caesar.cpp

BigC++的源码

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

using namespace std;

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

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

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

/**
   Encrypts a character using the Caesar cipher.
   @param ch the character to encrypt
   @param k the encryption key
   @return the encrypted character
*/
char encrypt(char ch, int k)
{  
   const int NLETTER = 'Z' - 'A' + 1;
   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;
}

/**
   Encrypts a stream using the Caesar cipher.
   @param in the stream to read from
   @param out the stream to write to
   @param k the encryption key
*/
void encrypt_file(istream& in, ostream& out, int k)
{  
   char ch;
   while (in.get(ch))
      out.put(encrypt(ch, k));
}

/** 
   Converts a string to an integer, e.g. "3" -> 3.
   @param s a string representing an integer
   @return the equivalent integer
*/   
int string_to_int(string s)
{  
   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 || argc > 5) usage(string(argv[0]));

   int i;
   for (i = 1; i < argc; i++)
   {  
      string arg = string(argv[i]);
      if (arg.length() >= 2 && 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;
}