[En|De]codeUnsignedVarint -> [En|De]codeVarint (firebase#817)

* [En|De]codeUnsignedVarint -> [En|De]codeVarint

The 'unsigned' portion was misleading, as these varints work with both
signed and unsigned integers. (The 'signed' varints also work with both
signed and unsigned integers, but use zig-zag encoding so that negative
numbers are encoded more efficiently. Note that 'signed' varints aren't
used in the Value proto, so won't appear in the serializer class for at
least the short term.)

Added some docstrings to help disambiguate this.

Author: rsgowman

Firestore/core/src/firebase/firestore/remote/serializer.cc

@@ -25,9 +25,16 @@ namespace remote {
 
 namespace {
 
-void EncodeUnsignedVarint(pb_ostream_t* stream,
-                          uint32_t field_number,
-                          uint64_t value) {
+/**
+ * Note that (despite the value parameter type) this works for bool, enum,
+ * int32, int64, uint32 and uint64 proto field types.
+ *
+ * Note: This is not expected to be called direclty, but rather only via the
+ * other Encode* methods (i.e. EncodeBool, EncodeLong, etc)
+ *
+ * @param value The value to encode, represented as a uint64_t.
+ */
+void EncodeVarint(pb_ostream_t* stream, uint32_t field_number, uint64_t value) {
   bool status = pb_encode_tag(stream, PB_WT_VARINT, field_number);
   if (!status) {
     // TODO(rsgowman): figure out error handling
@@ -41,7 +48,16 @@ void EncodeUnsignedVarint(pb_ostream_t* stream,
   }
 }
 
-uint64_t DecodeUnsignedVarint(pb_istream_t* stream) {
+/**
+ * Note that (despite the return type) this works for bool, enum, int32, int64,
+ * uint32 and uint64 proto field types.
+ *
+ * Note: This is not expected to be called direclty, but rather only via the
+ * other Decode* methods (i.e. DecodeBool, DecodeLong, etc)
+ *
+ * @return The decoded varint as a uint64_t.
+ */
+uint64_t DecodeVarint(pb_istream_t* stream) {
   uint64_t varint_value;
   bool status = pb_decode_varint(stream, &varint_value);
   if (!status) {
@@ -52,26 +68,25 @@ uint64_t DecodeUnsignedVarint(pb_istream_t* stream) {
 }
 
 void EncodeNull(pb_ostream_t* stream) {
-  return EncodeUnsignedVarint(stream,
-                              google_firestore_v1beta1_Value_null_value_tag,
-                              google_protobuf_NullValue_NULL_VALUE);
+  return EncodeVarint(stream, google_firestore_v1beta1_Value_null_value_tag,
+                      google_protobuf_NullValue_NULL_VALUE);
 }
 
 void DecodeNull(pb_istream_t* stream) {
-  uint64_t varint = DecodeUnsignedVarint(stream);
+  uint64_t varint = DecodeVarint(stream);
   if (varint != google_protobuf_NullValue_NULL_VALUE) {
     // TODO(rsgowman): figure out error handling
     abort();
   }
 }
 
 void EncodeBool(pb_ostream_t* stream, bool bool_value) {
-  return EncodeUnsignedVarint(
-      stream, google_firestore_v1beta1_Value_boolean_value_tag, bool_value);
+  return EncodeVarint(stream, google_firestore_v1beta1_Value_boolean_value_tag,
+                      bool_value);
 }
 
 bool DecodeBool(pb_istream_t* stream) {
-  uint64_t varint = DecodeUnsignedVarint(stream);
+  uint64_t varint = DecodeVarint(stream);
   switch (varint) {
     case 0:
       return false;