You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: bidirule.go
    path of file: ./repos/faygo/vendor/golang.org/x/text/secure/bidirule
    
    the code of the file until where you have to start completion: // Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package bidirule implements the Bidi Rule defined by RFC 5893.
//
// This package is under development. The API may change without n
    
func (t *Transformer) advanceString(s string) (n int, ok bool) {
	var e bidi.Properties
	var sz int
	for n < len(s) {
		if s[n] < utf8.RuneSelf {
			e, sz = asciiTable[s[n]], 1
		} else {
			e, sz = bidi.LookupString(s[n:])
			if sz <= 1 {
				if sz == 1 {
					return n, false // invalid UTF-8
				}
				return n, true // incomplete UTF-8 encoding
			}
		}
		// TODO: using CompactClass results in noticeable speedup.
		// See unicode/bidi/prop.go:Properties.CompactClass.
		c := uint16(1 << e.Class())
		t.seen |= c
		if t.seen&exclusiveRTL == exclusiveRTL {
			t.state = ruleInvalid
			return n, false
		}
		switch tr := transitions[t.state]; {
		case tr[0].mask&c != 0:
			t.state = tr[0].next
		case tr[1].mask&c != 0:
			t.state = tr[1].next
		default:
			t.state = ruleInvalid
			if t.isRTL() {
				return n, false
			}
		}
		n += sz
	}
	return n, true
}