Add patch format.

README.rdoc

@@ -72,6 +72,7 @@ Need a different output format?  We've got a few of those too.
   Differ.format = :ascii  # <- Default
   Differ.format = :color
   Differ.format = :html
+  Differ.format = :patch
 
   Differ.format = MyCustomFormatModule
 

differ.gemspec

@@ -25,6 +25,7 @@ Gem::Specification.new do |s|
     "lib/differ/format/ascii.rb",
     "lib/differ/format/color.rb",
     "lib/differ/format/html.rb",
+    "lib/differ/format/patch.rb",
     "lib/differ/string.rb",
     "spec/differ/change_spec.rb",
     "spec/differ/diff_spec.rb",

lib/differ.rb

@@ -3,6 +3,7 @@
 require 'differ/format/ascii'
 require 'differ/format/color'
 require 'differ/format/html'
+require 'differ/format/patch'
 
 module Differ
   class << self
@@ -49,6 +50,7 @@ def format_for(f)
       when :ascii then Format::Ascii
       when :color then Format::Color
       when :html  then Format::HTML
+      when :patch then Format::Patch
       when nil    then nil
       else raise "Unknown format type #{f.inspect}"
       end
@@ -80,4 +82,4 @@ def change(method, array, index)
       @diff.same(array.shift)
     end
   end
-end
+end

lib/differ/format/patch.rb

@@ -0,0 +1,28 @@
+module Differ
+  module Format
+    module Patch
+      class << self
+        def format(change)
+          (change.change? && as_change(change)) ||
+          (change.delete? && as_delete(change)) ||
+          (change.insert? && as_insert(change)) ||
+          ''
+        end
+
+      private
+
+        def as_insert(change)
+          "\033[32m+ #{change.insert}\033[0m"
+        end
+
+        def as_delete(change)
+          "\033[31m- #{change.delete}\033[0m"
+        end
+
+        def as_change(change)
+          [as_delete(change), as_insert(change)].join("\n")
+        end
+      end
+    end
+  end
+end

spec/differ/format/patch_spec.rb

@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe Differ::Format::Patch do
+  it 'should format inserts well' do
+    @expected = "\033[32m+ SAMPLE\033[0m"
+    Differ::Format::Patch.format(+'SAMPLE').should == @expected
+  end
+
+  it 'should format deletes well' do
+    @expected = "\033[31m- SAMPLE\033[0m"
+    Differ::Format::Patch.format(-'SAMPLE').should == @expected
+  end
+
+  it 'should format changes well' do
+    @expected = "\033[31m- THEN\033[0m\n\033[32m+ NOW\033[0m"
+    Differ::Format::Patch.format('THEN' >> 'NOW').should == @expected
+  end
+end