/* --------------------------------------------------------------------- 
** Figure 18.16:  Decrypting a number.
** --------------------------------------------------------------------- 
*/
#include "tools.h"              

#define AD  0x0E00       /* bits 4:6   goto 0:2   */ 
#define BD  0x001F       /* bits 11:15 goto 3:7   */ 
#define CD  0xF000       /* bits 0:3   goto 8:11  */ 
#define DD  0x01E0       /* bits 7:10  goto 12:15 */ 

unsigned short decrypt( unsigned short n );     

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

    printf( "Enter an encrypted short int: " );     
    scanf( "%hu", &in ); 
                        
    dcrypt = (signed) decrypt( in );                        
      
    printf( "\n The encrypted input number in base 10 is: %hu \n"       
               "Encrypted input number in hexadecimal is: %hx \n\n" 
               "The decrypted number in base 10 is:       %hi \n"           
               "The decrypted number in base 16 is:       %hx \n\n",    
               in, in, dcrypt, dcrypt );                            
} 
                                                                 
/* ----------------------------------------------------------------------- */
unsigned short                        /* use unsigned for bit manipulation */                     
decrypt( unsigned short n )
{   
    unsigned short a, b, c, d;        /* for the four parts of n */  
    
    a = (n & AD) << 4;     /* Isolate bits 4:6, shift to positions 0:2.    */    
    b = (n & BD) << 8;     /* Isolate bits 11:15, shift to positions 3:7.  */ 
    c = (n & CD) >> 8;     /* Isolate bits 0:3 shift to positions 8:11,.   */ 
    d = (n & DD) >> 5;     /* Isolate bits 7:10, shift to positions 12:15. */
    return a | b | c | d;            /* Pack the four parts back together. */
}