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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:9845

/* BEGIN block_swap_test.c */
/*
** block_swap swaps two contiguous subobjects.
** (base) is the address of the lower addressed subobject.
** (lo_size) is the size of the lower addressed subobject.
** (hi_size) is the size of the higher addressed subobject.
** If either (lo_size) or (hi_size) or both 
** compare equal to zero, then no swapping takes place.
*/
#include <stdio.h>

#define STRING  "0123456789A"

void block_swap(void *base, size_t lo_size, size_t hi_size);

int 
main(void)
{
    char string[] = STRING;
    size_t right = sizeof string;
    size_t left = 0;

    puts("/* BEGIN block_swap_test.c output */\n");
    do {
        --right;
        block_swap(string, left, right);
        puts(string);
        block_swap(string, right, left);
        puts(string);
        putchar('\n');
    } while (++left != sizeof string);
    puts("/* END block_swap_test.c output */");
    return 0;
}

void 
block_swap(void *base, size_t lo_size, size_t hi_size)
{
    size_t mod;
    unsigned char *end, *middle, swap;

    mod = lo_size != 0 && hi_size != 0;
    while (mod != 0) {
        end = base;
        middle = end + lo_size;
        if (hi_size > lo_size) {
            mod = hi_size % lo_size;
            base = end + hi_size - mod;
            do {
                swap = *end;
                *end++ = *middle;
                *middle++ = swap;
            } while (--hi_size != mod);
        } else {
            mod = lo_size % hi_size;
            end = middle + hi_size;
            do {
                swap = *--middle;
                *middle = *--end;
                *end = swap;
            } while (--lo_size != mod);
        }
    }
}

/* END block_swap_test.c */


/* BEGIN block_swap_test.c output */

0123456789A
0123456789A

123456789A0
0123456789A

23456789A01
0123456789A

3456789A012
0123456789A

456789A0123
0123456789A

56789A01234
0123456789A

6789A012345
0123456789A

789A0123456
0123456789A

89A01234567
0123456789A

9A012345678
0123456789A

A0123456789
0123456789A

0123456789A
0123456789A

/* END block_swap_test.c output */

-- 
pete