Add #reduce -> #sum mutations

Adds:
  * `a.reduce(:+)`     -> `a.sum`
  * `a.reduce(0, &:+)` -> `a.sum(0)`
  * ...

Changelog.md

@@ -1,5 +1,11 @@
 # Unreleased
 
+* [#1202](https://github.com/mbj/mutant/pull/1202)
+
+  * Add `#reduce` -> `#sum` mutations
+    * `a.reduce(:+)`     -> `a.sum`
+    * `a.reduce(0, &:+)` -> `a.sum(0)`
+
 * [#1201](https://github.com/mbj/mutant/pull/1201)
 
   * Add `/\Astatic/` -> `#start_with?` mutations:

lib/mutant/mutator/node/send.rb

@@ -88,6 +88,7 @@ def normal_dispatch
         end
 
         def emit_selector_specific_mutations
+          emit_reduce_to_sum_mutation
           emit_start_end_with_mutations
           emit_predicate_mutations
           emit_array_mutation
@@ -99,6 +100,21 @@ def emit_selector_specific_mutations
           emit_lambda_mutation
         end
 
+        def emit_reduce_to_sum_mutation
+          return unless selector.equal?(:reduce)
+
+          reducer = arguments.last
+
+          return unless reducer.eql?(s(:sym, :+)) || reducer.eql?(s(:block_pass, s(:sym, :+)))
+
+          if arguments.length > 1
+            initial_value = arguments.first
+            emit_type(receiver, :sum, initial_value)
+          else
+            emit_type(receiver, :sum)
+          end
+        end
+
         def emit_start_end_with_mutations # rubocop:disable Metrics/CyclomaticComplexity, Metrics/MethodLength
           return unless REGEXP_MATCH_METHODS.include?(selector) && arguments.one?
 

meta/send.rb

@@ -826,3 +826,61 @@
   mutation 'self.match(/foo\z/)'
   mutation "a.end_with?('foo')"
 end
+
+
+Mutant::Meta::Example.add :send do
+  source 'a.reduce(:+)'
+
+  singleton_mutations
+
+  mutation 'a'
+  mutation 'self.reduce(:+)'
+  mutation 'a.reduce'
+  mutation 'a.reduce(nil)'
+  mutation 'a.reduce(self)'
+  mutation 'a.reduce(:"+__mutant__")'
+  mutation ':+'
+  mutation 'a.sum'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'a.reduce(INITIAL, &:+)'
+
+  singleton_mutations
+
+  mutation 'a'
+  mutation 'a.reduce'
+  mutation 'a.reduce(nil, &:+)'
+  mutation 'a.reduce(self, &:+)'
+  mutation 'a.reduce(INITIAL, &nil)'
+  mutation 'a.reduce(INITIAL, &self)'
+  mutation 'a.reduce(INITIAL)'
+  mutation 'a.reduce(&:+)'
+  mutation 'self.reduce(INITIAL, &:+)'
+  mutation 'a.reduce(INITIAL, &:"+__mutant__")'
+  mutation 'a.sum(INITIAL)'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'reduce(:*)'
+
+  singleton_mutations
+
+  mutation 'reduce'
+  mutation ':*'
+  mutation 'reduce(nil)'
+  mutation 'reduce(self)'
+  mutation 'reduce(:"*__mutant__")'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'foo(:+)'
+
+  singleton_mutations
+
+  mutation 'foo'
+  mutation ':+'
+  mutation 'foo(nil)'
+  mutation 'foo(self)'
+  mutation 'foo(:"+__mutant__")'
+end