From 1b167707c2ae5ebc272afc3a7515f5b236083135 Mon Sep 17 00:00:00 2001
From: Eugen Rochko <eugen@zeonfederated.com>
Date: Fri, 15 Mar 2019 05:07:09 +0100
Subject: [PATCH] Fix language detection of non-latin alphabets even at few
 characters (#10276)

---
 app/lib/language_detector.rb       | 31 +++++++++++++++++++++++-------
 spec/lib/language_detector_spec.rb |  4 ++--
 2 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/app/lib/language_detector.rb b/app/lib/language_detector.rb
index 58c8e206940..70a9084d1b4 100644
--- a/app/lib/language_detector.rb
+++ b/app/lib/language_detector.rb
@@ -3,7 +3,8 @@
 class LanguageDetector
   include Singleton
 
-  CHARACTER_THRESHOLD = 140
+  CHARACTER_THRESHOLD    = 140
+  RELIABLE_CHARACTERS_RE = /[\p{Hebrew}\p{Arabic}\p{Syriac}\p{Thaana}\p{Nko}\p{Han}\p{Katakana}\p{Hiragana}\p{Hangul}]+/m
 
   def initialize
     @identifier = CLD3::NNetLanguageIdentifier.new(1, 2048)
@@ -11,15 +12,14 @@ class LanguageDetector
 
   def detect(text, account)
     input_text = prepare_text(text)
+
     return if input_text.blank?
 
     detect_language_code(input_text) || default_locale(account)
   end
 
   def language_names
-    @language_names =
-      CLD3::TaskContextParams::LANGUAGE_NAMES.map { |name| iso6391(name.to_s).to_sym }
-                                             .uniq
+    @language_names = CLD3::TaskContextParams::LANGUAGE_NAMES.map { |name| iso6391(name.to_s).to_sym }.uniq
   end
 
   private
@@ -29,12 +29,29 @@ class LanguageDetector
   end
 
   def unreliable_input?(text)
-    text.size < CHARACTER_THRESHOLD
+    !reliable_input?(text)
+  end
+
+  def reliable_input?(text)
+    sufficient_text_length?(text) || language_specific_character_set?(text)
+  end
+
+  def sufficient_text_length?(text)
+    text.size >= CHARACTER_THRESHOLD
+  end
+
+  def language_specific_character_set?(text)
+    words = text.scan(RELIABLE_CHARACTERS_RE)
+
+    if words.present?
+      words.reduce(0) { |acc, elem| acc + elem.size }.to_f / text.size.to_f > 0.3
+    else
+      false
+    end
   end
 
   def detect_language_code(text)
     return if unreliable_input?(text)
-
     result = @identifier.find_language(text)
     iso6391(result.language.to_s).to_sym if result.reliable?
   end
@@ -77,6 +94,6 @@ class LanguageDetector
   end
 
   def default_locale(account)
-    return account.user_locale&.to_sym || I18n.default_locale if account.local?
+    account.user_locale&.to_sym || I18n.default_locale if account.local?
   end
 end
diff --git a/spec/lib/language_detector_spec.rb b/spec/lib/language_detector_spec.rb
index 0fa2a59eff9..0cb70605ad4 100644
--- a/spec/lib/language_detector_spec.rb
+++ b/spec/lib/language_detector_spec.rb
@@ -106,11 +106,11 @@ describe LanguageDetector do
       end
 
       describe 'remote user' do
-        it 'nil for foreign user when language is not present' do
+        it 'detects Korean language' do
           string = '안녕하세요'
           result = described_class.instance.detect(string, account_remote)
 
-          expect(result).to eq nil
+          expect(result).to eq :ko
         end
       end