[feature] Set/show instance language(s); show post language on frontend (#2362)

* update go text, include text/display

* [feature] Set instance langs, show post lang on frontend

* go fmt

* WebGet

* set language for whole article, don't use FA icon

* mention instance languages + other optional config vars

* little tweak

* put languages in config properly

* warn log language parse

* change some naming around

* tidy up validate a bit

* lint

* rename LanguageTmpl in template

vendor/golang.org/x/text/language/display/dict.go

@@ -0,0 +1,92 @@
+// Copyright 2014 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 display
+
+// This file contains sets of data for specific languages. Users can use these
+// to create smaller collections of supported languages and reduce total table
+// size.
+
+// The variable names defined here correspond to those in package language.
+
+var (
+	Afrikaans            *Dictionary = &af        // af
+	Amharic              *Dictionary = &am        // am
+	Arabic               *Dictionary = &ar        // ar
+	ModernStandardArabic *Dictionary = Arabic     // ar-001
+	Azerbaijani          *Dictionary = &az        // az
+	Bulgarian            *Dictionary = &bg        // bg
+	Bengali              *Dictionary = &bn        // bn
+	Catalan              *Dictionary = &ca        // ca
+	Czech                *Dictionary = &cs        // cs
+	Danish               *Dictionary = &da        // da
+	German               *Dictionary = &de        // de
+	Greek                *Dictionary = &el        // el
+	English              *Dictionary = &en        // en
+	AmericanEnglish      *Dictionary = English    // en-US
+	BritishEnglish       *Dictionary = English    // en-GB
+	Spanish              *Dictionary = &es        // es
+	EuropeanSpanish      *Dictionary = Spanish    // es-ES
+	LatinAmericanSpanish *Dictionary = Spanish    // es-419
+	Estonian             *Dictionary = &et        // et
+	Persian              *Dictionary = &fa        // fa
+	Finnish              *Dictionary = &fi        // fi
+	Filipino             *Dictionary = &fil       // fil
+	French               *Dictionary = &fr        // fr
+	Gujarati             *Dictionary = &gu        // gu
+	Hebrew               *Dictionary = &he        // he
+	Hindi                *Dictionary = &hi        // hi
+	Croatian             *Dictionary = &hr        // hr
+	Hungarian            *Dictionary = &hu        // hu
+	Armenian             *Dictionary = &hy        // hy
+	Indonesian           *Dictionary = &id        // id
+	Icelandic            *Dictionary = &is        // is
+	Italian              *Dictionary = &it        // it
+	Japanese             *Dictionary = &ja        // ja
+	Georgian             *Dictionary = &ka        // ka
+	Kazakh               *Dictionary = &kk        // kk
+	Khmer                *Dictionary = &km        // km
+	Kannada              *Dictionary = &kn        // kn
+	Korean               *Dictionary = &ko        // ko
+	Kirghiz              *Dictionary = &ky        // ky
+	Lao                  *Dictionary = &lo        // lo
+	Lithuanian           *Dictionary = &lt        // lt
+	Latvian              *Dictionary = &lv        // lv
+	Macedonian           *Dictionary = &mk        // mk
+	Malayalam            *Dictionary = &ml        // ml
+	Mongolian            *Dictionary = &mn        // mn
+	Marathi              *Dictionary = &mr        // mr
+	Malay                *Dictionary = &ms        // ms
+	Burmese              *Dictionary = &my        // my
+	Nepali               *Dictionary = &ne        // ne
+	Dutch                *Dictionary = &nl        // nl
+	Norwegian            *Dictionary = &no        // no
+	Punjabi              *Dictionary = &pa        // pa
+	Polish               *Dictionary = &pl        // pl
+	Portuguese           *Dictionary = &pt        // pt
+	BrazilianPortuguese  *Dictionary = Portuguese // pt-BR
+	EuropeanPortuguese   *Dictionary = &ptPT      // pt-PT
+	Romanian             *Dictionary = &ro        // ro
+	Russian              *Dictionary = &ru        // ru
+	Sinhala              *Dictionary = &si        // si
+	Slovak               *Dictionary = &sk        // sk
+	Slovenian            *Dictionary = &sl        // sl
+	Albanian             *Dictionary = &sq        // sq
+	Serbian              *Dictionary = &sr        // sr
+	SerbianLatin         *Dictionary = &srLatn    // sr
+	Swedish              *Dictionary = &sv        // sv
+	Swahili              *Dictionary = &sw        // sw
+	Tamil                *Dictionary = &ta        // ta
+	Telugu               *Dictionary = &te        // te
+	Thai                 *Dictionary = &th        // th
+	Turkish              *Dictionary = &tr        // tr
+	Ukrainian            *Dictionary = &uk        // uk
+	Urdu                 *Dictionary = &ur        // ur
+	Uzbek                *Dictionary = &uz        // uz
+	Vietnamese           *Dictionary = &vi        // vi
+	Chinese              *Dictionary = &zh        // zh
+	SimplifiedChinese    *Dictionary = Chinese    // zh-Hans
+	TraditionalChinese   *Dictionary = &zhHant    // zh-Hant
+	Zulu                 *Dictionary = &zu        // zu
+)

vendor/golang.org/x/text/language/display/display.go

@@ -0,0 +1,420 @@
+// Copyright 2014 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.
+
+//go:generate go run maketables.go -output tables.go
+
+// Package display provides display names for languages, scripts and regions in
+// a requested language.
+//
+// The data is based on CLDR's localeDisplayNames. It includes the names of the
+// draft level "contributed" or "approved". The resulting tables are quite
+// large. The display package is designed so that users can reduce the linked-in
+// table sizes by cherry picking the languages one wishes to support. There is a
+// Dictionary defined for a selected set of common languages for this purpose.
+package display // import "golang.org/x/text/language/display"
+
+import (
+	"fmt"
+	"strings"
+
+	"golang.org/x/text/internal/format"
+	"golang.org/x/text/language"
+)
+
+/*
+TODO:
+All fairly low priority at the moment:
+  - Include alternative and variants as an option (using func options).
+  - Option for returning the empty string for undefined values.
+  - Support variants, currencies, time zones, option names and other data
+    provided in CLDR.
+  - Do various optimizations:
+    - Reduce size of offset tables.
+    - Consider compressing infrequently used languages and decompress on demand.
+*/
+
+// A Formatter formats a tag in the current language. It is used in conjunction
+// with the message package.
+type Formatter struct {
+	lookup func(tag int, x interface{}) string
+	x      interface{}
+}
+
+// Format implements "golang.org/x/text/internal/format".Formatter.
+func (f Formatter) Format(state format.State, verb rune) {
+	// TODO: there are a lot of inefficiencies in this code. Fix it when we
+	// language.Tag has embedded compact tags.
+	t := state.Language()
+	_, index, _ := matcher.Match(t)
+	str := f.lookup(index, f.x)
+	if str == "" {
+		// TODO: use language-specific punctuation.
+		// TODO: use codePattern instead of language?
+		if unknown := f.lookup(index, language.Und); unknown != "" {
+			fmt.Fprintf(state, "%v (%v)", unknown, f.x)
+		} else {
+			fmt.Fprintf(state, "[language: %v]", f.x)
+		}
+	} else {
+		state.Write([]byte(str))
+	}
+}
+
+// Language returns a Formatter that renders the name for lang in the
+// current language. x may be a language.Base or a language.Tag.
+// It renders lang in the default language if no translation for the current
+// language is supported.
+func Language(lang interface{}) Formatter {
+	return Formatter{langFunc, lang}
+}
+
+// Region returns a Formatter that renders the name for region in the current
+// language. region may be a language.Region or a language.Tag.
+// It renders region in the default language if no translation for the current
+// language is supported.
+func Region(region interface{}) Formatter {
+	return Formatter{regionFunc, region}
+}
+
+// Script returns a Formatter that renders the name for script in the current
+// language. script may be a language.Script or a language.Tag.
+// It renders script in the default language if no translation for the current
+// language is supported.
+func Script(script interface{}) Formatter {
+	return Formatter{scriptFunc, script}
+}
+
+// Tag returns a Formatter that renders the name for tag in the current
+// language. tag may be a language.Tag.
+// It renders tag in the default language if no translation for the current
+// language is supported.
+func Tag(tag interface{}) Formatter {
+	return Formatter{tagFunc, tag}
+}
+
+// A Namer is used to get the name for a given value, such as a Tag, Language,
+// Script or Region.
+type Namer interface {
+	// Name returns a display string for the given value. A Namer returns an
+	// empty string for values it does not support. A Namer may support naming
+	// an unspecified value. For example, when getting the name for a region for
+	// a tag that does not have a defined Region, it may return the name for an
+	// unknown region. It is up to the user to filter calls to Name for values
+	// for which one does not want to have a name string.
+	Name(x interface{}) string
+}
+
+var (
+	// Supported lists the languages for which names are defined.
+	Supported language.Coverage
+
+	// The set of all possible values for which names are defined. Note that not
+	// all Namer implementations will cover all the values of a given type.
+	// A Namer will return the empty string for unsupported values.
+	Values language.Coverage
+
+	matcher language.Matcher
+)
+
+func init() {
+	tags := make([]language.Tag, numSupported)
+	s := supported
+	for i := range tags {
+		p := strings.IndexByte(s, '|')
+		tags[i] = language.Raw.Make(s[:p])
+		s = s[p+1:]
+	}
+	matcher = language.NewMatcher(tags)
+	Supported = language.NewCoverage(tags)
+
+	Values = language.NewCoverage(langTagSet.Tags, supportedScripts, supportedRegions)
+}
+
+// Languages returns a Namer for naming languages. It returns nil if there is no
+// data for the given tag. The type passed to Name must be either language.Base
+// or language.Tag. Note that the result may differ between passing a tag or its
+// base language. For example, for English, passing "nl-BE" would return Flemish
+// whereas passing "nl" returns "Dutch".
+func Languages(t language.Tag) Namer {
+	if _, index, conf := matcher.Match(t); conf != language.No {
+		return languageNamer(index)
+	}
+	return nil
+}
+
+type languageNamer int
+
+func langFunc(i int, x interface{}) string {
+	return nameLanguage(languageNamer(i), x)
+}
+
+func (n languageNamer) name(i int) string {
+	return lookup(langHeaders[:], int(n), i)
+}
+
+// Name implements the Namer interface for language names.
+func (n languageNamer) Name(x interface{}) string {
+	return nameLanguage(n, x)
+}
+
+// nonEmptyIndex walks up the parent chain until a non-empty header is found.
+// It returns -1 if no index could be found.
+func nonEmptyIndex(h []header, index int) int {
+	for ; index != -1 && h[index].data == ""; index = int(parents[index]) {
+	}
+	return index
+}
+
+// Scripts returns a Namer for naming scripts. It returns nil if there is no
+// data for the given tag. The type passed to Name must be either a
+// language.Script or a language.Tag. It will not attempt to infer a script for
+// tags with an unspecified script.
+func Scripts(t language.Tag) Namer {
+	if _, index, conf := matcher.Match(t); conf != language.No {
+		if index = nonEmptyIndex(scriptHeaders[:], index); index != -1 {
+			return scriptNamer(index)
+		}
+	}
+	return nil
+}
+
+type scriptNamer int
+
+func scriptFunc(i int, x interface{}) string {
+	return nameScript(scriptNamer(i), x)
+}
+
+func (n scriptNamer) name(i int) string {
+	return lookup(scriptHeaders[:], int(n), i)
+}
+
+// Name implements the Namer interface for script names.
+func (n scriptNamer) Name(x interface{}) string {
+	return nameScript(n, x)
+}
+
+// Regions returns a Namer for naming regions. It returns nil if there is no
+// data for the given tag. The type passed to Name must be either a
+// language.Region or a language.Tag. It will not attempt to infer a region for
+// tags with an unspecified region.
+func Regions(t language.Tag) Namer {
+	if _, index, conf := matcher.Match(t); conf != language.No {
+		if index = nonEmptyIndex(regionHeaders[:], index); index != -1 {
+			return regionNamer(index)
+		}
+	}
+	return nil
+}
+
+type regionNamer int
+
+func regionFunc(i int, x interface{}) string {
+	return nameRegion(regionNamer(i), x)
+}
+
+func (n regionNamer) name(i int) string {
+	return lookup(regionHeaders[:], int(n), i)
+}
+
+// Name implements the Namer interface for region names.
+func (n regionNamer) Name(x interface{}) string {
+	return nameRegion(n, x)
+}
+
+// Tags returns a Namer for giving a full description of a tag. The names of
+// scripts and regions that are not already implied by the language name will
+// in appended within parentheses. It returns nil if there is not data for the
+// given tag. The type passed to Name must be a tag.
+func Tags(t language.Tag) Namer {
+	if _, index, conf := matcher.Match(t); conf != language.No {
+		return tagNamer(index)
+	}
+	return nil
+}
+
+type tagNamer int
+
+func tagFunc(i int, x interface{}) string {
+	return nameTag(languageNamer(i), scriptNamer(i), regionNamer(i), x)
+}
+
+// Name implements the Namer interface for tag names.
+func (n tagNamer) Name(x interface{}) string {
+	return nameTag(languageNamer(n), scriptNamer(n), regionNamer(n), x)
+}
+
+// lookup finds the name for an entry in a global table, traversing the
+// inheritance hierarchy if needed.
+func lookup(table []header, dict, want int) string {
+	for dict != -1 {
+		if s := table[dict].name(want); s != "" {
+			return s
+		}
+		dict = int(parents[dict])
+	}
+	return ""
+}
+
+// A Dictionary holds a collection of Namers for a single language. One can
+// reduce the amount of data linked in to a binary by only referencing
+// Dictionaries for the languages one needs to support instead of using the
+// generic Namer factories.
+type Dictionary struct {
+	parent *Dictionary
+	lang   header
+	script header
+	region header
+}
+
+// Tags returns a Namer for giving a full description of a tag. The names of
+// scripts and regions that are not already implied by the language name will
+// in appended within parentheses. It returns nil if there is not data for the
+// given tag. The type passed to Name must be a tag.
+func (d *Dictionary) Tags() Namer {
+	return dictTags{d}
+}
+
+type dictTags struct {
+	d *Dictionary
+}
+
+// Name implements the Namer interface for tag names.
+func (n dictTags) Name(x interface{}) string {
+	return nameTag(dictLanguages{n.d}, dictScripts{n.d}, dictRegions{n.d}, x)
+}
+
+// Languages returns a Namer for naming languages. It returns nil if there is no
+// data for the given tag. The type passed to Name must be either language.Base
+// or language.Tag. Note that the result may differ between passing a tag or its
+// base language. For example, for English, passing "nl-BE" would return Flemish
+// whereas passing "nl" returns "Dutch".
+func (d *Dictionary) Languages() Namer {
+	return dictLanguages{d}
+}
+
+type dictLanguages struct {
+	d *Dictionary
+}
+
+func (n dictLanguages) name(i int) string {
+	for d := n.d; d != nil; d = d.parent {
+		if s := d.lang.name(i); s != "" {
+			return s
+		}
+	}
+	return ""
+}
+
+// Name implements the Namer interface for language names.
+func (n dictLanguages) Name(x interface{}) string {
+	return nameLanguage(n, x)
+}
+
+// Scripts returns a Namer for naming scripts. It returns nil if there is no
+// data for the given tag. The type passed to Name must be either a
+// language.Script or a language.Tag. It will not attempt to infer a script for
+// tags with an unspecified script.
+func (d *Dictionary) Scripts() Namer {
+	return dictScripts{d}
+}
+
+type dictScripts struct {
+	d *Dictionary
+}
+
+func (n dictScripts) name(i int) string {
+	for d := n.d; d != nil; d = d.parent {
+		if s := d.script.name(i); s != "" {
+			return s
+		}
+	}
+	return ""
+}
+
+// Name implements the Namer interface for script names.
+func (n dictScripts) Name(x interface{}) string {
+	return nameScript(n, x)
+}
+
+// Regions returns a Namer for naming regions. It returns nil if there is no
+// data for the given tag. The type passed to Name must be either a
+// language.Region or a language.Tag. It will not attempt to infer a region for
+// tags with an unspecified region.
+func (d *Dictionary) Regions() Namer {
+	return dictRegions{d}
+}
+
+type dictRegions struct {
+	d *Dictionary
+}
+
+func (n dictRegions) name(i int) string {
+	for d := n.d; d != nil; d = d.parent {
+		if s := d.region.name(i); s != "" {
+			return s
+		}
+	}
+	return ""
+}
+
+// Name implements the Namer interface for region names.
+func (n dictRegions) Name(x interface{}) string {
+	return nameRegion(n, x)
+}
+
+// A SelfNamer implements a Namer that returns the name of language in this same
+// language. It provides a very compact mechanism to provide a comprehensive
+// list of languages to users in their native language.
+type SelfNamer struct {
+	// Supported defines the values supported by this Namer.
+	Supported language.Coverage
+}
+
+var (
+	// Self is a shared instance of a SelfNamer.
+	Self *SelfNamer = &self
+
+	self = SelfNamer{language.NewCoverage(selfTagSet.Tags)}
+)
+
+// Name returns the name of a given language tag in the language identified by
+// this tag. It supports both the language.Base and language.Tag types.
+func (n SelfNamer) Name(x interface{}) string {
+	t, _ := language.All.Compose(x)
+	base, scr, reg := t.Raw()
+	baseScript := language.Script{}
+	if (scr == language.Script{} && reg != language.Region{}) {
+		// For looking up in the self dictionary, we need to select the
+		// maximized script. This is even the case if the script isn't
+		// specified.
+		s1, _ := t.Script()
+		if baseScript = getScript(base); baseScript != s1 {
+			scr = s1
+		}
+	}
+
+	i, scr, reg := selfTagSet.index(base, scr, reg)
+	if i == -1 {
+		return ""
+	}
+
+	// Only return the display name if the script matches the expected script.
+	if (scr != language.Script{}) {
+		if (baseScript == language.Script{}) {
+			baseScript = getScript(base)
+		}
+		if baseScript != scr {
+			return ""
+		}
+	}
+
+	return selfHeaders[0].name(i)
+}
+
+// getScript returns the maximized script for a base language.
+func getScript(b language.Base) language.Script {
+	tag, _ := language.Raw.Compose(b)
+	scr, _ := tag.Script()
+	return scr
+}

vendor/golang.org/x/text/language/display/lookup.go

@@ -0,0 +1,253 @@
+// Copyright 2014 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 display
+
+// This file contains common lookup code that is shared between the various
+// implementations of Namer and Dictionaries.
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+
+	"golang.org/x/text/language"
+)
+
+type namer interface {
+	// name gets the string for the given index. It should walk the
+	// inheritance chain if a value is not present in the base index.
+	name(idx int) string
+}
+
+func nameLanguage(n namer, x interface{}) string {
+	t, _ := language.All.Compose(x)
+	for {
+		i, _, _ := langTagSet.index(t.Raw())
+		if s := n.name(i); s != "" {
+			return s
+		}
+		if t = t.Parent(); t == language.Und {
+			return ""
+		}
+	}
+}
+
+func nameScript(n namer, x interface{}) string {
+	t, _ := language.DeprecatedScript.Compose(x)
+	_, s, _ := t.Raw()
+	return n.name(scriptIndex.index(s.String()))
+}
+
+func nameRegion(n namer, x interface{}) string {
+	t, _ := language.DeprecatedRegion.Compose(x)
+	_, _, r := t.Raw()
+	return n.name(regionIndex.index(r.String()))
+}
+
+func nameTag(langN, scrN, regN namer, x interface{}) string {
+	t, ok := x.(language.Tag)
+	if !ok {
+		return ""
+	}
+	const form = language.All &^ language.SuppressScript
+	if c, err := form.Canonicalize(t); err == nil {
+		t = c
+	}
+	_, sRaw, rRaw := t.Raw()
+	i, scr, reg := langTagSet.index(t.Raw())
+	for i != -1 {
+		if str := langN.name(i); str != "" {
+			if hasS, hasR := (scr != language.Script{}), (reg != language.Region{}); hasS || hasR {
+				ss, sr := "", ""
+				if hasS {
+					ss = scrN.name(scriptIndex.index(scr.String()))
+				}
+				if hasR {
+					sr = regN.name(regionIndex.index(reg.String()))
+				}
+				// TODO: use patterns in CLDR or at least confirm they are the
+				// same for all languages.
+				if ss != "" && sr != "" {
+					return fmt.Sprintf("%s (%s, %s)", str, ss, sr)
+				}
+				if ss != "" || sr != "" {
+					return fmt.Sprintf("%s (%s%s)", str, ss, sr)
+				}
+			}
+			return str
+		}
+		scr, reg = sRaw, rRaw
+		if t = t.Parent(); t == language.Und {
+			return ""
+		}
+		i, _, _ = langTagSet.index(t.Raw())
+	}
+	return ""
+}
+
+// header contains the data and indexes for a single namer.
+// data contains a series of strings concatenated into one. index contains the
+// offsets for a string in data. For example, consider a header that defines
+// strings for the languages de, el, en, fi, and nl:
+//
+//	header{
+//		data: "GermanGreekEnglishDutch",
+//		index: []uint16{0, 6, 11, 18, 18, 23},
+//	}
+//
+// For a language with index i, the string is defined by
+// data[index[i]:index[i+1]]. So the number of elements in index is always one
+// greater than the number of languages for which header defines a value.
+// A string for a language may be empty, which means the name is undefined. In
+// the above example, the name for fi (Finnish) is undefined.
+type header struct {
+	data  string
+	index []uint16
+}
+
+// name looks up the name for a tag in the dictionary, given its index.
+func (h *header) name(i int) string {
+	if 0 <= i && i < len(h.index)-1 {
+		return h.data[h.index[i]:h.index[i+1]]
+	}
+	return ""
+}
+
+// tagSet is used to find the index of a language in a set of tags.
+type tagSet struct {
+	single tagIndex
+	long   []string
+}
+
+var (
+	langTagSet = tagSet{
+		single: langIndex,
+		long:   langTagsLong,
+	}
+
+	// selfTagSet is used for indexing the language strings in their own
+	// language.
+	selfTagSet = tagSet{
+		single: selfIndex,
+		long:   selfTagsLong,
+	}
+
+	zzzz = language.MustParseScript("Zzzz")
+	zz   = language.MustParseRegion("ZZ")
+)
+
+// index returns the index of the tag for the given base, script and region or
+// its parent if the tag is not available. If the match is for a parent entry,
+// the excess script and region are returned.
+func (ts *tagSet) index(base language.Base, scr language.Script, reg language.Region) (int, language.Script, language.Region) {
+	lang := base.String()
+	index := -1
+	if (scr != language.Script{} || reg != language.Region{}) {
+		if scr == zzzz {
+			scr = language.Script{}
+		}
+		if reg == zz {
+			reg = language.Region{}
+		}
+
+		i := sort.SearchStrings(ts.long, lang)
+		// All entries have either a script or a region and not both.
+		scrStr, regStr := scr.String(), reg.String()
+		for ; i < len(ts.long) && strings.HasPrefix(ts.long[i], lang); i++ {
+			if s := ts.long[i][len(lang)+1:]; s == scrStr {
+				scr = language.Script{}
+				index = i + ts.single.len()
+				break
+			} else if s == regStr {
+				reg = language.Region{}
+				index = i + ts.single.len()
+				break
+			}
+		}
+	}
+	if index == -1 {
+		index = ts.single.index(lang)
+	}
+	return index, scr, reg
+}
+
+func (ts *tagSet) Tags() []language.Tag {
+	tags := make([]language.Tag, 0, ts.single.len()+len(ts.long))
+	ts.single.keys(func(s string) {
+		tags = append(tags, language.Raw.MustParse(s))
+	})
+	for _, s := range ts.long {
+		tags = append(tags, language.Raw.MustParse(s))
+	}
+	return tags
+}
+
+func supportedScripts() []language.Script {
+	scr := make([]language.Script, 0, scriptIndex.len())
+	scriptIndex.keys(func(s string) {
+		scr = append(scr, language.MustParseScript(s))
+	})
+	return scr
+}
+
+func supportedRegions() []language.Region {
+	reg := make([]language.Region, 0, regionIndex.len())
+	regionIndex.keys(func(s string) {
+		reg = append(reg, language.MustParseRegion(s))
+	})
+	return reg
+}
+
+// tagIndex holds a concatenated lists of subtags of length 2 to 4, one string
+// for each length, which can be used in combination with binary search to get
+// the index associated with a tag.
+// For example, a tagIndex{
+//
+//	"arenesfrruzh",  // 6 2-byte tags.
+//	"barwae",        // 2 3-byte tags.
+//	"",
+//
+// }
+// would mean that the 2-byte tag "fr" had an index of 3, and the 3-byte tag
+// "wae" had an index of 7.
+type tagIndex [3]string
+
+func (t *tagIndex) index(s string) int {
+	sz := len(s)
+	if sz < 2 || 4 < sz {
+		return -1
+	}
+	a := t[sz-2]
+	index := sort.Search(len(a)/sz, func(i int) bool {
+		p := i * sz
+		return a[p:p+sz] >= s
+	})
+	p := index * sz
+	if end := p + sz; end > len(a) || a[p:end] != s {
+		return -1
+	}
+	// Add the number of tags for smaller sizes.
+	for i := 0; i < sz-2; i++ {
+		index += len(t[i]) / (i + 2)
+	}
+	return index
+}
+
+// len returns the number of tags that are contained in the tagIndex.
+func (t *tagIndex) len() (n int) {
+	for i, s := range t {
+		n += len(s) / (i + 2)
+	}
+	return n
+}
+
+// keys calls f for each tag.
+func (t *tagIndex) keys(f func(key string)) {
+	for i, s := range *t {
+		for ; s != ""; s = s[i+2:] {
+			f(s[:i+2])
+		}
+	}
+}