shuffle!

static VALUE
rb_ary_shuffle_bang(VALUE ary)
{
    VALUE *ptr;
    long i = RARRAY_LEN(ary);

    rb_ary_modify(ary);
    ptr = RARRAY_PTR(ary);
    while (i) {
        long j = (long)(rb_genrand_real()*i);
        VALUE tmp = ptr[--i];
        ptr[i] = ptr[j];
        ptr[j] = tmp;
    }
    return ary;
}