Suggest post language correction (#2486)

* feat: suggested language

* fix: wording correction

* Factor out SuggestedLanguage into a separate component

* Tighten the language-suggestion confidence to avoid false positives

* Tweak the copy and UI

* Add function fallbacks for safari

---------

Co-authored-by: Mary <pineapplecreamcheese@skiff.com>

3 files changed, 111 insertions, 0 deletions

diff --git a/src/locale/helpers.ts b/src/locale/helpers.ts
index 433dbaec6..8b3bf5f3d 100644
--- a/src/locale/helpers.ts
+++ b/src/locale/helpers.ts
@@ -22,6 +22,14 @@ export function code3ToCode2(lang: string): string {
   return lang
 }
 
+export function code3ToCode2Strict(lang: string): string | undefined {
+  if (lang.length === 3) {
+    return LANGUAGES_MAP_CODE3[lang]?.code2
+  }
+
+  return undefined
+}
+
 export function codeToLanguageName(lang: string): string {
   const lang2 = code3ToCode2(lang)
   return LANGUAGES_MAP_CODE2[lang2]?.name || lang
diff --git a/src/view/com/composer/Composer.tsx b/src/view/com/composer/Composer.tsx
index e24fdcf3e..1c2e126c8 100644
--- a/src/view/com/composer/Composer.tsx
+++ b/src/view/com/composer/Composer.tsx
@@ -45,6 +45,7 @@ import {Gallery} from './photos/Gallery'
 import {MAX_GRAPHEME_LENGTH} from 'lib/constants'
 import {LabelsBtn} from './labels/LabelsBtn'
 import {SelectLangBtn} from './select-language/SelectLangBtn'
+import {SuggestedLanguage} from './select-language/SuggestedLanguage'
 import {insertMentionAt} from 'lib/strings/mention-manip'
 import {Trans, msg} from '@lingui/macro'
 import {useLingui} from '@lingui/react'
@@ -454,6 +455,7 @@ export const ComposePost = observer(function ComposePost({
               ))}
           </View>
         ) : null}
+        <SuggestedLanguage text={richtext.text} />
         <View style={[pal.border, styles.bottomBar]}>
           {canSelectImages ? (
             <>
diff --git a/src/view/com/composer/select-language/SuggestedLanguage.tsx b/src/view/com/composer/select-language/SuggestedLanguage.tsx
new file mode 100644
index 000000000..987d89d36
--- /dev/null
+++ b/src/view/com/composer/select-language/SuggestedLanguage.tsx
@@ -0,0 +1,101 @@
+import React, {useEffect, useState} from 'react'
+import {StyleSheet, View} from 'react-native'
+import lande from 'lande'
+import {Trans, msg} from '@lingui/macro'
+import {useLingui} from '@lingui/react'
+import {Text} from '../../util/text/Text'
+import {Button} from '../../util/forms/Button'
+import {code3ToCode2Strict, codeToLanguageName} from '#/locale/helpers'
+import {
+  toPostLanguages,
+  useLanguagePrefs,
+  useLanguagePrefsApi,
+} from '#/state/preferences/languages'
+import {usePalette} from '#/lib/hooks/usePalette'
+import {s} from '#/lib/styles'
+import {
+  FontAwesomeIcon,
+  FontAwesomeIconStyle,
+} from '@fortawesome/react-native-fontawesome'
+
+// fallbacks for safari
+const onIdle = globalThis.requestIdleCallback || (cb => setTimeout(cb, 1))
+const cancelIdle = globalThis.cancelIdleCallback || clearTimeout
+
+export function SuggestedLanguage({text}: {text: string}) {
+  const [suggestedLanguage, setSuggestedLanguage] = useState<string>()
+  const langPrefs = useLanguagePrefs()
+  const setLangPrefs = useLanguagePrefsApi()
+  const pal = usePalette('default')
+  const {_} = useLingui()
+
+  useEffect(() => {
+    const textTrimmed = text.trim()
+
+    // Don't run the language model on small posts, the results are likely
+    // to be inaccurate anyway.
+    if (textTrimmed.length < 40) {
+      setSuggestedLanguage(undefined)
+      return
+    }
+
+    const idle = onIdle(() => {
+      // Only select languages that have a high confidence and convert to code2
+      const result = lande(textTrimmed).filter(
+        ([lang, value]) => value >= 0.97 && code3ToCode2Strict(lang),
+      )
+
+      setSuggestedLanguage(
+        result.length > 0 ? code3ToCode2Strict(result[0][0]) : undefined,
+      )
+    })
+
+    return () => cancelIdle(idle)
+  }, [text])
+
+  return suggestedLanguage &&
+    !toPostLanguages(langPrefs.postLanguage).includes(suggestedLanguage) ? (
+    <View style={[pal.border, styles.infoBar]}>
+      <FontAwesomeIcon
+        icon="language"
+        style={pal.text as FontAwesomeIconStyle}
+        size={24}
+      />
+      <Text style={[pal.text, s.flex1]}>
+        <Trans>
+          Are you writing in{' '}
+          <Text type="sm-bold" style={pal.text}>
+            {codeToLanguageName(suggestedLanguage)}
+          </Text>
+          ?
+        </Trans>
+      </Text>
+
+      <Button
+        type="default"
+        onPress={() => setLangPrefs.setPostLanguage(suggestedLanguage)}
+        accessibilityLabel={_(
+          msg`Change post language to ${codeToLanguageName(suggestedLanguage)}`,
+        )}
+        accessibilityHint="">
+        <Text type="button" style={[pal.link, s.fw600]}>
+          <Trans>Yes</Trans>
+        </Text>
+      </Button>
+    </View>
+  ) : null
+}
+
+const styles = StyleSheet.create({
+  infoBar: {
+    flexDirection: 'row',
+    alignItems: 'center',
+    gap: 10,
+    borderWidth: 1,
+    borderRadius: 6,
+    paddingHorizontal: 16,
+    paddingVertical: 12,
+    marginHorizontal: 10,
+    marginBottom: 10,
+  },
+})