shuffle.c

/**
 * A C source file that includes a type agnostic Fisher-Yates shuffle.
 *
 * @copyright (C) 2017 Henry Malinowski <malinowski.henry@gmail.com>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
#include "shuffle.h"

/**
 * @brief Byte-wise swap two items of size \p size.
 * This code is borrowed straight from the GCC qsort implementation of SWAP.
 */
#define SWAP(a, b, size)                \
    do                                  \
    {                                   \
        size_t __size = (size);			\
        char *__a = (a), *__b = (b);    \
        do                              \
        {                               \
            char __tmp = *__a;          \
            *__a++ = *__b;              \
            *__b++ = __tmp;             \
        } while (--__size > 0);         \
    } while (0)                         \


void
shuffle(void* base, size_t nmbers, size_t size, size_t (*r_func)(void))
{
    register char* right_ptr = (char*)base+(nmbers-1) * size;
    register char* rand_ptr;
    register size_t randint;

    while (nmbers > 0)
    {
        randint = r_func() % nmbers;
        rand_ptr = ((char*)base)+(randint*size);

        SWAP(rand_ptr, right_ptr, size);

        /* decrements the right pointer and number of members */
        right_ptr -= size;
        --nmbers;
    }

}