Add support for Welsh mutations

This commit addresses feedback from the Welsh translation changes in
issue #233. Specifically, we need to apply Welsh consonant mutations in
some cases. The mechanism this commit adds is possibly more than is
needed for the presenting issue, but we are expecting more mutions to be
needed in future.

This change addresses the feedback that some months should be mutated.
E.g:

    O Mai

should become:

    O Fai

app/lib/grammar.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+# Entry wrapper for applying generalised grammar rules. Mostly
+# this will be used for Welsh grammar actions, such as consonant
+# mutations
+class Grammar
+  def self.apply(options)
+    grammar_action = WelshGrammar.apply(options) if I18n.locale == :cy
+
+    grammar_action || GrammarAction.identity_action(options)
+  end
+end

app/lib/mutation.rb

@@ -14,6 +14,9 @@ def match?(options)
   end
 
   def apply(options)
-    options[:source]&.sub(pattern, replacement)
+    GrammarAction.new(
+      options,
+      options[:source]&.sub(pattern, replacement)
+    )
   end
-end
+end

app/lib/welsh_grammar.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-Mutation = Struct.new(:pattern, :replacement)
-
 MUTATIONS = {
   'o' =>
     [
@@ -17,19 +15,21 @@
 
 # Assistance with formulating correct Welsh grammar
 class WelshGrammar
+  def self.apply(options)
+    mutate(options)
+  end
+
   def self.mutate(options)
     result = nil
 
-    if I18n.locale == 'cy'
+    if I18n.locale == :cy
       result = mutate_prefix(options[:assuming_prefix], options) if options[:assuming_prefix]
     end
 
     result || GrammarAction.identity_action(options)
   end
 
-  private
-
-  def mutate_prefix(prefix, options)
+  def self.mutate_prefix(prefix, options)
     return nil unless (mutations = MUTATIONS[prefix])
 
     mutations

app/presenters/landing_state.rb

@@ -24,7 +24,15 @@ def period
     if month
       ref_month = to_value(month)
       date = Date.strptime(ref_month.value, '%Y-%m')
-      I18n.l(date, format: '%B %Y')
+
+      # Uncomfortable choice here: we know that, in the Welsh translation, the
+      # month will be preceded by 'O' (i.e. 'From'), which will trigger a mutation
+      # for some month names. The mutation will only happen in Welsh-lang mode, but
+      # we have to allow for that case so we apply the Grammar -> GrammarAction
+      # transform even in English mode (where it's not really needed)
+      Grammar
+        .apply(source: I18n.l(date, format: '%B %Y'), assuming_prefix: 'o')
+        .result
     else
       I18n.t('landing.not_available')
     end

test/lib/welsh_grammar_test.rb

@@ -2,15 +2,40 @@
 
 require 'test_helper'
 
+# Unit tests on Welsh grammar support
 class WelshGrammarTest < ActiveSupport::TestCase
   describe 'WelshGrammar' do
+    teardown do
+      I18n.locale = :en
+    end
+
     describe 'mutations' do
+      before do
+        I18n.locale = :en
+      end
+
+      it 'make no changes if language is not Welsh' do
+        _(
+          WelshGrammar
+            .mutate(source: 'Ionawr', assuming_prefix: 'o')
+            .result
+        ).must_equal('Ionawr')
+      end
+
       it 'should mutate months that are prefix by "O" if necessary' do
+        I18n.locale = :cy
+
         _(
           WelshGrammar
             .mutate(source: 'Ionawr', assuming_prefix: 'o')
             .result
         ).must_equal('Ionawr')
+
+        _(
+          WelshGrammar
+            .mutate(source: 'Mai', assuming_prefix: 'o')
+            .result
+        ).must_equal('Fai')
       end
     end
   end