feat(json): accept atom keys in from_decoded/2

Closes #373

Without atom-key support, callers that build maps in Elixir or use
`Jason.decode(..., keys: :atoms)` are forced to pre-convert keys to
strings before feeding the data to `Protobuf.JSON.from_decoded/2`.

Signed-off-by: Yordis Prieto <yordis.prieto@gmail.com>

Author: yordis

lib/protobuf/json.ex

@@ -362,6 +362,13 @@ defmodule Protobuf.JSON do
       iex> Protobuf.JSON.from_decoded(%{"color" => "GREEN","topSpeed" => 80.0}, Car)
       {:ok, %Car{color: :GREEN, top_speed: 80.0}}
 
+  Field names may also be supplied as atoms (matching either the proto field
+  name or the JSON name), which is convenient when consuming data produced by
+  `Jason.decode(..., keys: :atoms)` or built in Elixir:
+
+      iex> Protobuf.JSON.from_decoded(%{color: "RED"}, Car)
+      {:ok, %Car{color: :RED, top_speed: 0.0}}
+
   By default, an unknown enum string value is rejected:
 
       iex> {:error, %Protobuf.JSON.DecodeError{}} =

lib/protobuf/json/decode.ex

@@ -270,12 +270,27 @@ defmodule Protobuf.JSON.Decode do
   defp null_value?(nil, %Protobuf.FieldProps{type: {:enum, Google.Protobuf.NullValue}}), do: false
   defp null_value?(value, _props), do: is_nil(value)
 
-  defp fetch_field_value(%Protobuf.FieldProps{name: name_key, json_name: json_key}, data) do
+  defp fetch_field_value(
+         %Protobuf.FieldProps{name: name_key, json_name: json_key, name_atom: name_atom},
+         data
+       ) do
     case data do
       %{^json_key => value} -> {:ok, value}
       %{^name_key => value} -> {:ok, value}
+      %{^name_atom => value} -> {:ok, value}
+      _ -> fetch_by_existing_atom(json_key, data)
+    end
+  end
+
+  defp fetch_by_existing_atom(json_key, data) do
+    atom = :erlang.binary_to_existing_atom(json_key, :utf8)
+
+    case data do
+      %{^atom => value} -> {:ok, value}
       _ -> :error
     end
+  rescue
+    ArgumentError -> :error
   end
 
   defp decode_value(%{optional?: true, type: type}, nil, _opts)

test/protobuf/json/decode_test.exs

@@ -43,6 +43,33 @@ defmodule Protobuf.JSON.DecodeTest do
     end
   end
 
+  describe "atom keys" do
+    test "proto field name as atom key" do
+      data = %{string: "hello"}
+      decoded = %Scalars{string: "hello"}
+      assert decode(data, Scalars) == {:ok, decoded}
+    end
+
+    test "JSON (camelCase) field name as atom key" do
+      _ = :optionalString
+      data = %{optionalString: "hello"}
+      assert {:ok, decoded} = decode(data, TestAllTypesProto3)
+      assert decoded.optional_string == "hello"
+    end
+
+    test "string keys still win when both forms are present" do
+      data = %{"string" => "from-string", :string => "from-atom"}
+      decoded = %Scalars{string: "from-string"}
+      assert decode(data, Scalars) == {:ok, decoded}
+    end
+
+    test "mixed atom and string keys across fields" do
+      data = %{:string => "hi", "bool" => true}
+      decoded = %Scalars{string: "hi", bool: true}
+      assert decode(data, Scalars) == {:ok, decoded}
+    end
+  end
+
   describe "booleans" do
     test "actual boolean is valid" do
       data = %{"bool" => true}