idna/idna.go

// Code generated by running "go generate" in golang.org/x/text. DO NOT EDIT.

// 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 idna implements IDNA2008 using the compatibility processing
// defined by UTS (Unicode Technical Standard) #46, which defines a standard to
// deal with the transition from IDNA2003.
//
// IDNA2008 (Internationalized Domain Names for Applications), is defined in RFC
// 5890, RFC 5891, RFC 5892, RFC 5893 and RFC 5894.
// UTS #46 is defined in https://www.unicode.org/reports/tr46.
// See https://unicode.org/cldr/utility/idna.jsp for a visualization of the
// differences between these two standards.
package idna // import "golang.org/x/net/idna"

import (
	"fmt"
	"strings"
	"unicode"
	"unicode/utf8"

	"golang.org/x/text/secure/bidirule"
	"golang.org/x/text/unicode/bidi"
	"golang.org/x/text/unicode/norm"
)

const unicode16 = unicode.Version >= "16.0.0"

// NOTE: Unlike common practice in Go APIs, the functions will return a
// sanitized domain name in case of errors. Browsers sometimes use a partially
// evaluated string as lookup.
// TODO: the current error handling is, in my opinion, the least opinionated.
// Other strategies are also viable, though:
// Option 1) Return an empty string in case of error, but allow the user to
//    specify explicitly which errors to ignore.
// Option 2) Return the partially evaluated string if it is itself a valid
//    string, otherwise return the empty string in case of error.
// Option 3) Option 1 and 2.
// Option 4) Always return an empty string for now and implement Option 1 as
//    needed, and document that the return string may not be empty in case of
//    error in the future.
// I think Option 1 is best, but it is quite opinionated.

// ToASCII is a wrapper for Punycode.ToASCII.
func ToASCII(s string) (string, error) {
	return Punycode.process(s, true)
}

// ToUnicode is a wrapper for Punycode.ToUnicode.
func ToUnicode(s string) (string, error) {
	return Punycode.process(s, false)
}

// An Option configures a Profile at creation time.
type Option func(*options)

// Transitional sets a Profile to use the Transitional mapping as defined in UTS
// #46. This will cause, for example, "ß" to be mapped to "ss". Using the
// transitional mapping provides a compromise between IDNA2003 and IDNA2008
// compatibility. It is used by some browsers when resolving domain names. This
// option is only meaningful if combined with MapForLookup.
func Transitional(transitional bool) Option {
	return func(o *options) { o.transitional = transitional }
}

// VerifyDNSLength sets whether a Profile should fail if any of the IDN parts
// are longer than allowed by the RFC.
//
// This option corresponds to the VerifyDnsLength flag in UTS #46.
func VerifyDNSLength(verify bool) Option {
	return func(o *options) { o.verifyDNSLength = verify }
}

// RemoveLeadingDots removes leading label separators. Leading runes that map to
// dots, such as U+3002 IDEOGRAPHIC FULL STOP, are removed as well.
func RemoveLeadingDots(remove bool) Option {
	return func(o *options) { o.removeLeadingDots = remove }
}

// ValidateLabels sets whether to check the mandatory label validation criteria
// as defined in Section 5.4 of RFC 5891. This includes testing for correct use
// of hyphens ('-'), normalization, validity of runes, and the context rules.
// In particular, ValidateLabels also sets the CheckHyphens and CheckJoiners flags
// in UTS #46.
func ValidateLabels(enable bool) Option {
	return func(o *options) {
		// Don't override existing mappings, but set one that at least checks
		// normalization if it is not set.
		if o.mapping == nil && enable {
			o.mapping = normalize
		}
		o.trie = trie
		o.checkJoiners = enable
		o.checkHyphens = enable
		if enable {
			o.fromPuny = validateFromPunycode
		} else {
			o.fromPuny = nil
		}
	}
}

// validateLabels reports whether the ValidateLabels option is enabled.
func (p *Profile) validateLabels() bool {
	return p.fromPuny != nil
}

// CheckHyphens sets whether to check for correct use of hyphens ('-') in
// labels. Most web browsers do not have this option set, since labels such as
// "r3---sn-apo3qvuoxuxbt-j5pe" are in common use.
//
// This option corresponds to the CheckHyphens flag in UTS #46.
func CheckHyphens(enable bool) Option {
	return func(o *options) { o.checkHyphens = enable }
}

// CheckJoiners sets whether to check the ContextJ rules as defined in Appendix
// A of RFC 5892, concerning the use of joiner runes.
//
// This option corresponds to the CheckJoiners flag in UTS #46.
func CheckJoiners(enable bool) Option {
	return func(o *options) {
		o.trie = trie
		o.checkJoiners = enable
	}
}

// StrictDomainName limits the set of permissible ASCII characters to those
// allowed in domain names as defined in RFC 1034 (A-Z, a-z, 0-9 and the
// hyphen). This is set by default for MapForLookup and ValidateForRegistration,
// but is only useful if ValidateLabels is set.
//
// This option is useful, for instance, for browsers that allow characters
// outside this range, for example a '_' (U+005F LOW LINE). See
// http://www.rfc-editor.org/std/std3.txt for more details.
//
// This option corresponds to the UseSTD3ASCIIRules flag in UTS #46.
func StrictDomainName(use bool) Option {
	return func(o *options) { o.useSTD3Rules = use }
}

// NOTE: the following options pull in tables. The tables should not be linked
// in as long as the options are not used.

// BidiRule enables the Bidi rule as defined in RFC 5893. Any application
// that relies on proper validation of labels should include this rule.
//
// This option corresponds to the CheckBidi flag in UTS #46.
func BidiRule() Option {
	return func(o *options) { o.bidirule = bidirule.ValidString }
}

// ValidateForRegistration sets validation options to verify that a given IDN is
// properly formatted for registration as defined by Section 4 of RFC 5891.
func ValidateForRegistration() Option {
	return func(o *options) {
		o.mapping = validateRegistration
		StrictDomainName(true)(o)
		ValidateLabels(true)(o)
		VerifyDNSLength(true)(o)
		BidiRule()(o)
	}
}

// MapForLookup sets validation and mapping options such that a given IDN is
// transformed for domain name lookup according to the requirements set out in
// Section 5 of RFC 5891. The mappings follow the recommendations of RFC 5894,
// RFC 5895 and UTS 46. It does not add the Bidi Rule. Use the BidiRule option
// to add this check.
//
// The mappings include normalization and mapping case, width and other
// compatibility mappings.
func MapForLookup() Option {
	return func(o *options) {
		o.mapping = validateAndMap
		StrictDomainName(true)(o)
		ValidateLabels(true)(o)
	}
}

type options struct {
	transitional      bool
	useSTD3Rules      bool
	checkHyphens      bool
	checkJoiners      bool
	verifyDNSLength   bool
	removeLeadingDots bool

	trie *idnaTrie

	// fromPuny calls validation rules when converting A-labels to U-labels.
	fromPuny func(p *Profile, s string) error

	// mapping implements a validation and mapping step as defined in RFC 5895
	// or UTS 46, tailored to, for example, domain registration or lookup.
	mapping func(p *Profile, s string) (mapped string, isBidi bool, err error)

	// bidirule, if specified, checks whether s conforms to the Bidi Rule
	// defined in RFC 5893.
	bidirule func(s string) bool
}

// A Profile defines the configuration of an IDNA mapper.
type Profile struct {
	options
}

func apply(o *options, opts []Option) {
	for _, f := range opts {
		f(o)
	}
}

// New creates a new Profile.
//
// With no options, the returned Profile is the most permissive and equals the
// Punycode Profile. Options can be passed to further restrict the Profile. The
// MapForLookup and ValidateForRegistration options set a collection of options,
// for lookup and registration purposes respectively, which can be tailored by
// adding more fine-grained options, where later options override earlier
// options.
func New(o ...Option) *Profile {
	p := &Profile{}
	apply(&p.options, o)
	return p
}

// ToASCII converts a domain or domain label to its ASCII form. For example,
// ToASCII("bücher.example.com") is "xn--bcher-kva.example.com", and
// ToASCII("golang") is "golang". If an error is encountered it will return
// an error and a (partially) processed result.
func (p *Profile) ToASCII(s string) (string, error) {
	return p.process(s, true)
}

// ToUnicode converts a domain or domain label to its Unicode form. For example,
// ToUnicode("xn--bcher-kva.example.com") is "bücher.example.com", and
// ToUnicode("golang") is "golang". If an error is encountered it will return
// an error and a (partially) processed result.
func (p *Profile) ToUnicode(s string) (string, error) {
	pp := *p
	pp.transitional = false
	return pp.process(s, false)
}

// String reports a string with a description of the profile for debugging
// purposes. The string format may change with different versions.
func (p *Profile) String() string {
	s := ""
	if p.transitional {
		s = "Transitional"
	} else {
		s = "NonTransitional"
	}
	if p.useSTD3Rules {
		s += ":UseSTD3Rules"
	}
	if p.checkHyphens {
		s += ":CheckHyphens"
	}
	if p.checkJoiners {
		s += ":CheckJoiners"
	}
	if p.verifyDNSLength {
		s += ":VerifyDNSLength"
	}
	return s
}

// Transitional processing is disabled by default as of Go 1.18.
// https://golang.org/issue/47510
const transitionalLookup = false

var (
	// Punycode is a Profile that does raw punycode processing with a minimum
	// of validation.
	Punycode *Profile = punycode

	// Lookup is the recommended profile for looking up domain names, according
	// to Section 5 of RFC 5891. The exact configuration of this profile may
	// change over time.
	Lookup *Profile = lookup

	// Display is the recommended profile for displaying domain names.
	// The configuration of this profile may change over time.
	Display *Profile = display

	// Registration is the recommended profile for checking whether a given
	// IDN is valid for registration, according to Section 4 of RFC 5891.
	Registration *Profile = registration

	punycode = &Profile{}
	lookup   = &Profile{options{
		transitional: transitionalLookup,
		useSTD3Rules: true,
		checkHyphens: true,
		checkJoiners: true,
		trie:         trie,
		fromPuny:     validateFromPunycode,
		mapping:      validateAndMap,
		bidirule:     bidirule.ValidString,
	}}
	display = &Profile{options{
		useSTD3Rules: true,
		checkHyphens: true,
		checkJoiners: true,
		trie:         trie,
		fromPuny:     validateFromPunycode,
		mapping:      validateAndMap,
		bidirule:     bidirule.ValidString,
	}}
	registration = &Profile{options{
		useSTD3Rules:    true,
		verifyDNSLength: true,
		checkHyphens:    true,
		checkJoiners:    true,
		trie:            trie,
		fromPuny:        validateFromPunycode,
		mapping:         validateRegistration,
		bidirule:        bidirule.ValidString,
	}}

	// TODO: profiles
	// Register: recommended for approving domain names: don't do any mappings
	// but rather reject on invalid input. Bundle or block deviation characters.
)

type labelError struct{ label, code_ string }

func (e labelError) code() string { return e.code_ }
func (e labelError) Error() string {
	return fmt.Sprintf("idna: invalid label %q", e.label)
}

type runeError struct {
	r     rune
	code_ string
}

func (e runeError) code() string { return e.code_ }
func (e runeError) Error() string {
	return fmt.Sprintf("idna: disallowed rune %U", e.r)
}

// code16 returns old for Unicode < 16, new for Unicode >= 16.
func code16(old, new string) string {
	if unicode16 {
		return new
	}
	return old
}

// process10 implements the algorithm described in section 4 of UTS #46.
// It implements both the Unicode 10 algorithm
// (https://www.unicode.org/reports/tr46/tr46-19.html)
// and the Unicode 16 algorithm
// (https://www.unicode.org/reports/tr46/tr46-35.html)
// depending on unicode16, which in turn depends on unicode.Version.
func (p *Profile) process(s string, toASCII bool) (string, error) {
	var err error
	var isBidi bool
	if p.mapping != nil {
		s, isBidi, err = p.mapping(p, s)
	}
	// Remove leading empty labels.
	if p.removeLeadingDots {
		for ; len(s) > 0 && s[0] == '.'; s = s[1:] {
		}
	}
	// TODO: allow for a quick check of the tables data.
	// It seems like we should only create this error on ToASCII, but the
	// UTS 46 conformance tests suggests we should always check this.
	labelCode := "X4_2"
	if !unicode16 || toASCII {
		labelCode = "A4"
	}
	if err == nil && p.verifyDNSLength && s == "" {
		err = labelError{s, labelCode}
	}
	labels := labelIter{orig: s}
	for ; !labels.done(); labels.next() {
		label := labels.label()
		if label == "" {
			// Empty labels are not okay. The label iterator skips the last
			// label if it is empty.
			if err == nil && p.verifyDNSLength {
				err = labelError{s, labelCode}
			}
			continue
		}
		if strings.HasPrefix(label, acePrefix) {
			enc := label[len(acePrefix):]
			u, err2 := decode(enc)
			if err2 != nil {
				if err == nil {
					err = err2
				}
				// Spec says keep the old label.
				continue
			}
			if unicode16 && err == nil && len(u) > 0 && isASCII(u) {
				err = punyError(enc)
			}
			isBidi = isBidi || bidirule.DirectionString(u) != bidi.LeftToRight
			labels.set(u)
			if err == nil && p.fromPuny != nil {
				err = p.fromPuny(p, u)
			}
			if err == nil {
				// This should be called on NonTransitional, according to the
				// spec, but that currently does not have any effect. Use the
				// original profile to preserve options.
				err = p.validateLabel(u, labelCode)
			}
		} else if err == nil {
			err = p.validateLabel(label, labelCode)
		}
	}
	if isBidi && p.bidirule != nil && err == nil {
		for labels.reset(); !labels.done(); labels.next() {
			if !p.bidirule(labels.label()) {
				err = labelError{s, "B"}
				break
			}
		}
	}
	if toASCII {
		for labels.reset(); !labels.done(); labels.next() {
			label := labels.label()
			if !ascii(label) {
				a, err2 := encode(acePrefix, label)
				if err == nil {
					err = err2
				}
				label = a
				labels.set(a)
			}
			n := len(label)
			if p.verifyDNSLength && err == nil && (n == 0 || n > 63) {
				err = labelError{label, labelCode}
			}
		}
	}
	s = labels.result()
	if toASCII && p.verifyDNSLength && err == nil {
		if unicode16 && strings.HasSuffix(s, ".") {
			err = labelError{s, labelCode}
		}
		// Compute the length of the domain name minus the root label and its dot.
		n := len(s)
		if n > 0 && s[n-1] == '.' {
			n--
		}
		if len(s) < 1 || n > 253 {
			err = labelError{s, labelCode}
		}
	}
	return s, err
}

func isASCII(s string) bool {
	for _, c := range []byte(s) {
		if c >= 0x80 {
			return false
		}
	}
	return true
}

func normalize(p *Profile, s string) (mapped string, isBidi bool, err error) {
	// TODO: consider first doing a quick check to see if any of these checks
	// need to be done. This will make it slower in the general case, but
	// faster in the common case.
	mapped = norm.NFC.String(s)
	isBidi = bidirule.DirectionString(mapped) == bidi.RightToLeft
	return mapped, isBidi, nil
}

func validateRegistration(p *Profile, s string) (idem string, bidi bool, err error) {
	// TODO: filter need for normalization in loop below.
	if !norm.NFC.IsNormalString(s) {
		return s, false, labelError{s, "V1"}
	}
	for i := 0; i < len(s); {
		v, sz := trie.lookupString(s[i:])
		if sz == 0 {
			return s, bidi, runeError{utf8.RuneError, "P1"}
		}
		bidi = bidi || info(v).isBidi(s[i:])
		// Copy bytes not copied so far.
		switch p.simplify(info(v).category()) {
		// TODO: handle the NV8 defined in the Unicode idna data set to allow
		// for strict conformance to IDNA2008.
		case valid, deviation:
			if sz == 1 && p.useSTD3Rules && !allowedSTD3(rune(s[i])) {
				return s, bidi, runeError{rune(s[i]), "P1"}
			}
		case disallowed, mapped, unknown, ignored:
			r, _ := utf8.DecodeRuneInString(s[i:])
			return s, bidi, runeError{r, "P1"}
		}
		i += sz
	}
	return s, bidi, nil
}

func (c info) isBidi(s string) bool {
	if !c.isMapped() {
		return c&attributesMask == rtl
	}
	// TODO: also store bidi info for mapped data. This is possible, but a bit
	// cumbersome and not for the common case.
	p, _ := bidi.LookupString(s)
	switch p.Class() {
	case bidi.R, bidi.AL, bidi.AN:
		return true
	}
	return false
}

func validateAndMap(p *Profile, s string) (vm string, bidi bool, err error) {
	var (
		b []byte
		k int
	)
	// combinedInfoBits contains the or-ed bits of all runes. We use this
	// to derive the mayNeedNorm bit later. This may trigger normalization
	// overeagerly, but it will not do so in the common case. The end result
	// is another 10% saving on BenchmarkProfile for the common case.
	var combinedInfoBits info
	for i := 0; i < len(s); {
		v, sz := trie.lookupString(s[i:])
		if sz == 0 {
			b = append(b, s[k:i]...)
			b = append(b, "\ufffd"...)
			k = len(s)
			if err == nil {
				err = runeError{utf8.RuneError, "P1"}
			}
			break
		}
		combinedInfoBits |= info(v)
		bidi = bidi || info(v).isBidi(s[i:])
		start := i
		i += sz
		// Copy bytes not copied so far.
		switch p.simplify(info(v).category()) {
		case valid:
			continue
		case disallowed:
			// Unicode 16 delays the error until validateLabels.
			// Unicode 10 gave an error now.
			if !unicode16 && err == nil {
				r, _ := utf8.DecodeRuneInString(s[start:])
				err = runeError{r, "P1"}
			}
			continue
		case deviation:
			if unicode16 && !p.transitional {
				break
			}
			fallthrough
		case mapped:
			b = append(b, s[k:start]...)
			// Unicode 16 requires a special case to handle ẞ -> ss in transitional mode.
			if unicode16 && p.transitional && s[start:start+sz] == "ẞ" {
				b = append(b, "ss"...)
			} else {
				b = info(v).appendMapping(b, s[start:i])
			}
		case ignored:
			b = append(b, s[k:start]...)
			// drop the rune
		case unknown:
			b = append(b, s[k:start]...)
			b = append(b, "\ufffd"...)
		}
		k = i
	}
	if k == 0 {
		// No changes so far.
		if combinedInfoBits&mayNeedNorm != 0 {
			s = norm.NFC.String(s)
		}
	} else {
		b = append(b, s[k:]...)
		if norm.NFC.QuickSpan(b) != len(b) {
			b = norm.NFC.Bytes(b)
		}
		// TODO: the punycode converters require strings as input.
		s = string(b)
	}
	return s, bidi, err
}

// A labelIter allows iterating over domain name labels.
type labelIter struct {
	orig     string
	slice    []string
	curStart int
	curEnd   int
	i        int
}

func (l *labelIter) reset() {
	l.curStart = 0
	l.curEnd = 0
	l.i = 0
}

func (l *labelIter) done() bool {
	return l.curStart >= len(l.orig)
}

func (l *labelIter) result() string {
	if l.slice != nil {
		return strings.Join(l.slice, ".")
	}
	return l.orig
}

func (l *labelIter) label() string {
	if l.slice != nil {
		return l.slice[l.i]
	}
	p := strings.IndexByte(l.orig[l.curStart:], '.')
	l.curEnd = l.curStart + p
	if p == -1 {
		l.curEnd = len(l.orig)
	}
	return l.orig[l.curStart:l.curEnd]
}

// next sets the value to the next label. It skips the last label if it is empty.
func (l *labelIter) next() {
	l.i++
	if l.slice != nil {
		if l.i >= len(l.slice) || l.i == len(l.slice)-1 && l.slice[l.i] == "" {
			l.curStart = len(l.orig)
		}
	} else {
		l.curStart = l.curEnd + 1
		if l.curStart == len(l.orig)-1 && l.orig[l.curStart] == '.' {
			l.curStart = len(l.orig)
		}
	}
}

func (l *labelIter) set(s string) {
	if l.slice == nil {
		l.slice = strings.Split(l.orig, ".")
	}
	l.slice[l.i] = s
}

// acePrefix is the ASCII Compatible Encoding prefix.
const acePrefix = "xn--"

func (p *Profile) simplify(cat category) category {
	switch cat {
	case disallowedSTD3Mapped: // only happens for pre-Unicode 16
		if p.useSTD3Rules {
			cat = disallowed
		} else {
			cat = mapped
		}
	case disallowedSTD3Valid: // only happens for pre-Unicode 16
		if p.useSTD3Rules {
			cat = disallowed
		} else {
			cat = valid
		}
	case deviation:
		if !p.transitional {
			cat = valid
		}
	case validNV8, validXV8:
		// TODO: handle V2008
		cat = valid
	}
	return cat
}

func validateFromPunycode(p *Profile, s string) error {
	if !norm.NFC.IsNormalString(s) {
		return labelError{s, "V1"}
	}
	// TODO: detect whether string may have to be normalized in the following
	// loop.
	for i := 0; i < len(s); {
		v, sz := trie.lookupString(s[i:])
		if sz == 0 {
			return runeError{utf8.RuneError, "P1"}
		}
		cat := info(v).category()
		if c := p.simplify(cat); c != valid && c != deviation {
			return labelError{s, code16("V6", "V7")}
		}
		i += sz
	}
	return nil
}

const (
	zwnj = "\u200c"
	zwj  = "\u200d"
)

type joinState int8

const (
	stateStart joinState = iota
	stateVirama
	stateBefore
	stateBeforeVirama
	stateAfter
	stateFAIL
)

var joinStates = [][numJoinTypes]joinState{
	stateStart: {
		joiningL:   stateBefore,
		joiningD:   stateBefore,
		joinZWNJ:   stateFAIL,
		joinZWJ:    stateFAIL,
		joinVirama: stateVirama,
	},
	stateVirama: {
		joiningL: stateBefore,
		joiningD: stateBefore,
	},
	stateBefore: {
		joiningL:   stateBefore,
		joiningD:   stateBefore,
		joiningT:   stateBefore,
		joinZWNJ:   stateAfter,
		joinZWJ:    stateFAIL,
		joinVirama: stateBeforeVirama,
	},
	stateBeforeVirama: {
		joiningL: stateBefore,
		joiningD: stateBefore,
		joiningT: stateBefore,
	},
	stateAfter: {
		joiningL:   stateFAIL,
		joiningD:   stateBefore,
		joiningT:   stateAfter,
		joiningR:   stateStart,
		joinZWNJ:   stateFAIL,
		joinZWJ:    stateFAIL,
		joinVirama: stateAfter, // no-op as we can't accept joiners here
	},
	stateFAIL: {
		0:          stateFAIL,
		joiningL:   stateFAIL,
		joiningD:   stateFAIL,
		joiningT:   stateFAIL,
		joiningR:   stateFAIL,
		joinZWNJ:   stateFAIL,
		joinZWJ:    stateFAIL,
		joinVirama: stateFAIL,
	},
}

// allowedSTD3 reports whether r is a rune that can appear in a domain name
// according to STD3. We allow all non-ASCII runes and then letters, digits, hyphens.
// We also add dot so that this can be run against the whole name and not just
// a single name element (label). The surrounding code checks dots well enough.
func allowedSTD3(r rune) bool {
	return r >= 0x80 || 'a' <= r && r <= 'z' || '0' <= r && r <= '9' || r == '-' || r == '.'
}

// validateLabel validates the criteria from Section 4.1. Item 1, 4, and 6 are
// already implicitly satisfied by the overall implementation.
func (p *Profile) validateLabel(s string, labelCode string) (err error) {
	if s == "" {
		if p.verifyDNSLength {
			return labelError{s, labelCode}
		}
		return nil
	}
	if p.checkHyphens {
		if len(s) > 4 && s[2] == '-' && s[3] == '-' {
			return labelError{s, "V2"}
		}
		if s[0] == '-' || s[len(s)-1] == '-' {
			return labelError{s, "V3"}
		}
	}

	// Unicode 16's TR 46 delays the rune validity checks until after the label is decoded.
	// (validateAndMap did not reject them earlier.)
	if unicode16 && p.validateLabels() {
		for i := 0; i < len(s); {
			v, sz := trie.lookupString(s[i:])
			if sz == 0 {
				return runeError{utf8.RuneError, "P1"}
			}
			cat := info(v).category()
			if c := p.simplify(cat); c != valid && (!p.transitional || c != deviation) {
				return labelError{s, "V7"}
			}
			if sz == 1 && p.useSTD3Rules && !allowedSTD3(rune(s[i])) {
				return runeError{rune(s[i]), "U1"}
			}
			i += sz
		}
	}

	if !p.checkJoiners {
		return nil
	}
	trie := p.trie // p.checkJoiners is only set if trie is set.
	// TODO: merge the use of this in the trie.
	v, sz := trie.lookupString(s)
	x := info(v)
	if x.isModifier() {
		return labelError{s, code16("V5", "V6")}
	}
	// Quickly return in the absence of zero-width (non) joiners.
	if strings.Index(s, zwj) == -1 && strings.Index(s, zwnj) == -1 {
		return nil
	}
	st := stateStart
	for i := 0; ; {
		jt := x.joinType()
		if s[i:i+sz] == zwj {
			jt = joinZWJ
		} else if s[i:i+sz] == zwnj {
			jt = joinZWNJ
		}
		st = joinStates[st][jt]
		if x.isViramaModifier() {
			st = joinStates[st][joinVirama]
		}
		if i += sz; i == len(s) {
			break
		}
		v, sz = trie.lookupString(s[i:])
		x = info(v)
	}
	if st == stateFAIL || st == stateAfter {
		return labelError{s, "C"}
	}

	return nil
}

func ascii(s string) bool {
	for i := 0; i < len(s); i++ {
		if s[i] >= utf8.RuneSelf {
			return false
		}
	}
	return true
}

// appendMapping appends the mapping for the respective rune. isMapped must be
// true. A mapping is a categorization of a rune as defined in UTS #46.
func (c info) appendMapping(b []byte, s string) []byte {
	index := int(c >> indexShift)
	if c&xorBit == 0 {
		p := index
		return append(b, mappings[mappingIndex[p]:mappingIndex[p+1]]...)
	}
	b = append(b, s...)
	if c&inlineXOR == inlineXOR {
		// TODO: support and handle two-byte inline masks
		b[len(b)-1] ^= byte(index)
	} else {
		for p := len(b) - int(xorData[index]); p < len(b); p++ {
			index++
			b[p] ^= xorData[index]
		}
	}
	return b
}