encode.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int index_of(char* needle, char* haystack[], int length) {
  int i;

  for (i = 0; i < length; i++) {
    if (strcmp(haystack[i], needle) == 0) {
      return i;
    }
  }

  return -1;
}

int decrypt(int argc, char* argv[]) {
  if (index_of("-d", argv, argc) > -1) {
    return 1; 
  }

  return 0;
}

int offset(int argc, char* argv[]) {
  int delta = 1;
  int flag  = index_of("-n", argv, argc);

  if (flag > -1) {
    delta = atoi(argv[flag + 1]);
  }

  if (decrypt(argc, argv)) {
    delta *= -1; 
  }

  return delta;
}

FILE* input(int argc, char* argv[]) {
  FILE* file = stdin;
  int flag   = index_of("-f", argv, argc);

  if (flag > -1) {
    file = fopen(argv[flag + 1], "r");

    if (!file) {
      printf("Invalid filename.");
      exit(1);
    }
  }

  return file;
}

void encode(FILE* data, int delta) {
  char c;

  setbuf(stdout, NULL); // print immediately

  while ((c = fgetc(data)) != EOF) {
    printf("%c", c + delta);
  }
}

int main(int argc, char* argv[]) {
  FILE* data = input(argc, argv);
  encode(data, offset(argc, argv));
  fclose(data);
  return 0;
}