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// Copyright 2013 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 language
import (
"errors"
"strings"
"golang.org/x/text/internal/language"
)
// A MatchOption configures a Matcher.
type MatchOption func(*matcher)
// PreferSameScript will, in the absence of a match, result in the first
// preferred tag with the same script as a supported tag to match this supported
// tag. The default is currently true, but this may change in the future.
func PreferSameScript(preferSame bool) MatchOption {
return func(m *matcher) { m.preferSameScript = preferSame }
}
// TODO(v1.0.0): consider making Matcher a concrete type, instead of interface.
// There doesn't seem to be too much need for multiple types.
// Making it a concrete type allows MatchStrings to be a method, which will
// improve its discoverability.
// MatchStrings parses and matches the given strings until one of them matches
// the language in the Matcher. A string may be an Accept-Language header as
// handled by ParseAcceptLanguage. The default language is returned if no
// other language matched.
func MatchStrings(m Matcher, lang ...string) (tag Tag, index int) {
for _, accept := range lang {
desired, _, err := ParseAcceptLanguage(accept)
if err != nil {
continue
}
if tag, index, conf := m.Match(desired...); conf != No {
return tag, index
}
}
tag, index, _ = m.Match()
return
}
// Matcher is the interface that wraps the Match method.
//
// Match returns the best match for any of the given tags, along with
// a unique index associated with the returned tag and a confidence
// score.
type Matcher interface {
Match(t ...Tag) (tag Tag, index int, c Confidence)
}
// Comprehends reports the confidence score for a speaker of a given language
// to being able to comprehend the written form of an alternative language.
func Comprehends(speaker, alternative Tag) Confidence {
_, _, c := NewMatcher([]Tag{alternative}).Match(speaker)
return c
}
// NewMatcher returns a Matcher that matches an ordered list of preferred tags
// against a list of supported tags based on written intelligibility, closeness
// of dialect, equivalence of subtags and various other rules. It is initialized
// with the list of supported tags. The first element is used as the default
// value in case no match is found.
//
// Its Match method matches the first of the given Tags to reach a certain
// confidence threshold. The tags passed to Match should therefore be specified
// in order of preference. Extensions are ignored for matching.
//
// The index returned by the Match method corresponds to the index of the
// matched tag in t, but is augmented with the Unicode extension ('u')of the
// corresponding preferred tag. This allows user locale options to be passed
// transparently.
func NewMatcher(t []Tag, options ...MatchOption) Matcher {
return newMatcher(t, options)
}
func (m *matcher) Match(want ...Tag) (t Tag, index int, c Confidence) {
var tt language.Tag
match, w, c := m.getBest(want...)
if match != nil {
tt, index = match.tag, match.index
} else {
// TODO: this should be an option
tt = m.default_.tag
if m.preferSameScript {
outer:
for _, w := range want {
script, _ := w.Script()
if script.scriptID == 0 {
// Don't do anything if there is no script, such as with
// private subtags.
continue
}
for i, h := range m.supported {
if script.scriptID == h.maxScript {
tt, index = h.tag, i
break outer
}
}
}
}
// TODO: select first language tag based on script.
}
if w.RegionID != tt.RegionID && w.RegionID != 0 {
if w.RegionID != 0 && tt.RegionID != 0 && tt.RegionID.Contains(w.RegionID) {
tt.RegionID = w.RegionID
tt.RemakeString()
} else if r := w.RegionID.String(); len(r) == 2 {
// TODO: also filter macro and deprecated.
tt, _ = tt.SetTypeForKey("rg", strings.ToLower(r)+"zzzz")
}
}
// Copy options from the user-provided tag into the result tag. This is hard
// to do after the fact, so we do it here.
// TODO: add in alternative variants to -u-va-.
// TODO: add preferred region to -u-rg-.
if e := w.Extensions(); len(e) > 0 {
b := language.Builder{}
b.SetTag(tt)
for _, e := range e {
b.AddExt(e)
}
tt = b.Make()
}
return makeTag(tt), index, c
}
// ErrMissingLikelyTagsData indicates no information was available
// to compute likely values of missing tags.
var ErrMissingLikelyTagsData = errors.New("missing likely tags data")
// func (t *Tag) setTagsFrom(id Tag) {
// t.LangID = id.LangID
// t.ScriptID = id.ScriptID
// t.RegionID = id.RegionID
// }
// Tag Matching
// CLDR defines an algorithm for finding the best match between two sets of language
// tags. The basic algorithm defines how to score a possible match and then find
// the match with the best score
// (see https://www.unicode.org/reports/tr35/#LanguageMatching).
// Using scoring has several disadvantages. The scoring obfuscates the importance of
// the various factors considered, making the algorithm harder to understand. Using
// scoring also requires the full score to be computed for each pair of tags.
//
// We will use a different algorithm which aims to have the following properties:
// - clarity on the precedence of the various selection factors, and
// - improved performance by allowing early termination of a comparison.
//
// Matching algorithm (overview)
// Input:
// - supported: a set of supported tags
// - default: the default tag to return in case there is no match
// - desired: list of desired tags, ordered by preference, starting with
// the most-preferred.
//
// Algorithm:
// 1) Set the best match to the lowest confidence level
// 2) For each tag in "desired":
// a) For each tag in "supported":
// 1) compute the match between the two tags.
// 2) if the match is better than the previous best match, replace it
// with the new match. (see next section)
// b) if the current best match is Exact and pin is true the result will be
// frozen to the language found thusfar, although better matches may
// still be found for the same language.
// 3) If the best match so far is below a certain threshold, return "default".
//
// Ranking:
// We use two phases to determine whether one pair of tags are a better match
// than another pair of tags. First, we determine a rough confidence level. If the
// levels are different, the one with the highest confidence wins.
// Second, if the rough confidence levels are identical, we use a set of tie-breaker
// rules.
//
// The confidence level of matching a pair of tags is determined by finding the
// lowest confidence level of any matches of the corresponding subtags (the
// result is deemed as good as its weakest link).
// We define the following levels:
// Exact - An exact match of a subtag, before adding likely subtags.
// MaxExact - An exact match of a subtag, after adding likely subtags.
// [See Note 2].
// High - High level of mutual intelligibility between different subtag
// variants.
// Low - Low level of mutual intelligibility between different subtag
// variants.
// No - No mutual intelligibility.
//
// The following levels can occur for each type of subtag:
// Base: Exact, MaxExact, High, Low, No
// Script: Exact, MaxExact [see Note 3], Low, No
// Region: Exact, MaxExact, High
// Variant: Exact, High
// Private: Exact, No
//
// Any result with a confidence level of Low or higher is deemed a possible match.
// Once a desired tag matches any of the supported tags with a level of MaxExact
// or higher, the next desired tag is not considered (see Step 2.b).
// Note that CLDR provides languageMatching data that defines close equivalence
// classes for base languages, scripts and regions.
//
// Tie-breaking
// If we get the same confidence level for two matches, we apply a sequence of
// tie-breaking rules. The first that succeeds defines the result. The rules are
// applied in the following order.
// 1) Original language was defined and was identical.
// 2) Original region was defined and was identical.
// 3) Distance between two maximized regions was the smallest.
// 4) Original script was defined and was identical.
// 5) Distance from want tag to have tag using the parent relation [see Note 5.]
// If there is still no winner after these rules are applied, the first match
// found wins.
//
// Notes:
// [2] In practice, as matching of Exact is done in a separate phase from
// matching the other levels, we reuse the Exact level to mean MaxExact in
// the second phase. As a consequence, we only need the levels defined by
// the Confidence type. The MaxExact confidence level is mapped to High in
// the public API.
// [3] We do not differentiate between maximized script values that were derived
// from suppressScript versus most likely tag data. We determined that in
// ranking the two, one ranks just after the other. Moreover, the two cannot
// occur concurrently. As a consequence, they are identical for practical
// purposes.
// [4] In case of deprecated, macro-equivalents and legacy mappings, we assign
// the MaxExact level to allow iw vs he to still be a closer match than
// en-AU vs en-US, for example.
// [5] In CLDR a locale inherits fields that are unspecified for this locale
// from its parent. Therefore, if a locale is a parent of another locale,
// it is a strong measure for closeness, especially when no other tie
// breaker rule applies. One could also argue it is inconsistent, for
// example, when pt-AO matches pt (which CLDR equates with pt-BR), even
// though its parent is pt-PT according to the inheritance rules.
//
// Implementation Details:
// There are several performance considerations worth pointing out. Most notably,
// we preprocess as much as possible (within reason) at the time of creation of a
// matcher. This includes:
// - creating a per-language map, which includes data for the raw base language
// and its canonicalized variant (if applicable),
// - expanding entries for the equivalence classes defined in CLDR's
// languageMatch data.
// The per-language map ensures that typically only a very small number of tags
// need to be considered. The pre-expansion of canonicalized subtags and
// equivalence classes reduces the amount of map lookups that need to be done at
// runtime.
// matcher keeps a set of supported language tags, indexed by language.
type matcher struct {
default_ *haveTag
supported []*haveTag
index map[language.Language]*matchHeader
passSettings bool
preferSameScript bool
}
// matchHeader has the lists of tags for exact matches and matches based on
// maximized and canonicalized tags for a given language.
type matchHeader struct {
haveTags []*haveTag
original bool
}
// haveTag holds a supported Tag and its maximized script and region. The maximized
// or canonicalized language is not stored as it is not needed during matching.
type haveTag struct {
tag language.Tag
// index of this tag in the original list of supported tags.
index int
// conf is the maximum confidence that can result from matching this haveTag.
// When conf < Exact this means it was inserted after applying a CLDR equivalence rule.
conf Confidence
// Maximized region and script.
maxRegion language.Region
maxScript language.Script
// altScript may be checked as an alternative match to maxScript. If altScript
// matches, the confidence level for this match is Low. Theoretically there
// could be multiple alternative scripts. This does not occur in practice.
altScript language.Script
// nextMax is the index of the next haveTag with the same maximized tags.
nextMax uint16
}
func makeHaveTag(tag language.Tag, index int) (haveTag, language.Language) {
max := tag
if tag.LangID != 0 || tag.RegionID != 0 || tag.ScriptID != 0 {
max, _ = canonicalize(All, max)
max, _ = max.Maximize()
max.RemakeString()
}
return haveTag{tag, index, Exact, max.RegionID, max.ScriptID, altScript(max.LangID, max.ScriptID), 0}, max.LangID
}
// altScript returns an alternative script that may match the given script with
// a low confidence. At the moment, the langMatch data allows for at most one
// script to map to another and we rely on this to keep the code simple.
func altScript(l language.Language, s language.Script) language.Script {
for _, alt := range matchScript {
// TODO: also match cases where language is not the same.
if (language.Language(alt.wantLang) == l || language.Language(alt.haveLang) == l) &&
language.Script(alt.haveScript) == s {
return language.Script(alt.wantScript)
}
}
return 0
}
// addIfNew adds a haveTag to the list of tags only if it is a unique tag.
// Tags that have the same maximized values are linked by index.
func (h *matchHeader) addIfNew(n haveTag, exact bool) {
h.original = h.original || exact
// Don't add new exact matches.
for _, v := range h.haveTags {
if equalsRest(v.tag, n.tag) {
return
}
}
// Allow duplicate maximized tags, but create a linked list to allow quickly
// comparing the equivalents and bail out.
for i, v := range h.haveTags {
if v.maxScript == n.maxScript &&
v.maxRegion == n.maxRegion &&
v.tag.VariantOrPrivateUseTags() == n.tag.VariantOrPrivateUseTags() {
for h.haveTags[i].nextMax != 0 {
i = int(h.haveTags[i].nextMax)
}
h.haveTags[i].nextMax = uint16(len(h.haveTags))
break
}
}
h.haveTags = append(h.haveTags, &n)
}
// header returns the matchHeader for the given language. It creates one if
// it doesn't already exist.
func (m *matcher) header(l language.Language) *matchHeader {
if h := m.index[l]; h != nil {
return h
}
h := &matchHeader{}
m.index[l] = h
return h
}
func toConf(d uint8) Confidence {
if d <= 10 {
return High
}
if d < 30 {
return Low
}
return No
}
// newMatcher builds an index for the given supported tags and returns it as
// a matcher. It also expands the index by considering various equivalence classes
// for a given tag.
func newMatcher(supported []Tag, options []MatchOption) *matcher {
m := &matcher{
index: make(map[language.Language]*matchHeader),
preferSameScript: true,
}
for _, o := range options {
o(m)
}
if len(supported) == 0 {
m.default_ = &haveTag{}
return m
}
// Add supported languages to the index. Add exact matches first to give
// them precedence.
for i, tag := range supported {
tt := tag.tag()
pair, _ := makeHaveTag(tt, i)
m.header(tt.LangID).addIfNew(pair, true)
m.supported = append(m.supported, &pair)
}
m.default_ = m.header(supported[0].lang()).haveTags[0]
// Keep these in two different loops to support the case that two equivalent
// languages are distinguished, such as iw and he.
for i, tag := range supported {
tt := tag.tag()
pair, max := makeHaveTag(tt, i)
if max != tt.LangID {
m.header(max).addIfNew(pair, true)
}
}
// update is used to add indexes in the map for equivalent languages.
// update will only add entries to original indexes, thus not computing any
// transitive relations.
update := func(want, have uint16, conf Confidence) {
if hh := m.index[language.Language(have)]; hh != nil {
if !hh.original {
return
}
hw := m.header(language.Language(want))
for _, ht := range hh.haveTags {
v := *ht
if conf < v.conf {
v.conf = conf
}
v.nextMax = 0 // this value needs to be recomputed
if v.altScript != 0 {
v.altScript = altScript(language.Language(want), v.maxScript)
}
hw.addIfNew(v, conf == Exact && hh.original)
}
}
}
// Add entries for languages with mutual intelligibility as defined by CLDR's
// languageMatch data.
for _, ml := range matchLang {
update(ml.want, ml.have, toConf(ml.distance))
if !ml.oneway {
update(ml.have, ml.want, toConf(ml.distance))
}
}
// Add entries for possible canonicalizations. This is an optimization to
// ensure that only one map lookup needs to be done at runtime per desired tag.
// First we match deprecated equivalents. If they are perfect equivalents
// (their canonicalization simply substitutes a different language code, but
// nothing else), the match confidence is Exact, otherwise it is High.
for i, lm := range language.AliasMap {
// If deprecated codes match and there is no fiddling with the script or
// or region, we consider it an exact match.
conf := Exact
if language.AliasTypes[i] != language.Macro {
if !isExactEquivalent(language.Language(lm.From)) {
conf = High
}
update(lm.To, lm.From, conf)
}
update(lm.From, lm.To, conf)
}
return m
}
// getBest gets the best matching tag in m for any of the given tags, taking into
// account the order of preference of the given tags.
func (m *matcher) getBest(want ...Tag) (got *haveTag, orig language.Tag, c Confidence) {
best := bestMatch{}
for i, ww := range want {
w := ww.tag()
var max language.Tag
// Check for exact match first.
h := m.index[w.LangID]
if w.LangID != 0 {
if h == nil {
continue
}
// Base language is defined.
max, _ = canonicalize(Legacy|Deprecated|Macro, w)
// A region that is added through canonicalization is stronger than
// a maximized region: set it in the original (e.g. mo -> ro-MD).
if w.RegionID != max.RegionID {
w.RegionID = max.RegionID
}
// TODO: should we do the same for scripts?
// See test case: en, sr, nl ; sh ; sr
max, _ = max.Maximize()
} else {
// Base language is not defined.
if h != nil {
for i := range h.haveTags {
have := h.haveTags[i]
if equalsRest(have.tag, w) {
return have, w, Exact
}
}
}
if w.ScriptID == 0 && w.RegionID == 0 {
// We skip all tags matching und for approximate matching, including
// private tags.
continue
}
max, _ = w.Maximize()
if h = m.index[max.LangID]; h == nil {
continue
}
}
pin := true
for _, t := range want[i+1:] {
if w.LangID == t.lang() {
pin = false
break
}
}
// Check for match based on maximized tag.
for i := range h.haveTags {
have := h.haveTags[i]
best.update(have, w, max.ScriptID, max.RegionID, pin)
if best.conf == Exact {
for have.nextMax != 0 {
have = h.haveTags[have.nextMax]
best.update(have, w, max.ScriptID, max.RegionID, pin)
}
return best.have, best.want, best.conf
}
}
}
if best.conf <= No {
if len(want) != 0 {
return nil, want[0].tag(), No
}
return nil, language.Tag{}, No
}
return best.have, best.want, best.conf
}
// bestMatch accumulates the best match so far.
type bestMatch struct {
have *haveTag
want language.Tag
conf Confidence
pinnedRegion language.Region
pinLanguage bool
sameRegionGroup bool
// Cached results from applying tie-breaking rules.
origLang bool
origReg bool
paradigmReg bool
regGroupDist uint8
origScript bool
}
// update updates the existing best match if the new pair is considered to be a
// better match. To determine if the given pair is a better match, it first
// computes the rough confidence level. If this surpasses the current match, it
// will replace it and update the tie-breaker rule cache. If there is a tie, it
// proceeds with applying a series of tie-breaker rules. If there is no
// conclusive winner after applying the tie-breaker rules, it leaves the current
// match as the preferred match.
//
// If pin is true and have and tag are a strong match, it will henceforth only
// consider matches for this language. This corresponds to the nothing that most
// users have a strong preference for the first defined language. A user can
// still prefer a second language over a dialect of the preferred language by
// explicitly specifying dialects, e.g. "en, nl, en-GB". In this case pin should
// be false.
func (m *bestMatch) update(have *haveTag, tag language.Tag, maxScript language.Script, maxRegion language.Region, pin bool) {
// Bail if the maximum attainable confidence is below that of the current best match.
c := have.conf
if c < m.conf {
return
}
// Don't change the language once we already have found an exact match.
if m.pinLanguage && tag.LangID != m.want.LangID {
return
}
// Pin the region group if we are comparing tags for the same language.
if tag.LangID == m.want.LangID && m.sameRegionGroup {
_, sameGroup := regionGroupDist(m.pinnedRegion, have.maxRegion, have.maxScript, m.want.LangID)
if !sameGroup {
return
}
}
if c == Exact && have.maxScript == maxScript {
// If there is another language and then another entry of this language,
// don't pin anything, otherwise pin the language.
m.pinLanguage = pin
}
if equalsRest(have.tag, tag) {
} else if have.maxScript != maxScript {
// There is usually very little comprehension between different scripts.
// In a few cases there may still be Low comprehension. This possibility
// is pre-computed and stored in have.altScript.
if Low < m.conf || have.altScript != maxScript {
return
}
c = Low
} else if have.maxRegion != maxRegion {
if High < c {
// There is usually a small difference between languages across regions.
c = High
}
}
// We store the results of the computations of the tie-breaker rules along
// with the best match. There is no need to do the checks once we determine
// we have a winner, but we do still need to do the tie-breaker computations.
// We use "beaten" to keep track if we still need to do the checks.
beaten := false // true if the new pair defeats the current one.
if c != m.conf {
if c < m.conf {
return
}
beaten = true
}
// Tie-breaker rules:
// We prefer if the pre-maximized language was specified and identical.
origLang := have.tag.LangID == tag.LangID && tag.LangID != 0
if !beaten && m.origLang != origLang {
if m.origLang {
return
}
beaten = true
}
// We prefer if the pre-maximized region was specified and identical.
origReg := have.tag.RegionID == tag.RegionID && tag.RegionID != 0
if !beaten && m.origReg != origReg {
if m.origReg {
return
}
beaten = true
}
regGroupDist, sameGroup := regionGroupDist(have.maxRegion, maxRegion, maxScript, tag.LangID)
if !beaten && m.regGroupDist != regGroupDist {
if regGroupDist > m.regGroupDist {
return
}
beaten = true
}
paradigmReg := isParadigmLocale(tag.LangID, have.maxRegion)
if !beaten && m.paradigmReg != paradigmReg {
if !paradigmReg {
return
}
beaten = true
}
// Next we prefer if the pre-maximized script was specified and identical.
origScript := have.tag.ScriptID == tag.ScriptID && tag.ScriptID != 0
if !beaten && m.origScript != origScript {
if m.origScript {
return
}
beaten = true
}
// Update m to the newly found best match.
if beaten {
m.have = have
m.want = tag
m.conf = c
m.pinnedRegion = maxRegion
m.sameRegionGroup = sameGroup
m.origLang = origLang
m.origReg = origReg
m.paradigmReg = paradigmReg
m.origScript = origScript
m.regGroupDist = regGroupDist
}
}
func isParadigmLocale(lang language.Language, r language.Region) bool {
for _, e := range paradigmLocales {
if language.Language(e[0]) == lang && (r == language.Region(e[1]) || r == language.Region(e[2])) {
return true
}
}
return false
}
// regionGroupDist computes the distance between two regions based on their
// CLDR grouping.
func regionGroupDist(a, b language.Region, script language.Script, lang language.Language) (dist uint8, same bool) {
const defaultDistance = 4
aGroup := uint(regionToGroups[a]) << 1
bGroup := uint(regionToGroups[b]) << 1
for _, ri := range matchRegion {
if language.Language(ri.lang) == lang && (ri.script == 0 || language.Script(ri.script) == script) {
group := uint(1 << (ri.group &^ 0x80))
if 0x80&ri.group == 0 {
if aGroup&bGroup&group != 0 { // Both regions are in the group.
return ri.distance, ri.distance == defaultDistance
}
} else {
if (aGroup|bGroup)&group == 0 { // Both regions are not in the group.
return ri.distance, ri.distance == defaultDistance
}
}
}
}
return defaultDistance, true
}
// equalsRest compares everything except the language.
func equalsRest(a, b language.Tag) bool {
// TODO: don't include extensions in this comparison. To do this efficiently,
// though, we should handle private tags separately.
return a.ScriptID == b.ScriptID && a.RegionID == b.RegionID && a.VariantOrPrivateUseTags() == b.VariantOrPrivateUseTags()
}
// isExactEquivalent returns true if canonicalizing the language will not alter
// the script or region of a tag.
func isExactEquivalent(l language.Language) bool {
for _, o := range notEquivalent {
if o == l {
return false
}
}
return true
}
var notEquivalent []language.Language
func init() {
// Create a list of all languages for which canonicalization may alter the
// script or region.
for _, lm := range language.AliasMap {
tag := language.Tag{LangID: language.Language(lm.From)}
if tag, _ = canonicalize(All, tag); tag.ScriptID != 0 || tag.RegionID != 0 {
notEquivalent = append(notEquivalent, language.Language(lm.From))
}
}
// Maximize undefined regions of paradigm locales.
for i, v := range paradigmLocales {
t := language.Tag{LangID: language.Language(v[0])}
max, _ := t.Maximize()
if v[1] == 0 {
paradigmLocales[i][1] = uint16(max.RegionID)
}
if v[2] == 0 {
paradigmLocales[i][2] = uint16(max.RegionID)
}
}
}