END]); // child won't write anyone anyway

    // read from the pipe's read end
    // one char at a time into c;
    // for each read, write c to stdout
    // finish with newline
    char c;
    if (verbose) {
      while (read(fd[READ_END], &c, 1) > 0) { write(STDOUT_FILENO, &c, 1); }
      write(STDOUT_FILENO, "\n", 1);
    }
    else { while (read(fd[READ_END], &c, 1) > 0) ; }
    close(fd[READ_END]); // child done reading
    print_time();
    exit(EXIT_SUCCESS);
  }
  else { /* THE PARENT */
    if (verbose) {
      printf(" Parent speaking: child has PID %d.\n", fork_ret);
    }
    close(fd[READ_END]); // parent won't read anything

    // write the first argument passed at invocation
    // or, if none, write a simple greeting
    char* s = (arg_str_index ? argv[arg_str_index] : "Message: Hi");
    print_time();
    write(fd[WRITE_END], s, strlen(s));
    close(fd[WRITE_END]); // parent done writing

    wait(NULL); // parent will wait for the child to terminate
    exit(EXIT_SUCCESS);
  }
  return 0;
}