swap_bytes_impl.h

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/*
    swap_bytes: Swap byte order of samples in a buffer.
 
    copyright 2018 by the mpg123 project
    licensed under the terms of the LGPL 2.1
    see COPYING and AUTHORS files in distribution or http://mpg123.org
 
    initially written by Thomas Orgis
 
    This is C source to include in your code to get the function named
    swap_bytes. It is serves intentional duplication in libmpg123 and
    libsyn123 to avoid introducing a dependency between them. This function
    is too small for that.
*/
 
/* Other headers are already included! */
 
/* Optionally use platform-specific byteswap macros. */
#ifdef HAVE_BYTESWAP_H
#include <byteswap.h>
#endif
 
/* Plain stupid swapping of elements in a byte array. */
/* This is the fallback when there is no native bswap macro. */
#define SWAP(a,b) tmp = p[a]; p[a] = p[b]; p[b] = tmp;
 
/* Convert samplecount elements of samplesize bytes each in buffer buf. */
static void swap_bytes(void *buf, size_t samplesize, size_t samplecount)
{
    unsigned char *p = buf;
    unsigned char *pend = (unsigned char*)buf+samplesize*samplecount;
    unsigned char tmp;
 
    if(samplesize < 2)
        return;
    switch(samplesize)
    {
        case 2: /* AB -> BA */
#ifdef HAVE_BYTESWAP_H
        {
            uint16_t* pp = (uint16_t*)p;
            for(; pp<(uint16_t*)pend; ++pp)
                *pp = bswap_16(*pp);
        }
#else
            for(; p<pend; p+=2)
            {
                SWAP(0,1)
            }
#endif
        break;
        case 3: /* ABC -> CBA */
            for(; p<pend; p+=3)
            {
                SWAP(0,2)
            }
        break;
        case 4: /* ABCD -> DCBA */
#ifdef HAVE_BYTESWAP_H
        {
            uint32_t* pp = (uint32_t*)p;
            for(; pp<(uint32_t*)pend; ++pp)
                *pp = bswap_32(*pp);
        }
#else
            for(; p<pend; p+=4)
            {
                SWAP(0,3)
                SWAP(1,2)
            }
#endif
        break;
        case 8: /* ABCDEFGH -> HGFEDCBA */
#ifdef HAVE_BYTESWAP_H
        {
            uint64_t* pp = (uint64_t*)p;
            for(; pp<(uint64_t*)pend; ++pp)
                *pp = bswap_64(*pp);
        }
#else
            for(; p<pend; p+=8)
            {
                SWAP(0,7)
                SWAP(1,6)
                SWAP(2,5)
                SWAP(3,4)
            }
#endif
        break;
        /* All the weird choices with the full nested loop. */
        default:
            for(; p<pend; p+=samplesize)
            {
                size_t j;
                for(j=0; j<samplesize/2; ++j)
                {
                    SWAP(j, samplesize-j-1)
                }
            }
    }
}
 
#undef SWAP