#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int usage(char *argv0) {
  printf("Usage: %s [-n number] [file ...]\n", argv0);
  return 2;
}

int main(int argc, char *argv[]) {
  char ch;
  int opt;
  FILE *file = stdin;

  int number = 10;
  int line = 0;

  bool multiple_files = false;

  while ((opt = getopt(argc, argv, "n:")) != -1) {
    switch (opt) {
      case 'n':
        if (optarg[0] == '-') {
          optarg++;
        }
        for (size_t i = 0; i < strlen(optarg); i++) {
          if (!isdigit(optarg[i])) {
            fprintf(stderr, "%s: invalid number of lines: '%s'\n", argv[0], optarg);
            return 1;
          }
        }
        number = atoi(optarg);
        break;
      case '?':
        fprintf(stderr, "%s: invalid option -- '%c'\n", argv[0], optopt);
        return usage(argv[0]);
    }
  }

  if (optind == argc) { // FIXME: this code is not clean
    goto READ_STDIN;
  }

  if (argc - optind > 1) {
    multiple_files = true;
  }

  for (; optind < argc; optind++) {
    if (strcmp(argv[optind], "-")) {
      file = fopen(argv[optind], "r");
      if (file == NULL) {
        fprintf(stderr, "%s: ", argv[0]);
        perror(argv[optind]);
        return -1;
      }
      if (multiple_files) {
        printf("%s==> %s <==\n", (optind - 1 > argc - optind ? "\n" : ""), argv[optind]);
      }
    } else {
      file = stdin;
      if (multiple_files) {
        printf("%s==> %s <==\n", (optind - 1 > argc - optind ? "\n" : ""), "standard input");
      }
    }
READ_STDIN:
    line = 0;
    while ((ch = fgetc(file)) != EOF && line < number) {
      putchar(ch);
      if (ch == '\n') {
        line++;
      }
    }
    if (file != NULL && file != stdin) {
      fclose(file);
    }
  }
  return 0;
}