From 2f53ef0d01b0e86ecc0c634344711ad24265aa38 Mon Sep 17 00:00:00 2001
From: John Backus <jbackus@stanford.edu>
Date: Thu, 20 Aug 2015 01:42:09 -0700
Subject: [PATCH] Add mutations from `and`/`or` into `if`/`else`

Closes #233
---
 Changelog.md                      |  1 +
 lib/mutant/mutator/node/binary.rb | 17 +++++++++++++++++
 meta/and.rb                       |  1 +
 meta/or.rb                        |  1 +
 4 files changed, 20 insertions(+)

diff --git a/Changelog.md b/Changelog.md
index a1b4e6e1..df33fb4c 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -3,6 +3,7 @@
 * Remove invalid mutation `super(...)` to `super`
 * Add mutation from `def foo(a = true); end` to `def foo(a = true); a = true; end` #419
 * Add mutation from `def foo; end` to `remove_method :foo` #413
+* Add mutation from `a or b` to `b unless a` and `a and b` to `b if a` #430
 
 # v0.8.2 2015-08-11
 
diff --git a/lib/mutant/mutator/node/binary.rb b/lib/mutant/mutator/node/binary.rb
index 41fe8f67..9a46c9d6 100644
--- a/lib/mutant/mutator/node/binary.rb
+++ b/lib/mutant/mutator/node/binary.rb
@@ -24,6 +24,7 @@ module Mutant
           emit_singletons
           emit_promotions
           emit_operator_mutations
+          emit_conditional_mutations
           emit_left_negation
           emit_left_mutations
           emit_right_mutations
@@ -38,6 +39,22 @@ module Mutant
           emit(s(INVERSE.fetch(node.type), left, right))
         end
 
+        # Mutate `and` and `or` into `if` and `unless`
+        #
+        # - `a or b` becomes `b unless a`
+        # - `a and b` becomes `b if a`
+        #
+        # @return [undefined]
+        #
+        # @api private
+        def emit_conditional_mutations
+          if n_or?(node)
+            emit(s(:if, left, nil, right))
+          else
+            emit(s(:if, left, right, nil))
+          end
+        end
+
         # Emit promotions
         #
         # @return [undefined]
diff --git a/meta/and.rb b/meta/and.rb
index db7c71d2..278dabdb 100644
--- a/meta/and.rb
+++ b/meta/and.rb
@@ -10,4 +10,5 @@ Mutant::Meta::Example.add do
   mutation 'false and false'
   mutation 'true and true'
   mutation '!true and false'
+  mutation 'false if true'
 end
diff --git a/meta/or.rb b/meta/or.rb
index c452ad7b..833cc06f 100644
--- a/meta/or.rb
+++ b/meta/or.rb
@@ -10,4 +10,5 @@ Mutant::Meta::Example.add do
   mutation 'true or true'
   mutation 'true and false'
   mutation '!true or false'
+  mutation 'false unless true'
 end