Implementation tweaks:

1. Rename keyOrder to actualAndExpectedKeys.

The keyOrder name suggests that ordering is significant, when in fact:
- the ProtoTruth logic doesn't take order into consideration
- map fields do not have defined order: https://developers.google.com/protocol-buffers/docs/proto3#maps

2. Extract a method for retrieving the value of a field, substituting the default value. Use it in toProtoMap().

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=281590847

extensions/proto/src/main/java/com/google/common/truth/extensions/proto/ProtoTruthMessageDifferencer.java

@@ -116,14 +116,14 @@ private DiffResult diffMessages(Message actual, Message expected, FluentEquality
  Map<Object, Object> actualMap = toProtoMap(actualFields.get(fieldDescriptor));
  Map<Object, Object> expectedMap = toProtoMap(expectedFields.get(fieldDescriptor));
 
- ImmutableSet<Object> keyOrder =
+ ImmutableSet<Object> actualAndExpectedKeys =
  Sets.union(actualMap.keySet(), expectedMap.keySet()).immutableCopy();
  builder.addAllSingularFields(
  fieldDescriptor.getNumber(),
  compareMapFieldsByKey(
  actualMap,
  expectedMap,
- keyOrder,
+ actualAndExpectedKeys,
  fieldDescriptor,
  config.subScope(rootDescriptor, fieldDescriptorOrUnknown)));
  } else {
@@ -206,20 +206,17 @@ private static ImmutableMap<Object, Object> toProtoMap(@NullableDecl Object cont
  Map<Object, Object> retVal = Maps.newHashMap();
  for (Object entry : entryMessages) {
  Message message = (Message) entry;
- Object key = message.getAllFields().get(message.getDescriptorForType().findFieldByNumber(1));
- if (key == null) {
- key = message.getDescriptorForType().findFieldByNumber(1).getDefaultValue();
- }
- Object value =
- message.getAllFields().get(message.getDescriptorForType().findFieldByNumber(2));
- if (value == null) {
- value = message.getDescriptorForType().findFieldByNumber(2).getDefaultValue();
- }
- retVal.put(key, value);
+ retVal.put(valueAtFieldNumber(message, 1), valueAtFieldNumber(message, 2));
  }
  return ImmutableMap.copyOf(retVal);
  }
 
+ private static Object valueAtFieldNumber(Message message, int fieldNumber) {
+ FieldDescriptor field = message.getDescriptorForType().findFieldByNumber(fieldNumber);
+ Object value = message.getAllFields().get(field);
+ return value != null ? value : field.getDefaultValue();
+ }
+
  // Takes a List<Object> or null, and returns the casted list in the first case, an empty list in
  // the latter case.
  private static List<?> toProtoList(@NullableDecl Object container) {
@@ -232,7 +229,7 @@ private static List<?> toProtoList(@NullableDecl Object container) {
  private List<SingularField> compareMapFieldsByKey(
  Map<Object, Object> actualMap,
  Map<Object, Object> expectedMap,
- Set<Object> keyOrder,
+ Set<Object> actualAndExpectedKeys,
  FieldDescriptor mapFieldDescriptor,
  FluentEqualityConfig mapConfig) {
  FieldDescriptor keyFieldDescriptor = mapFieldDescriptor.getMessageType().findFieldByNumber(1);
@@ -257,8 +254,8 @@ private List<SingularField> compareMapFieldsByKey(
  mapConfig.subScope(rootDescriptor, valueFieldDescriptorOrUnknown);
 
  ImmutableList.Builder<SingularField> builder =
- ImmutableList.builderWithExpectedSize(keyOrder.size());
- for (Object key : keyOrder) {
+ ImmutableList.builderWithExpectedSize(actualAndExpectedKeys.size());
+ for (Object key : actualAndExpectedKeys) {
  @NullableDecl Object actualValue = actualMap.get(key);
  @NullableDecl Object expectedValue = expectedMap.get(key);
  if (ignoreExtraRepeatedFieldElements && !expectedMap.isEmpty() && expectedValue == null) {