/* --------------------------------------------------------------------- 
**  Figure 18.16:  Encrypting a number.
** --------------------------------------------------------------------- 
*/
#include "tools.h"   
           
#define AE  0xE000       /* bits 0:2   end up as 4:6   */ 
#define BE  0x1F00       /* bits 3:7   end up as 11:15 */ 
#define CE  0x00F0       /* bits 8:11  end up as 0:3   */ 
#define DE  0x000F       /* bits 12:15 end up as 7:10  */ 

unsigned short encrypt( unsigned short n );     

void main( void )                                      
{   
    short in;         
    unsigned short crypt;                           

    printf( "Enter a short number to encrypt: " );      
    scanf( "%hi", &in );                      
    
    /* Cast the int to unsigned before calling encrypt. */  
    crypt = encrypt( (unsigned short) in );                        
    
    printf( "\n The input number in base 10 is:     %hi \n"       
              " The input number in hexadecimal is: %hx \n\n" 
       
              " The encrypted number in base 10 is: %hu \n"           
              " The encrypted number in base 16 is: %hx \n\n",    
              in, in, crypt, crypt );                            
}                                                 
                 
/* --------------------------------------------------------------------- */
unsigned short                      /* use unsigned for bit manipulation */                     
encrypt( unsigned short n )
{
    unsigned short a, b, c, d;      /* for the four parts of n */  
    
    a = (n & AE) >> 4;       /* Isolate bits 0:2, shift to positions 4:6. */    
    b = (n & BE) >> 8;       /* Isolate bits 3:7, shift to positions 11:15. */ 
    c = (n & CE) << 8;       /* Isolate bits 8:11, shift to positions 0:3.  */ 
    d = (n & DE) << 5;       /* Isolate bits 12:15, shift to positions 7:10. */
    return c | a | d | b;    /* Pack the four parts back together. */
}