i18n: Avoid rebuilding the Translations map for every lookup

```bash
benchmark                                                    old ns/op     new ns/op     delta
BenchmarkI18nTranslate/all-present-4                         764           757           -0.92%
BenchmarkI18nTranslate/present-in-default-4                  2578          1457          -43.48%
BenchmarkI18nTranslate/present-in-current-4                  764           766           +0.26%
BenchmarkI18nTranslate/missing-4                             3362          1103          -67.19%
BenchmarkI18nTranslate/file-missing-4                        4646          3611          -22.28%
BenchmarkI18nTranslate/context-provided-4                    2013          2014          +0.05%
BenchmarkI18nTranslate/same-id-and-translation-4             1961          784           -60.02%
BenchmarkI18nTranslate/same-id-and-translation-default-4     3717          1405          -62.20%
BenchmarkI18nTranslate/unknown-language-code-4               1775          1787          +0.68%

benchmark                                                    old allocs     new allocs     delta
BenchmarkI18nTranslate/all-present-4                         6              6              +0.00%
BenchmarkI18nTranslate/present-in-default-4                  16             10             -37.50%
BenchmarkI18nTranslate/present-in-current-4                  6              6              +0.00%
BenchmarkI18nTranslate/missing-4                             20             8              -60.00%
BenchmarkI18nTranslate/file-missing-4                        27             21             -22.22%
BenchmarkI18nTranslate/context-provided-4                    15             15             +0.00%
BenchmarkI18nTranslate/same-id-and-translation-4             12             6              -50.00%
BenchmarkI18nTranslate/same-id-and-translation-default-4     22             10             -54.55%
BenchmarkI18nTranslate/unknown-language-code-4               13             13             +0.00%

benchmark                                                    old bytes     new bytes     delta
BenchmarkI18nTranslate/all-present-4                         152           152           +0.00%
BenchmarkI18nTranslate/present-in-default-4                  1144          216           -81.12%
BenchmarkI18nTranslate/present-in-current-4                  152           152           +0.00%
BenchmarkI18nTranslate/missing-4                             2008          152           -92.43%
BenchmarkI18nTranslate/file-missing-4                        1208          600           -50.33%
BenchmarkI18nTranslate/context-provided-4                    704           704           +0.00%
BenchmarkI18nTranslate/same-id-and-translation-4             1080          152           -85.93%
BenchmarkI18nTranslate/same-id-and-translation-default-4     2073          216           -89.58%
BenchmarkI18nTranslate/unknown-language-code-4               696           696           +0.00%
```

Fixes #5892

2 files changed, 55 insertions, 13 deletions

diff --git a/i18n/i18n.go b/i18n/i18n.go
index 25f2d300e..5beef8683 100644
--- a/i18n/i18n.go
+++ b/i18n/i18n.go
@@ -19,6 +19,7 @@ import (
 	"github.com/gohugoio/hugo/helpers"
 
 	"github.com/nicksnyder/go-i18n/i18n/bundle"
+	"github.com/nicksnyder/go-i18n/i18n/translation"
 )
 
 var (
@@ -64,6 +65,8 @@ func (t Translator) initFuncs(bndl *bundle.Bundle) {
 		t.logger.INFO.Printf("No translation bundle found for default language %q", defaultContentLanguage)
 	}
 
+	translations := bndl.Translations()
+
 	enableMissingTranslationPlaceholders := t.cfg.GetBool("enableMissingTranslationPlaceholders")
 	for _, lang := range bndl.LanguageTags() {
 		currentLang := lang
@@ -82,7 +85,7 @@ func (t Translator) initFuncs(bndl *bundle.Bundle) {
 			// then Tfunc returns translationID itself.
 			// But if user set same translationID and translation, we should check
 			// if it really untranslated:
-			if isIDTranslated(currentLang, translationID, bndl) {
+			if isIDTranslated(translations, currentLang, translationID) {
 				return translated
 			}
 
@@ -97,7 +100,7 @@ func (t Translator) initFuncs(bndl *bundle.Bundle) {
 				if translated != translationID {
 					return translated
 				}
-				if isIDTranslated(defaultContentLanguage, translationID, bndl) {
+				if isIDTranslated(translations, defaultContentLanguage, translationID) {
 					return translated
 				}
 			}
@@ -106,9 +109,9 @@ func (t Translator) initFuncs(bndl *bundle.Bundle) {
 	}
 }
 
-// If bndl contains the translationID for specified currentLang,
+// If the translation map contains translationID for specified currentLang,
 // then the translationID is actually translated.
-func isIDTranslated(lang, id string, b *bundle.Bundle) bool {
-	_, contains := b.Translations()[lang][id]
+func isIDTranslated(translations map[string]map[string]translation.Translation, lang, id string) bool {
+	_, contains := translations[lang][id]
 	return contains
 }
diff --git a/i18n/i18n_test.go b/i18n/i18n_test.go
index 5db539d30..b67cabc55 100644
--- a/i18n/i18n_test.go
+++ b/i18n/i18n_test.go
@@ -23,18 +23,19 @@ import (
 	"github.com/gohugoio/hugo/htesting"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/spf13/afero"
+	"github.com/spf13/viper"
 
 	"github.com/gohugoio/hugo/deps"
 
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/hugofs"
-	"github.com/spf13/viper"
 	"github.com/stretchr/testify/require"
 )
 
 var logger = loggers.NewErrorLogger()
 
 type i18nTest struct {
+	name                             string
 	data                             map[string][]byte
 	args                             interface{}
 	lang, id, expected, expectedFlag string
@@ -43,6 +44,7 @@ type i18nTest struct {
 var i18nTests = []i18nTest{
 	// All translations present
 	{
+		name: "all-present",
 		data: map[string][]byte{
 			"en.toml": []byte("[hello]\nother = \"Hello, World!\""),
 			"es.toml": []byte("[hello]\nother = \"¡Hola, Mundo!\""),
@@ -55,6 +57,7 @@ var i18nTests = []i18nTest{
 	},
 	// Translation missing in current language but present in default
 	{
+		name: "present-in-default",
 		data: map[string][]byte{
 			"en.toml": []byte("[hello]\nother = \"Hello, World!\""),
 			"es.toml": []byte("[goodbye]\nother = \"¡Adiós, Mundo!\""),
@@ -67,6 +70,7 @@ var i18nTests = []i18nTest{
 	},
 	// Translation missing in default language but present in current
 	{
+		name: "present-in-current",
 		data: map[string][]byte{
 			"en.toml": []byte("[goodbye]\nother = \"Goodbye, World!\""),
 			"es.toml": []byte("[hello]\nother = \"¡Hola, Mundo!\""),
@@ -79,6 +83,7 @@ var i18nTests = []i18nTest{
 	},
 	// Translation missing in both default and current language
 	{
+		name: "missing",
 		data: map[string][]byte{
 			"en.toml": []byte("[goodbye]\nother = \"Goodbye, World!\""),
 			"es.toml": []byte("[goodbye]\nother = \"¡Adiós, Mundo!\""),
@@ -91,6 +96,7 @@ var i18nTests = []i18nTest{
 	},
 	// Default translation file missing or empty
 	{
+		name: "file-missing",
 		data: map[string][]byte{
 			"en.toml": []byte(""),
 		},
@@ -102,6 +108,7 @@ var i18nTests = []i18nTest{
 	},
 	// Context provided
 	{
+		name: "context-provided",
 		data: map[string][]byte{
 			"en.toml": []byte("[wordCount]\nother = \"Hello, {{.WordCount}} people!\""),
 			"es.toml": []byte("[wordCount]\nother = \"¡Hola, {{.WordCount}} gente!\""),
@@ -119,6 +126,7 @@ var i18nTests = []i18nTest{
 	// Same id and translation in current language
 	// https://github.com/gohugoio/hugo/issues/2607
 	{
+		name: "same-id-and-translation",
 		data: map[string][]byte{
 			"es.toml": []byte("[hello]\nother = \"hello\""),
 			"en.toml": []byte("[hello]\nother = \"hi\""),
@@ -131,6 +139,7 @@ var i18nTests = []i18nTest{
 	},
 	// Translation missing in current language, but same id and translation in default
 	{
+		name: "same-id-and-translation-default",
 		data: map[string][]byte{
 			"es.toml": []byte("[bye]\nother = \"bye\""),
 			"en.toml": []byte("[hello]\nother = \"hello\""),
@@ -143,6 +152,7 @@ var i18nTests = []i18nTest{
 	},
 	// Unknown language code should get its plural spec from en
 	{
+		name: "unknown-language-code",
 		data: map[string][]byte{
 			"en.toml": []byte(`[readingTime]
 one ="one minute read"
@@ -159,24 +169,29 @@ other = "{{ .Count }} minuttar lesing"`),
 	},
 }
 
-func doTestI18nTranslate(t *testing.T, test i18nTest, cfg config.Provider) string {
+func doTestI18nTranslate(t testing.TB, test i18nTest, cfg config.Provider) string {
+	tp := prepareTranslationProvider(t, test, cfg)
+	f := tp.t.Func(test.lang)
+	return f(test.id, test.args)
+
+}
+
+func prepareTranslationProvider(t testing.TB, test i18nTest, cfg config.Provider) *TranslationProvider {
 	assert := require.New(t)
 	fs := hugofs.NewMem(cfg)
-	tp := NewTranslationProvider()
 
 	for file, content := range test.data {
 		err := afero.WriteFile(fs.Source, filepath.Join("i18n", file), []byte(content), 0755)
 		assert.NoError(err)
 	}
 
+	tp := NewTranslationProvider()
 	depsCfg := newDepsConfig(tp, cfg, fs)
 	d, err := deps.New(depsCfg)
 	assert.NoError(err)
-
 	assert.NoError(d.LoadResources())
-	f := tp.t.Func(test.lang)
-	return f(test.id, test.args)
 
+	return tp
 }
 
 func newDepsConfig(tp *TranslationProvider, cfg config.Provider, fs *hugofs.Fs) deps.DepsCfg {
@@ -193,8 +208,7 @@ func newDepsConfig(tp *TranslationProvider, cfg config.Provider, fs *hugofs.Fs)
 	}
 }
 
-func TestI18nTranslate(t *testing.T) {
-	var actual, expected string
+func getConfig() *viper.Viper {
 	v := viper.New()
 	v.SetDefault("defaultContentLanguage", "en")
 	v.Set("contentDir", "content")
@@ -205,6 +219,13 @@ func TestI18nTranslate(t *testing.T) {
 	v.Set("assetDir", "assets")
 	v.Set("resourceDir", "resources")
 	v.Set("publishDir", "public")
+	return v
+
+}
+
+func TestI18nTranslate(t *testing.T) {
+	var actual, expected string
+	v := getConfig()
 
 	// Test without and with placeholders
 	for _, enablePlaceholders := range []bool{false, true} {
@@ -221,3 +242,21 @@ func TestI18nTranslate(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkI18nTranslate(b *testing.B) {
+	v := getConfig()
+	for _, test := range i18nTests {
+		b.Run(test.name, func(b *testing.B) {
+			tp := prepareTranslationProvider(b, test, v)
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				f := tp.t.Func(test.lang)
+				actual := f(test.id, test.args)
+				if actual != test.expected {
+					b.Fatalf("expected %v got %v", test.expected, actual)
+				}
+			}
+		})
+	}
+
+}