From 9c56a0daa2e2dba32370851224c185c7aeb94dac Mon Sep 17 00:00:00 2001
From: Jonathon Jones <jonathon_d_jones@yahoo.com>
Date: Fri, 2 Mar 2012 10:44:42 -0500
Subject: [PATCH] Empty hash assignments no longer make the mutation fail

---
 lib/mutant/literal.rb                        | 12 ++++++++
 lib/mutant/mutatee.rb                        |  3 +-
 lib/mutant/mutation.rb                       |  2 +-
 lib/mutant/node.rb                           |  4 +++
 spec/functional/instance_method/hash_spec.rb | 30 ++++++++++++++++++++
 5 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/lib/mutant/literal.rb b/lib/mutant/literal.rb
index 4bd9277..c84fde9 100644
--- a/lib/mutant/literal.rb
+++ b/lib/mutant/literal.rb
@@ -13,7 +13,15 @@ def swap
       @class.new(@node).swap
     end
 
+    def changes_on_swap?
+      @class.changes_on_swap?(@node)
+    end
+
     class BaseLiteral
+      def self.changes_on_swap?(node)
+        true
+      end
+
       def initialize(node)
         @node = node
       end
@@ -83,6 +91,10 @@ def swap
     end
 
     class HashLiteral < BaseLiteral
+      def self.changes_on_swap?(node)
+        node.array != []
+      end
+
       def swap
         new_body = @node.array.each_slice(2).inject([]) do |body, (key, value)|
           new_value = literal_class(value).new(value.clone).swap
diff --git a/lib/mutant/mutatee.rb b/lib/mutant/mutatee.rb
index d44cc09..0cf138e 100644
--- a/lib/mutant/mutatee.rb
+++ b/lib/mutant/mutatee.rb
@@ -21,7 +21,8 @@ def clean
 
     def set_mutations
       nodes.each do |node|
-        @mutations << Mutation.new(node, body.array)
+        mutation = Mutation.new(node, body.array)
+        @mutations << mutation if mutation.mutatable?
       end
     end
 
diff --git a/lib/mutant/mutation.rb b/lib/mutant/mutation.rb
index fa38ee1..67bbf5f 100644
--- a/lib/mutant/mutation.rb
+++ b/lib/mutant/mutation.rb
@@ -10,7 +10,7 @@ def initialize(node, array)
       @mutated = false
     end
 
-    def_delegators :@node, :line, :from, :to
+    def_delegators :@node, :line, :from, :to, :mutatable?
 
     def mutated?
       @mutated
diff --git a/lib/mutant/node.rb b/lib/mutant/node.rb
index 82db97b..ca9a9ef 100644
--- a/lib/mutant/node.rb
+++ b/lib/mutant/node.rb
@@ -22,5 +22,9 @@ def to
     def swap
       @copy = Literal.new(copy).swap
     end
+
+    def mutatable?
+      Literal.new(@copy).changes_on_swap?
+    end
   end
 end
diff --git a/spec/functional/instance_method/hash_spec.rb b/spec/functional/instance_method/hash_spec.rb
index 8af30c1..65891ec 100644
--- a/spec/functional/instance_method/hash_spec.rb
+++ b/spec/functional/instance_method/hash_spec.rb
@@ -2,6 +2,36 @@
 
 describe 'Mutating hashes' do
   context 'for an instance method' do
+    context 'that contains {}' do
+      before do
+        write_file 'thing.rb', """
+          class Thing
+            def to_hash
+              {}
+            end
+          end
+        """
+      end
+
+      context 'with an expectation that the method returns {}' do
+        before do
+          write_file 'spec/thing_spec.rb', """
+            $: << '.'
+            require 'thing'
+
+            describe 'Thing#to_hash' do
+              specify { Thing.new.to_hash.should === {} }
+            end
+          """
+          mutate 'Thing#to_hash spec/thing_spec.rb'
+        end
+
+        specify 'there are no possible mutations' do
+          all_output.should include('no possible mutations')
+        end
+      end
+    end
+
     context 'that contains {:foo => {:bar => 3}}' do
       before do
         write_file 'thing.rb', """