Array#&

static VALUE
rb_ary_and(VALUE ary1, VALUE ary2)
{
    ary2 = to_ary(ary2);
    VALUE ary3 = rb_ary_new();
    if (RARRAY_LEN(ary1) == 0 || RARRAY_LEN(ary2) == 0) return ary3;

    if (RARRAY_LEN(ary1) <= SMALL_ARRAY_LEN && RARRAY_LEN(ary2) <= SMALL_ARRAY_LEN) {
        for (rb_len_t i = 0; i < RARRAY_LEN(ary1); i++) {
            VALUE v = RARRAY_AREF(ary1, i);
            if (!rb_ary_includes_by_eql(ary2, v)) continue;
            if (rb_ary_includes_by_eql(ary3, v)) continue;
            rb_ary_push(ary3, v);
        }
        return ary3;
    }

    VALUE set = ary_to_set(ary2);

    for (rb_len_t i = 0; i < RARRAY_LEN(ary1); i++) {
        VALUE v = RARRAY_AREF(ary1, i);
        if (rb_set_delete_no_check(set, v)) {
            rb_ary_push(ary3, v);
        }
    }

    return ary3;
}