/* Implements a Caesar cipher for encrypting and decrypting a string 
   message (see ch. 4 in D'Orazio).  Includes the syntax for getline
   using string objects (note the difference with using getline with
   C strings).  Also note the integer arithmetic to implement the 
   cipher.
*/

#include<iostream.h>
#include<ctype.h>
#include<string>
#include<iomanip.h>

void get_data (string&,int&);
void cipher(string&,int);

void main(){

  string message;
  int offset;

  get_data(message, offset);
  cipher (message, offset);
  cout << "The encrypted message is:" << endl;
  cout << message << endl << endl;
  cipher (message, -offset);
  cout << "The deciphered message is:" << endl;
  cout << message << endl << endl;
}

void get_data(string& m, int& n){

  cout << "Enter several lines of text terminated by a #:" << endl;

  getline(cin,m,'#');
  /* reads in data until a '#' is found, instead of waiting for a '\n' */

  cin.ignore(2000,'\n');

  cout << "Enter an integer offset: ";
  cin >> n;
}

void cipher(string& m, int n){

  int len, i;

  n = ((n%26)+26)%26;  /* ensures n is nonnegative */

  len = m.length();

  for(i=0; i<len; i++){
    if (isupper(m[i]))
      m[i] = (m[i] - 'A' + n)%26 + 'A';
    else if (islower(m[i]))
      m[i] = (m[i] - 'a' + n)%26 + 'a';
  }
}