#define _GNU_SOURCE
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

// You may assume that lines are no longer than 1024 bytes
#define LINELEN 1024

static void usage (void) __attribute__ ((unused));

int
main (int argc, char *argv[])
{
  int opt;
  int n = 5;

  while ((opt = getopt (argc, argv, "n:")) != -1)
    {
      switch (opt)
        {
        case 'n':
          n = strtol (optarg, NULL, 10);
          break;
        default:
          printf ("./bin/head: invalid option -- \'%c\'\n", optopt);
          exit (EXIT_FAILURE);
        }
    }

  FILE *file = optind < argc ? fopen (argv[optind], "r") : stdin;
  if (file == NULL)
    {
      perror ("Failed to open input file");
      exit (EXIT_FAILURE);
    }

  char buffer[LINELEN];
  int count = 0;

  while (count < n && fgets (buffer, sizeof (buffer), file) != NULL)
    {
      printf ("%s", buffer);
      count++;
    }

  return EXIT_SUCCESS;
}

static void
usage (void)
{
  printf ("head, prints the first few lines of a file\n");
  printf ("usage: head [FLAG] FILE\n");
  printf ("FLAG can be:\n");
  printf ("  -n N     show the first N lines (default 5)\n");
  printf ("If no FILE specified, read from STDIN\n");
}