Array#union

static VALUE
rb_ary_union_multi(int argc, VALUE *argv, VALUE ary)
{
    rb_len_t sum = RARRAY_LEN(ary);
    for (int i = 0; i < argc; i++) {
        argv[i] = to_ary(argv[i]);
        sum += RARRAY_LEN(argv[i]);
    }

    if (sum <= SMALL_ARRAY_LEN) {
        VALUE ary_union = rb_ary_new();

        rb_ary_union(ary_union, ary);
        for (int i = 0; i < argc; i++) rb_ary_union(ary_union, argv[i]);

        return ary_union;
    }

    VALUE set = rb_obj_hide(rb_set_new_capa(sum));
    rb_ary_union_set(set, ary);
    for (int i = 0; i < argc; i++) rb_ary_union_set(set, argv[i]);

    return rb_set_to_a(set);
}