Allow deriving Jason.Encoder with sorted keys

lib/encoder.ex

@@ -10,6 +10,7 @@ defprotocol Jason.Encoder do
 
     * `:only` - encodes only values of specified keys.
     * `:except` - encodes all struct fields except specified keys.
+    * `:sort_keys` - sorts the keys, optionally with a custom function.
 
   By default all keys except the `:__struct__` key are encoded.
 
@@ -51,6 +52,13 @@ defprotocol Jason.Encoder do
   The actually generated implementations are more efficient computing some data
   during compilation similar to the macros from the `Jason.Helpers` module.
 
+  ## Sorting keys
+
+  You can also sort the keys in the output JSON, either according to Erlang's
+  term ordering with `@derive {Jason.Encoder, sort_keys: true}` or using a custom
+  sort function (as passed to `Enum.sort/2`) e.g.
+  `@derive {Jason.Encoder, sort_keys: &(&1>=&2)}`
+
   ## Explicit implementation
 
   If you wish to implement the protocol fully yourself, it is advised to
@@ -138,7 +146,15 @@ defimpl Jason.Encoder, for: Any do
   end
 
   defp fields_to_encode(struct, opts) do
-    fields = Map.keys(struct)
+    fields =  case Keyword.get(opts, :sort_keys) do
+      true ->
+        Map.keys(struct) |> Enum.sort()
+      sort_fn when is_function(sort_fn, 2) ->
+        Map.keys(struct)
+        |> Enum.sort(sort_fn)
+      nil ->
+        Map.keys(struct)
+    end
 
     cond do
       only = Keyword.get(opts, :only) ->

test/encode_test.exs

@@ -135,6 +135,18 @@ defmodule Jason.EncoderTest do
     defstruct name: "", size: 0
   end
 
+  defmodule DerivedUsingSortKeys do
+    @derive {Encoder, sort_keys: true}
+    defstruct a: 1, b: "", c: nil, d: 4
+  end
+
+
+  defmodule DerivedUsingCustomSortKeys do
+    @derive {Encoder, sort_keys: &(&1>=&2)}
+    defstruct a: 1, b: "", c: nil, d: 4
+  end
+
+
   defmodule DerivedWeirdKey do
     @derive Encoder
     defstruct [:_]
@@ -160,6 +172,12 @@ defmodule Jason.EncoderTest do
     derived_using_except = %DerivedUsingExcept{name: "derived using :except", size: 10}
     assert to_json(derived_using_except) == ~s({"size":10})
 
+    derived_using_sort_keys = %DerivedUsingSortKeys{}
+    assert to_json(derived_using_sort_keys) == ~s({"a":1,"b":"","c":null,"d":4})
+
+    derived_using_custom_sort_keys = %DerivedUsingCustomSortKeys{}
+    assert to_json(derived_using_custom_sort_keys) == ~s({"d":4,"c":null,"b":"","a":1})
+
     derived_weird_key = %DerivedWeirdKey{}
     assert to_json(derived_weird_key) == ~s({"_":null})
   end