#include <stddef.h>

/* Byte-wise swap two items of size 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)                         \


/**
 * Performs a Fisher-Yates shuffle shuffle on an arbitrarily typed array.
 * @param base      [in,out] A pointer to the beginning of the array.
 * @param nmbers    [in]    The number of items in the array.
 * @param size      [in]    The size of the individual elements of the array.
 * @param r_func    [in]    The random number generator for shuffling the data.
 *
 * Example usage:
 * @code{.c}
 *  shuffle(array, sizeof(array)/sizeof(array[0]), sizeof(array[0]), rand_wrapper)
 * @endcode
 * @see <a href="https://en.wikipedia.org/wiki/Fisher?Yates_shuffle">
 * Fisher-Yates shuffle | Wikipedia</a>
 * @copyright GNU Public License.
 */
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;
    }
}