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/*
** Public domain demo by Ray Gardner, 7 dec 88
**
** Here's an old programming trick that (I bet) will be new to at least a
** few of you out there, even some "old hands". I don't remember where I
** saw this; it might have been Jon Bentley's "Programming Pearls" column in
** Communications of the ACM.
**
** Have you ever wanted to exchange two adjacent areas of storage which
** might be of two different lengths? There are some tricky and complicated
** "efficient" methods to do this without using a lot of extra temporary
** storage. But there is also an old and simple way: Assume that the buffer
** looks like this:
**
** |...... head .......|.................. tail .................|
**
** You reverse the head, reverse the tail, then reverse the entire buffer.
** That's all there is to it. It will leave you with:
**
** |.................. tail .................|...... head .......|
**
** Here's code:
*/
#include <stdlib.h>
/*
** reverse "count" bytes starting at "buf"
*/
void memrev(char *buf, size_t count)
{
char *r;
for (r = buf + count - 1; buf < r; buf++, r--)
{
*buf ^= *r;
*r ^= *buf;
*buf ^= *r;
}
}
/*
** swap "head" bytes with "tail" bytes at "buf"
*/
void aswap(char *buf, size_t head, size_t tail)
{
memrev(buf, head);
memrev(buf + head, tail);
memrev(buf, head + tail);
}
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