Implement decode_query in URI

lib/uri.ex

@@ -21,6 +21,33 @@ defmodule URI do
   """
   def encode_query(l), do: Enum.join(Enum.map(l, pair(&1)), "&")
 
+  @doc """
+  Given a query string of the form "key1=value1&key=value2...", produces an
+  orddict with one entry for each key-value pair. Each key and value will be a
+  binary. It also does percent-unescaping of both keys and values.
+
+  Returns nil if the query string is malformed.
+  """
+  def decode_query(q) do
+    parts = Regex.split %r/&/, q
+    # Set up a try block to return quickly from List.foldl in case of an error
+    try do
+      List.foldl parts, Orddict.new, fn(kvstr, dict) ->
+        pair = case Regex.split(%r/=/, kvstr) do
+        match: [ key, value ]
+          { decode(key), decode(value) }
+        match: [ key ]
+          { decode(key), "" }
+        else:
+          raise "bad query"
+        end
+        Dict.Orddict.Record.put dict, pair
+      end
+    rescue: RuntimeError
+      nil
+    end
+  end
+
   defp pair({k, v}) do
     encode(to_binary(k)) <> "=" <> encode(to_binary(v))
   end

test/elixir/uri_test.exs

@@ -28,6 +28,15 @@ defmodule URITest do
     assert URI.encode_query([{:foo, 'bar'}]) == "foo=bar"
   end
 
+  test :decode_query do
+    assert URI.decode_query("q=search%20query&cookie=ab%26cd&block%20buster=") ==
+                Orddict.new [{"block buster", ""}, {"cookie", "ab&cd"}, {"q", "search query"}]
+    assert URI.decode_query("") == Orddict.new
+
+    assert URI.decode_query("something=weird=happening") == nil
+    assert URI.decode_query("something=weird%3Dhappening") == Orddict.new [{"something", "weird=happening"}]
+  end
+
   test :decode do
     data_to_be_decoded = "%26%3C%3E%22+%E3%82%86%E3%82%93%E3%82%86%E3%82%93"
     assert URI.decode(data_to_be_decoded) == "&<>\" ゆんゆん"