Sort encodings by server preference. Implements #1184.

3 files changed, 16 insertions, 8 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7eea265c..f60641f4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,7 @@ All notable changes to this project will be documented in this file. For info on
 - `Request#ssl?` is true for the `wss` scheme (secure websockets). ([@jeremyevans](https://github.com/jeremyevans))
 - `Rack::HeaderHash` is memoized by default. ([#1549](https://github.com/rack/rack/pull/1549), [@ioquatix](https://github.com/ioquatix))
 - `Rack::Directory` allow directory traversal inside root directory. ([#1417](https://github.com/rack/rack/pull/1417), [@ThomasSevestre](https://github.com/ThomasSevestre))
+- Sort encodings by server preference. ([#1184](https://github.com/rack/rack/pull/1184), [@ioquatix](https://github.com/ioquatix), [@wjordan](https://github.com/wjordan))
 
 ### Removed
 
diff --git a/lib/rack/utils.rb b/lib/rack/utils.rb
index 0fef215d..f033c900 100644
--- a/lib/rack/utils.rb
+++ b/lib/rack/utils.rb
@@ -174,17 +174,23 @@ module Rack
     def select_best_encoding(available_encodings, accept_encoding)
       # http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html
 
-      expanded_accept_encoding =
-        accept_encoding.each_with_object([]) do |(m, q), list|
-          if m == "*"
-            (available_encodings - accept_encoding.map(&:first))
-              .each { |m2| list << [m2, q] }
-          else
-            list << [m, q]
+      expanded_accept_encoding = []
+
+      accept_encoding.each do |m, q|
+        preference = available_encodings.index(m) || available_encodings.size
+
+        if m == "*"
+          (available_encodings - accept_encoding.map(&:first)).each do |m2|
+            expanded_accept_encoding << [m2, q, preference]
           end
+        else
+          expanded_accept_encoding << [m, q, preference]
         end
+      end
 
-      encoding_candidates = expanded_accept_encoding.sort_by { |_, q| -q }.map!(&:first)
+      encoding_candidates = expanded_accept_encoding
+        .sort_by { |_, q, p| [-q, p] }
+        .map!(&:first)
 
       unless encoding_candidates.include?("identity")
         encoding_candidates.push("identity")
diff --git a/test/spec_utils.rb b/test/spec_utils.rb
index e25070bc..9a2d29e6 100644
--- a/test/spec_utils.rb
+++ b/test/spec_utils.rb
@@ -444,6 +444,7 @@ describe Rack::Utils do
 
     helper.call(%w(compress gzip identity), [["compress", 1.0], ["gzip", 1.0]]).must_equal "compress"
     helper.call(%w(compress gzip identity), [["compress", 0.5], ["gzip", 1.0]]).must_equal "gzip"
+    helper.call(%w(compress gzip identity), [["gzip", 1.0], ["compress", 1.0]]).must_equal "compress"
 
     helper.call(%w(foo bar identity), []).must_equal "identity"
     helper.call(%w(foo bar identity), [["*", 1.0]]).must_equal "foo"