HeaderHash.new avoids unnecessary object creation

Creating a new HeaderHash is an O(n) operation in addition to
the cost of allocating a new object.  When using multiple pieces
of middleware, this can lead to unnecessary memory allocation
and iteration overhead.   We now explicitly define the
HeaderHash.new class method to return its original argument if
it is already a HeaderHash to avoid repeating work.

Signed-off-by: Joshua Peek <josh@joshpeek.com>

2 files changed, 12 insertions, 0 deletions

diff --git a/lib/rack/utils.rb b/lib/rack/utils.rb
index b5aa8a19..4956177e 100644
--- a/lib/rack/utils.rb
+++ b/lib/rack/utils.rb
@@ -263,6 +263,10 @@ module Rack
     # A case-insensitive Hash that preserves the original case of a
     # header when set.
     class HeaderHash < Hash
+      def self.new(hash={})
+        HeaderHash === hash ? hash : super(hash)
+      end
+
       def initialize(hash={})
         super()
         @names = {}
diff --git a/test/spec_rack_utils.rb b/test/spec_rack_utils.rb
index 0a73d18c..755a1619 100644
--- a/test/spec_rack_utils.rb
+++ b/test/spec_rack_utils.rb
@@ -273,6 +273,14 @@ context "Rack::Utils::HeaderHash" do
     h = Rack::Utils::HeaderHash.new("foo" => "bar")
     h.delete("Hello").should.be.nil
   end
+
+  specify "should avoid unnecessary object creation if possible" do
+    a = Rack::Utils::HeaderHash.new("foo" => "bar")
+    b = Rack::Utils::HeaderHash.new(a)
+    b.object_id.should.equal(a.object_id)
+    b.should.equal(a)
+  end
+
 end
 
 context "Rack::Utils::Context" do