shuffle!

static VALUE
rb_ary_shuffle_bang(int argc, VALUE *argv, VALUE ary)
{
    VALUE opts, randgen = rb_cRandom;
    long i, len;

    if (OPTHASH_GIVEN_P(opts)) {
        VALUE rnd;
        ID keyword_ids[1];

        keyword_ids[0] = id_random;
        rb_get_kwargs(opts, keyword_ids, 0, 1, &rnd);
        if (rnd != Qundef) {
            randgen = rnd;
        }
    }
    rb_check_arity(argc, 0, 0);
    rb_ary_modify(ary);
    i = len = RARRAY_LEN(ary);
    RARRAY_PTR_USE(ary, ptr, {
        while (i) {
            long j = RAND_UPTO(i);
            VALUE tmp;
            if (len != RARRAY_LEN(ary) || ptr != RARRAY_CONST_PTR(ary)) {
                rb_raise(rb_eRuntimeError, "modified during shuffle");
            }
            tmp = ptr[--i];
            ptr[i] = ptr[j];
            ptr[j] = tmp;
        }
    }); /* WB: no new reference */
    return ary;
}