// this file contains some functions that do bitwise operation
//  X. Meng  9/91
// bitwise operators: 
// << shift left
// >> shift right
// &  bitwise AND
// |  bitwise OR
// ^  bitwise XOR  (exclusive OR)
// ~  complement   (invert all bits)

#include <iostream.h>
#include <limits.h>

#define BITSinCHAR 8
#define LOWPOS     1     // the lowest position can be 1 or 0

unsigned short setbit(unsigned short m, int pos, int value);
unsigned short getbit(unsigned short m, int pos);
main()
{
   unsigned short x;
   int position, value;  // which bit? what value (0,1)?

   cout << "input value of x \n";
   cin >> x;
   cout << "which position \n";
   cin >> position;
   cout << "what value \n";
   cin >> value;

   x = setbit(x, position, value);

   cout << "value of x now is " << x << '\n';

   cout << "input value of x \n";
   cin >> x;
   cout << "which position \n";
   cin >> position;

   x = getbit(x, position);

   cout << "value of bit at position " << position << " is " << x << '\n';
   cout << "value of negation " << ~x << endl;
}

unsigned short setbit(unsigned short m, int pos, int value)
// set 'pos' bit of 'm' to be 'value', either 0 or 1
// 'pos' being from right to left, i.e. 87654321
{
     unsigned short mask;

     if (pos >= (sizeof(unsigned short)*BITSinCHAR)+LOWPOS || pos < LOWPOS)
     {
       cerr << " position out of range \n";
       exit(1);
     }
     if (value == 1)       // set mask to be, for example 000000100000
     {
       mask = 1;
       mask = mask << (pos - LOWPOS);
       return (m | mask);
     }
     if (value == 0)       // set mask to be, for example 111111011111
     {
       mask = 1;
       mask = ~(mask << pos+1-LOWPOS);
       return (m & mask);
     }
     else
       return 0;
}

unsigned short getbit(unsigned short m, int pos)
// getbit returns the value of bit 'pos' in number 'm'
{
    unsigned short mask;

    mask = 0;                     // all 0's
    mask = setbit(mask,pos,1);    // set bit 'pos' to be one
    m = m & mask;                 // only bit 'pos' keeps original value
    m = m >> (pos-LOWPOS);
    return (m);
}