/****************************************************************************
** COPYRIGHT (C):    1996 Cay S. Horstmann. All Rights Reserved.
** PROJECT:          Computing Concepts with C++
** FILE:             caesar.cpp -- modified by JGM
                     data file:  crypt_in.txt  (input)
** UPDATED:          RJB Tue Apr 17 15:10:41 PDT 2001
** STATUS:           Compiles clean but untested
****************************************************************************/

#include <iostream>
#include <fstream>
#include <string>
#include <strstream>

using namespace std;


void usage(string program_name)
{  cout << "Usage: " << program_name
		<< " [-d] [-kn] infile outfile\n";
	exit(EXIT_FAILURE);
}

void open_file_error(string filename)
{  cout << "Error opening file " << filename << "\n";
	exit(EXIT_FAILURE);
}

int remainder( int a, int n)
{
	if( a >= 0)
		return a % n;
	else
		return n -1 - ( -a -1) %n;
}

char encrypt(char ch, int k)
{  const int NLETTER = 26; //leters in alphabet
   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(fstream& in, fstream& out, int k)
{  char ch;
   while( in.get(ch))
	out.put(encrypt(ch, k));
}

int intvalue(string s)
{	//p460 with a different name
	istrstream instr(s.c_str());
	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 */
	fstream infile;
	fstream outfile;

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

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

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

	if (decrypt) key = 26 - key;

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

/*   SAMPLE run
$more crypt_in.txt
The quick brown fox jumped over the lazy dog.
This input file will be encrypted.

a.out -k11 crypt_in.txt crypt_out.txt
$ more crypt_out.txt
Esp bftnv mczhy qzi ufxapo zgpc esp wlkj ozr.
Estd tyafe qtwp htww mp pyncjaepo.

$ a.out -d -k11 crypt_out.txt decrypt_out.txt
$ more decrypt_out.txt
The quick brown fox jumped over the lazy dog.
This input file will be encrypted.

*/