Re: [Algorithms] Byte Swapping

[brian s. <br...@hu...>]

> From: Charles Bloom <cb...@cb...>
> 
> Jumping in late : I'm sure you don't need it now, but
> here's a crazy ANSI C byte-swapper :
> 
> let x = a 32 bit ulong , ABCD
> then y = DCBA
> 
> y = (x = x>>16 | x<<16) ^ (x = (x^x>>8) & 0x00FF00FF) ^ (x<<8);
> 

wow, I'll say that's crazy...but unfortunately non-functional.  see the
results of this program:

#include <stdio.h>

long swap1(long x)
{
    return (x = x>>16 | x<<16) ^ (x = (x^x>>8) & 0x00FF00FF) ^ (x<<8);
}

long swap2(long x)
{
    return         ((x >> 24) & 0x000000FF)
            |    ((x >> 8 ) & 0x0000FF00)
            |    ((x << 8 ) & 0x00FF0000)
            |    ((x << 24) & 0xFF000000)
            ;
}

int main(int argc, char **argv)
{
    long test = 0x01020304;

    printf("swap1: 0x%08X 0x%08X 0x%08X \n", test, swap1(test),
swap1(swap1(test)));

    printf("swap2: 0x%08X 0x%08X 0x%08X \n", test, swap2(test),
swap2(swap2(test)));
}

swap1: 0x01020304 0x07000300 0x03000700
swap2: 0x01020304 0x04030201 0x01020304



anyway, in case anyone hasn't gotten their fill of this stuff, here is our
comparatively lame byteswapping code that uses those fast swappers on
PowerPC:

#ifndef _MBSWAP_H_
#define _MBSWAP_H_

// swapping macros/inlines
#include <Runtime/MBBase.h>

inline MBint16 MBSwap(const MBint16 & swap)
{
#if defined(MB_MAC) && defined(MB_POWERPC)
    return __lhbrx(const_cast<MBint16 *>(&swap), 0);
#else
    return         ((swap >> 8 ) & 0x00FF)
            |    ((swap << 8 ) & 0xFF00)
            ;
#endif
}

inline MBuint16 MBSwap(const MBuint16 & swap)
{
#if defined(MB_MAC) && defined(MB_POWERPC)
    return __lhbrx(const_cast<MBuint16 *>(&swap), 0);
#else
    return         ((swap >> 8 ) & 0x00FF)
            |    ((swap << 8 ) & 0xFF00)
            ;
#endif
}

inline MBint32 MBSwap(const MBint32 & swap)
{
#if defined(MB_MAC) && defined(MB_POWERPC)
    return __lwbrx(const_cast<MBint32 *>(&swap), 0);
#else
    return         ((swap >> 24) & 0x000000FF)
            |    ((swap >> 8 ) & 0x0000FF00)
            |    ((swap << 8 ) & 0x00FF0000)
            |    ((swap << 24) & 0xFF000000)
            ;
#endif
}

inline MBuint32 MBSwap(const MBuint32 & swap)
{
#if defined(MB_MAC) && defined(MB_POWERPC)
    return __lwbrx(const_cast<MBuint32 *>(&swap), 0);
#else
    return         ((swap >> 24) & 0x000000FF)
            |    ((swap >> 8 ) & 0x0000FF00)
            |    ((swap << 8 ) & 0x00FF0000)
            |    ((swap << 24) & 0xFF000000)
            ;
#endif
}

inline MBfloat MBSwap(const MBfloat & swap)
{
    MBuint32 temp = *(const MBuint32*)&swap;
    temp = MBSwap(temp);
    return *(const MBfloat *)&temp;
}

#ifdef MB_MSB

    #define MBSwapMSBToNative( s ) s
    #define MBSwapNativeToMSB( s ) s
    
    #define MBSwapLSBToNative MBSwap
    #define MBSwapNativeToLSB MBSwap
    
#elif defined(MB_LSB)

    #define MBSwapLSBToNative( s ) s
    #define MBSwapNativeToLSB( s ) s
    
    #define MBSwapMSBToNative MBSwap
    #define MBSwapNativeToMSB MBSwap
    
#endif

#endif // header guard