/******
 * bit_ops.c
 *
 * sample bit operations in C
 *
 * In all this I'm passing in characters explicitly, and returning characters
 * where appropriate so that these routines have no side effects.  If you'd
 * rather directly on a bit in a larger buffer, make the inputs pointers
 * (i.e. "char* c") instead.

 * $ make bit_ops
 * gcc -march=athlon64 -O2    bit_ops.c   -o bit_ops
 * $ ./bit_ops
 *  'a' is ascii '97' ('141' octal) which has '3' 1's in binary. 
 *  'a' base 2 is '01100001'
 *  flipped bit 3 : 01101001
 *  flipped bit 3 again : 01100001
 *  set bit 2 to 1 : 01100101
 *  set bit 2 to 1 again : 01100101
 *  set bit 2 to 0 : 01100001
 *  set bit 2 to 0 again : 01100001
 * 
 * Jim M, April 4 2007
 *******/

#include <stdio.h>

typedef int bool;  // declare boolean type (for convenience)

// Return the number of 1's in the binary representation of c.
int bitcount(char c){
  int count;
  for (count=0; c != 0; c>>=1){
    if (c & 1){
      count++;
    }
  }
  return count;
}

// Return true (i.e. 1) if the n'th bit (0 is rightmost) of c is 1.
bool bittest(char c, int n){
  return ((1<<n & c) != 0);
}

// Return a copy of a char with the n'th bit flipped.
char bitflip(char c, int n){
  // First, 1<<n is 00000001 shifted left n times, e.g. 00000100 for n=2 .
  // Second, ^ is the XOR operator.
  return ((1<<n) ^ c);
}

// Return a copy of a char with the n'th bit set to a given value (0 or 1).
char bitset(char c, int n, int value){
  if (value){
    return ((1<<n) | c);         // e.g. (00000100 OR c) for n=2
  }
  else {
    return ( (~(1<<n)) & c);     // e.g. (11111011 AND c) for n=2
  }
}

// Print a base2 representation of a character.
void print_base2(char c){
  int i;
  for (i=7; i>=0; i--){
    printf("%1i", bittest(c,i));
  }
}

// -------------------------------------------------------

int main(){
  char a = 'a';

  printf(" '%c' is ascii '%i' ('%o' octal) which has '%i' 1's in binary. \n", 
	 a, (int)a, (int)a, bitcount(a));

  printf(" '%c' base 2 is '", a);
  print_base2(a);
  printf("'\n");

  a = bitflip(a, 3);
  printf(" flipped bit 3 : ");
  print_base2(a);
  printf("\n");

  a = bitflip(a, 3);
  printf(" flipped bit 3 again : ");
  print_base2(a);
  printf("\n");
  
  a = bitset(a, 2, 1);
  printf(" set bit 2 to 1 : ");  
  print_base2(a);
  printf("\n");

  a = bitset(a, 2, 1);
  printf(" set bit 2 to 1 again : ");  
  print_base2(a);
  printf("\n");

  a = bitset(a, 2, 0);
  printf(" set bit 2 to 0 : ");  
  print_base2(a);
  printf("\n");

  a = bitset(a, 2, 0);
  printf(" set bit 2 to 0 again : ");  
  print_base2(a);
  printf("\n");


}