crc_rev.cpp

#include <bits/stdc++.h>
using namespace std;
#define FOR(i, n) for (int i = 0; i < n; i++)

unsigned int    message  = 0 ;
unsigned int    POLYNOMIAL =  0 ;
unsigned int    rem = 0 ;

unsigned int CRC(  )
{
     rem = message;

      for ( int  bit = 31; bit > 0; --bit)
        {

               if (rem & (1<<31) ) // checking the left most bit
                  rem ^= POLYNOMIAL; // xor the remainder with the  polynomial

              rem = (rem << 1); // left shift at each iteration
         }


    return (rem >> 4); // only last four bits to be returned ;

}

int main (  )
  {
     string s ;
     cout<<"Enter the received string" <<  endl ;
     cin >> s ;
     int n = s.length( ) ;
     vector<int> A( n ) ;

     for( int  i = n - 1 ; i >= 0 ; i -- )
                 if(  s[i] == '1' )
                message+=(1<<(n-1-i)) ;

     cin >> s ;
     n =  s.length() ;

          for( int  i = n - 1 ; i >= 0 ; i -- )
                 if(  s[i] == '1' )
                POLYNOMIAL+=(1<<(n-1-i)) ;

       cout<< CRC( ) << endl ;



  }