Thread: Swap a bit

FWIW, I found some code lying around (from the Swap Byte thread) that compared several methods.

Code:

#include <stdio.h>
#include <limits.h>
#include <time.h>

unsigned char foo(unsigned char value)
{
   unsigned char mask = 1 << (CHAR_BIT - 1), result = 0;
   while ( value ) /* skip most significant bits that are zero */
   {
      if ( value & 1 ) /* replace mod (machine dependency) */
      {
         result |= mask;
      }
      mask  >>= 1;
      value >>= 1;
   }
   return result;
}

unsigned char bar(unsigned char num)
{
   unsigned char byte = 0;
   int i = 0;
   do
   {
      if ( num % 2 )
      {
         byte = byte << 1;
         byte |= 0x01;
      }
      else
         byte = byte << 1;

      num = num >> 1;
      i++;
   } while ( i & 7 );
   return byte;
}

unsigned char baz(unsigned char value)
{
   union byteu
   {
      unsigned char byte;
      struct
      {
         unsigned int u0:1;
         unsigned int u1:1;
         unsigned int u2:1;
         unsigned int u3:1;
         unsigned int u4:1;
         unsigned int u5:1;
         unsigned int u6:1;
         unsigned int u7:1;
      } sbyte;
   } a, b;

   a.byte = value;

   b.sbyte.u7 = a.sbyte.u0;
   b.sbyte.u6 = a.sbyte.u1;
   b.sbyte.u5 = a.sbyte.u2;
   b.sbyte.u4 = a.sbyte.u3;
   b.sbyte.u3 = a.sbyte.u4;
   b.sbyte.u2 = a.sbyte.u5;
   b.sbyte.u1 = a.sbyte.u6;
   b.sbyte.u0 = a.sbyte.u7;

   return b.byte;
}

unsigned char qux(unsigned char value)
{
   unsigned char result = 0;
   if ( value & 0x80 ) result |= 0x01;
   if ( value & 0x40 ) result |= 0x02;
   if ( value & 0x20 ) result |= 0x04;
   if ( value & 0x10 ) result |= 0x08;
   if ( value & 0x08 ) result |= 0x10;
   if ( value & 0x04 ) result |= 0x20;
   if ( value & 0x02 ) result |= 0x40;
   if ( value & 0x01 ) result |= 0x80;
   return result;
}

unsigned char zot(unsigned char value)
{
   value = (value & 0x0f) <<  4  |  (value & 0xf0) >>  4;
   value = (value & 0x33) <<  2  |  (value & 0xcc) >>  2;
   value = (value & 0x55) <<  1  |  (value & 0xaa) >>  1;
   return value;
}

unsigned char fum(unsigned char value)
{
   static const unsigned char table[256] =
   {
      0x00,0x80,0x40,0xC0,0x20,0xA0,0x60,0xE0,
      0x10,0x90,0x50,0xD0,0x30,0xB0,0x70,0xF0,
      0x08,0x88,0x48,0xC8,0x28,0xA8,0x68,0xE8,
      0x18,0x98,0x58,0xD8,0x38,0xB8,0x78,0xF8,
      0x04,0x84,0x44,0xC4,0x24,0xA4,0x64,0xE4,
      0x14,0x94,0x54,0xD4,0x34,0xB4,0x74,0xF4,
      0x0C,0x8C,0x4C,0xCC,0x2C,0xAC,0x6C,0xEC,
      0x1C,0x9C,0x5C,0xDC,0x3C,0xBC,0x7C,0xFC,
      0x02,0x82,0x42,0xC2,0x22,0xA2,0x62,0xE2,
      0x12,0x92,0x52,0xD2,0x32,0xB2,0x72,0xF2,
      0x0A,0x8A,0x4A,0xCA,0x2A,0xAA,0x6A,0xEA,
      0x1A,0x9A,0x5A,0xDA,0x3A,0xBA,0x7A,0xFA,
      0x06,0x86,0x46,0xC6,0x26,0xA6,0x66,0xE6,
      0x16,0x96,0x56,0xD6,0x36,0xB6,0x76,0xF6,
      0x0E,0x8E,0x4E,0xCE,0x2E,0xAE,0x6E,0xEE,
      0x1E,0x9E,0x5E,0xDE,0x3E,0xBE,0x7E,0xFE,
      0x01,0x81,0x41,0xC1,0x21,0xA1,0x61,0xE1,
      0x11,0x91,0x51,0xD1,0x31,0xB1,0x71,0xF1,
      0x09,0x89,0x49,0xC9,0x29,0xA9,0x69,0xE9,
      0x19,0x99,0x59,0xD9,0x39,0xB9,0x79,0xF9,
      0x05,0x85,0x45,0xC5,0x25,0xA5,0x65,0xE5,
      0x15,0x95,0x55,0xD5,0x35,0xB5,0x75,0xF5,
      0x0D,0x8D,0x4D,0xCD,0x2D,0xAD,0x6D,0xED,
      0x1D,0x9D,0x5D,0xDD,0x3D,0xBD,0x7D,0xFD,
      0x03,0x83,0x43,0xC3,0x23,0xA3,0x63,0xE3,
      0x13,0x93,0x53,0xD3,0x33,0xB3,0x73,0xF3,
      0x0B,0x8B,0x4B,0xCB,0x2B,0xAB,0x6B,0xEB,
      0x1B,0x9B,0x5B,0xDB,0x3B,0xBB,0x7B,0xFB,
      0x07,0x87,0x47,0xC7,0x27,0xA7,0x67,0xE7,
      0x17,0x97,0x57,0xD7,0x37,0xB7,0x77,0xF7,
      0x0F,0x8F,0x4F,0xCF,0x2F,0xAF,0x6F,0xEF,
      0x1F,0x9F,0x5F,0xDF,0x3F,0xBF,0x7F,0xFF,
   };
   return table[value];
}

#define CYCLES 100000000

int main(void)
{
   const char *name[] = { "foo", "bar", "baz", "qux", "zot", "fum" };
   unsigned char (*const function[])(unsigned char) =
   {
      foo, bar, baz, qux, zot, fum
   };
   size_t i,j;
   fflush(stdout);
   for ( i = 0; i < sizeof(function)/sizeof(*function); ++i )
   {
      clock_t end, start;
      printf("%s(%X) = %X", name [ i ], 0x5C, function [ i ] (0x5C));
      fflush(stdout);
      start = clock();
      for ( j = 0; j < CYCLES; ++j )
      {
         function [ i ] (j);
      }
      end = clock();
      printf(", elapsed time = %g\n", (end - start) / (double)CLOCKS_PER_SEC);
   }

   return 0;
}

/* my output (Borland C++ 5.5.1 for Win32)
foo(5C) = 3A, elapsed time = 12.789
bar(5C) = 3A, elapsed time = 18.366
baz(5C) = 3A, elapsed time = 24.646
qux(5C) = 3A, elapsed time = 4.496
zot(5C) = 3A, elapsed time = 9.684
fum(5C) = 3A, elapsed time = 1.592
*/