From 2f2cb2c4fcb2b4c5a6cefb9277c2eabb4f282c96 Mon Sep 17 00:00:00 2001
From: Alex Gleason <alex@alexgleason.me>
Date: Tue, 4 Feb 2025 15:05:52 -0600
Subject: [PATCH] detectLanguage: check the text's script for definitive
 language categorization for some languages

---
 src/utils/language.test.ts | 15 +++++++++++++++
 src/utils/language.ts      | 30 ++++++++++++++++++++++++------
 2 files changed, 39 insertions(+), 6 deletions(-)

diff --git a/src/utils/language.test.ts b/src/utils/language.test.ts
index 255f6b58..f4025290 100644
--- a/src/utils/language.test.ts
+++ b/src/utils/language.test.ts
@@ -26,3 +26,18 @@ Deno.test('Detect English language', () => {
     'en',
   );
 });
+
+Deno.test('Detects definitive texts', () => {
+  // NOTE: pass `1` as min confidence to test only the definitive patterns
+
+  // unambiguous
+  assertEquals(detectLanguage('안녕하세요.', 1), 'ko');
+  assertEquals(detectLanguage('Γειά σου!', 1), 'el');
+  assertEquals(detectLanguage('שלום!', 1), 'he');
+  assertEquals(detectLanguage('こんにちは。', 1), 'ja');
+
+  // ambiguous
+  assertEquals(detectLanguage('你好', 1), undefined);
+  assertEquals(detectLanguage('Привет', 1), undefined);
+  assertEquals(detectLanguage('Hello', 1), undefined);
+});
diff --git a/src/utils/language.ts b/src/utils/language.ts
index 8af8ddf9..4b6e3807 100644
--- a/src/utils/language.ts
+++ b/src/utils/language.ts
@@ -4,8 +4,9 @@ import linkify from 'linkifyjs';
 
 linkify.registerCustomProtocol('nostr', true);
 
-/** Returns the detected language if the confidence is greater or equal than 'minConfidence'
- * 'minConfidence' must be a number between 0 and 1, such as 0.95
+/**
+ * Returns the detected language if the confidence is greater or equal than 'minConfidence'.
+ * 'minConfidence' must be a number between 0 and 1, such as 0.95.
  */
 export function detectLanguage(text: string, minConfidence: number): LanguageCode | undefined {
   // It's better to remove the emojis first
@@ -15,13 +16,31 @@ export function detectLanguage(text: string, minConfidence: number): LanguageCod
       .replaceAll(/[\s\uFEFF\u00A0\u200B-\u200D\u{0FE0E}]+/gu, ' '),
   ).reduce((acc, { t, v }) => t === 'text' ? acc + v : acc, '').trim();
 
+  // Definite patterns for some languages.
+  // Text which matches MUST unambiguously be in the given language.
+  // This is only possible for some languages.
+  // All patterns match the full text, so mixed scripts would fail these tests.
+  const languagePatterns: Partial<Record<LanguageCode, RegExp>> = {
+    ko: /^[\p{Script=Hangul}\s]+$/u, // Korean (Hangul only)
+    el: /^[\p{Script=Greek}\s]+$/u, // Greek
+    he: /^[\p{Script=Hebrew}\s]+$/u, // Hebrew
+    ja: /^(?=.*[\p{Script=Hiragana}\p{Script=Katakana}])[\p{Script=Hiragana}\p{Script=Katakana}\p{Script=Han}\s]+$/u, // Japanese (requires at least one Kana)
+    // zh: not possible to detect unambiguously
+  };
+
+  // If any pattern matches, the language is known.
+  for (const [lang, pattern] of Object.entries(languagePatterns) as [LanguageCode, RegExp][]) {
+    if (pattern.test(text.replace(/[\p{P}\p{S}]/gu, ''))) { // strip punctuation and symbols before checking
+      return lang;
+    }
+  }
+
   if (sanitizedText.length < 10) { // heuristics
     return;
   }
 
-  const [topResult] = lande(
-    sanitizedText,
-  );
+  const [topResult] = lande(sanitizedText);
+
   if (topResult) {
     const [iso6393, confidence] = topResult;
     const locale = new Intl.Locale(iso6393);
@@ -30,5 +49,4 @@ export function detectLanguage(text: string, minConfidence: number): LanguageCod
       return locale.language as LanguageCode;
     }
   }
-  return;
 }