Re: [Algorithms] Byte Swapping

[Aaron D. <ri...@ho...>]

It looks like the way this is meant to work is that it swaps the top and
bottom 16 bits then proceeds to do the old 3 xor byte swapping  trick
(x^=y^=x) for each of the two 16 bit segments but I think the code is a
little bit broken. I can't see how that second bit works.


-----Original Message-----
From: brian sharon <br...@hu...>
To: gda...@li...
<gda...@li...>
Date: Tuesday, 18 July 2000 12:02
Subject: Re: [Algorithms] Byte Swapping


>
>> 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
>
>
>
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