/****************************************************************************
** COPYRIGHT (C):    1996 Cay S. Horstmann. All Rights Reserved.
** PROJECT:          Computing Concepts with C++
** FILE:             caesar.cpp
** NOTE TO STUDENTS: This file has been edited to be compatible with older
** compilers. If your compiler fully supports the ANSI/ISO C++
** standard, remove the line #include "ccc_ansi.cpp". You can also remove
** the lines #ifndef CCC_ANSI_H and #endif
****************************************************************************/

   #include "ccc_ansi.cpp"
   #ifndef CCC_ANSI_H

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>

using namespace std;

   #endif

void usage(string program_name)
/* PURPOSE:  Prints usage instructions
   RECEIVES: program_name - the name of this program
*/
{  cout << "Usage: " << program_name
      << " [-d] [-kn] infile outfile\n";
   exit(1);
}

void open_file_error(string filename)
/* PURPOSE:  Prints file opening error message
   RECEIVES: filename - the name of the file that could not be opened
*/
{  cout << "Error opening file " << filename << "\n";
   exit(1);
}

int remainder(int a, int n)
/* PURPOSE:  Compute correct remainder for negative dividend
   RECEIVES: a - an integer
             n - an integer > 0
   RETURNS:  the mathematically correct remainder r such that
             a - r is divisible by n and 0 <= r and r < n
*/
{  if (a >= 0)
      return a % n;
   else
      return n - 1 - (-a - 1) % n;
}

char encrypt(char ch, int k)
/* PURPOSE:  Encrypt a character using the Caesar cipher
   RECEIVES: ch - the character to encrypt
             k - the encryption key
   RETURNS:  the encrypted character
*/
{  const int NLETTER = 26;
   if ('A' <= ch && ch <= 'Z')
      return static_cast<char>('A' + remainder(ch - 'A' + k, NLETTER));
   if ('a' <= ch && ch <= 'z')
      return static_cast<char>('a' + remainder(ch - 'a' + k, NLETTER));
   return ch;
}

void encrypt_file(ifstream& in, ofstream& out, int k)
/* PURPOSE:  Encrypt a file using the Caesar cipher
   RECEIVES: in - the file to read from
             out- the file to write to
             k - the encryption key
*/
{  char ch;
   while (in.get(ch))
      out.put(encrypt(ch, k));
}

int string_to_int(string s)
/* PURPOSE:  Convert a string to an integer, e.g. "3" -> 3
   RECEIVES: s - a string representing an integer
   RETURNS:  the equivalent integer
*/   
{  istringstream instr(s);
   int n;
   instr >> n;
   return n;
}

int main(int argc, char* argv[])
{  bool decrypt = false;
   int key = 3;
   int nfile = 0; /* the number of files specified */
   ifstream infile;
   ofstream outfile;

   if (argc < 3 or argc > 5) usage(string(argv[0]));

   int i;
   for (i = 1; i < argc; i++)
   {  string arg = string(argv[i]);
      if (arg.length() >= 2 and arg[0] == '-')
      /* it is a command line option */
      {  char option = arg[1];
         if (option == 'd')
         decrypt = true;
         else if (option == 'k')
         key = string_to_int(arg.substr(2, arg.length() - 2));
      }
      else
      {  nfile++;
         if (nfile == 1)
         {  infile.open(arg.c_str());
            if (infile.fail()) open_file_error(arg);
         }
         else if (nfile == 2)
         {  outfile.open(arg.c_str());
            if (outfile.fail()) open_file_error(arg);
         }
      }
   }

   if(nfile != 2) usage(string(argv[0]));

   if (decrypt) key = -key;

   encrypt_file(infile, outfile, key);
   infile.close();
   outfile.close();
   return 0;
}