shuffle!

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

    if (OPTHASH_GIVEN_P(opts)) {
        randgen = rb_hash_lookup2(opts, sym_random, randgen);
    }
    if (argc > 0) {
        rb_raise(rb_eArgError, "wrong number of arguments (%d for 0)", argc);
    }
    rb_ary_modify(ary);
    i = RARRAY_LEN(ary);
    ptr = RARRAY_PTR(ary);
    snap_len = i;
    snap_ptr = ptr;
    while (i) {
        long j = RAND_UPTO(i);
        VALUE tmp;
        if (snap_len != RARRAY_LEN(ary) || snap_ptr != RARRAY_PTR(ary)) {
            rb_raise(rb_eRuntimeError, "modified during shuffle");
        }
        tmp = ptr[--i];
        ptr[i] = ptr[j];
        ptr[j] = tmp;
    }
    return ary;
}