Improve locale selection algorithm

Don't include locales which only have rails translations in
the candidates, and ensure that user specified options take
priority over less specific variants of earlier choices.

.rubocop_todo.yml

@@ -18,7 +18,7 @@ require:
 # Configuration parameters: AutoCorrect, AllowHeredoc, AllowURI, URISchemes, IgnoreCopDirectives, IgnoredPatterns.
 # URISchemes: http, https
 Layout/LineLength:
-  Max: 260
+  Max: 370
 
 # Offense count: 35
 # Configuration parameters: AllowSafeAssignment.

lib/locale.rb

@@ -15,7 +15,11 @@ class Locale < I18n::Locale::Tag::Rfc4646
     end
 
     def expand
-      map(&:candidates).flatten.uniq << Locale.default
+      List.new(reverse.each_with_object([]) do |locale, expanded|
+                 locale.candidates.uniq.reverse_each do |candidate|
+                   expanded << candidate if candidate == locale || !expanded.include?(candidate)
+                 end
+               end.reverse.uniq << Locale.default)
     end
   end
 
@@ -28,17 +32,23 @@ class Locale < I18n::Locale::Tag::Rfc4646
   end
 
   def self.available
-    @available ||= List.new(I18n.available_locales)
+    @available ||= List.new(I18n.available_locales).reject!(&:invalid?)
+  end
+
+  def invalid?
+    I18n.t("activerecord.models.acl", :locale => self, :fallback => false, :raise => true).nil?
+  rescue I18n::MissingTranslationData
+    true
   end
 
   def candidates
-    [self.class.new(language, script, region, variant),
-     self.class.new(language, script, region),
-     self.class.new(language, script, nil, variant),
-     self.class.new(language, script),
-     self.class.new(language, nil, region, variant),
-     self.class.new(language, nil, region),
-     self.class.new(language, nil, nil, variant),
-     self.class.new(language)]
+    List.new([self.class.new(language, script, region, variant),
+              self.class.new(language, script, region),
+              self.class.new(language, script, nil, variant),
+              self.class.new(language, script),
+              self.class.new(language, nil, region, variant),
+              self.class.new(language, nil, region),
+              self.class.new(language, nil, nil, variant),
+              self.class.new(language)])
   end
 end

test/integration/user_creation_test.rb

@@ -32,7 +32,7 @@ class UserCreationTest < ActionDispatch::IntegrationTest
   end
 
   def test_user_create_submit_duplicate_email
-    I18n.available_locales.each do |locale|
+    Locale.available.each do |locale|
       dup_email = create(:user).email
       display_name = "#{locale}_new_tester"
       assert_difference("User.count", 0) do
@@ -50,7 +50,7 @@ class UserCreationTest < ActionDispatch::IntegrationTest
       end
       assert_response :success
       assert_template "users/new"
-      assert_equal locale.to_s, response.headers["Content-Language"] unless locale == :root
+      assert_equal locale.to_s, response.headers["Content-Language"]
       assert_select "form > fieldset > div.standard-form-row > input.field_with_errors#user_email"
       assert_no_missing_translations
     end

test/lib/locale_test.rb

@@ -80,7 +80,7 @@ class LocaleTest < ActiveSupport::TestCase
   end
 
   def test_available
-    assert_equal I18n.available_locales.count, Locale.available.count
+    assert Locale.available.count <= I18n.available_locales.count
   end
 
   def test_preferred
@@ -91,9 +91,7 @@ class LocaleTest < ActiveSupport::TestCase
     assert_equal "de", Locale.available.preferred(Locale.list("zh-Hant", "de")).to_s
     assert_equal "zh-TW", Locale.available.preferred(Locale.list("zh-Hant-TW", "de")).to_s
     assert_equal "zh-TW", Locale.available.preferred(Locale.list("zh-TW", "de")).to_s
-    assert_equal "zh-HK", Locale.available.preferred(Locale.list("yue", "zh-HK", "de")).to_s
-    assert_equal "zh-yue", Locale.available.preferred(Locale.list("yue", "zh-yue", "zh-HK", "de")).to_s
-    assert_equal "zh-yue", Locale.available.preferred(Locale.list("yue", "zh-YUE", "zh-HK", "de")).to_s
+    assert_equal "zh-TW", Locale.available.preferred(Locale.list("zh-HK", "zh-hk", "zh-Hant", "zh-hant", "zh-TW", "zh-tw", "zh", "zh-yue", "yue", "yue-Hant", "yue-HK", "yue-Hans", "zh-classical", "lzh", "ja-Hani", "ko-Hani", "ko_hanja", "vi-Hani", "ja-hani", "ko-hani", "vi-hani", "en-HK", "en-hk", "en-SG", "en-sg", "en-GB", "en-gb", "en-US", "en-us", "en", "ja")).to_s
     assert_equal "en", Locale.available.preferred(Locale.list("yue")).to_s
   end
 end