Add nthref mutator

Closes #20

lib/mutant.rb

@@ -73,6 +73,7 @@ require 'mutant/mutator/node/send/binary'
 require 'mutant/mutator/node/when'
 require 'mutant/mutator/node/define'
 require 'mutant/mutator/node/mlhs'
+require 'mutant/mutator/node/nthref'
 require 'mutant/mutator/node/masgn'
 require 'mutant/mutator/node/return'
 require 'mutant/mutator/node/block'

lib/mutant/mutator/node/generic.rb

@@ -15,7 +15,7 @@ module Mutant
           :blockarg, :op_asgn, :and_asgn,
           :regopt, :restarg, :resbody, :retry, :arg_expr,
           :kwrestarg, :kwoptarg, :kwarg, :undef, :module, :empty,
-          :alias, :for, :xstr, :back_ref, :nth_ref, :class,
+          :alias, :for, :xstr, :back_ref, :class,
           :sclass, :match_with_lvasgn, :match_current_line, :or_asgn, :kwbegin
         )
 

lib/mutant/mutator/node/nthref.rb

@@ -0,0 +1,27 @@
+module Mutant
+  class Mutator
+    class Node
+      # Mutator for nth-ref nodes
+      class NthRef < self
+
+        handle :nth_ref
+
+        children :number
+
+      private
+
+        # Perform dispatch
+        #
+        # @return [undefined]
+        #
+        # @api private
+        #
+        def dispatch
+          emit_number(number - 1)
+          emit_number(number + 1)
+        end
+
+      end # NthRef
+    end # Node
+  end # Mutator
+end # Mutant

spec/unit/mutant/mutator/node/nthref/mutation_spec.rb

@@ -0,0 +1,19 @@
+# encoding: utf-8
+
+require 'spec_helper'
+
+describe Mutant::Mutator, 'nthref' do
+  context '$1' do
+    let(:source)    { '$1' }
+    let(:mutations) { ['$2', '$0']   }
+
+    it_should_behave_like 'a mutator'
+  end
+
+  context '$2' do
+    let(:source)    { '$2' }
+    let(:mutations) { ['$3', '$1']   }
+
+    it_should_behave_like 'a mutator'
+  end
+end