Tidy up options

BUILD.bazel

@@ -12,7 +12,6 @@ go_library(
     srcs = [
         "hasher.go",
         "hashing.go",
-        "protoreflecthash.go",
     ],
     importpath = "github.com/stackb/protoreflecthash",
     visibility = ["//visibility:public"],
@@ -26,7 +25,6 @@ go_test(
     name = "protoreflecthash_test",
     srcs = [
         "hasher_test.go",
-        "protoreflecthash_test.go",
     ],
     data = glob(["testdata/**"]),
     embed = [":protoreflecthash"],

README.md

@@ -17,8 +17,8 @@ protoreflecthash is a re-implementation of
 <https://github.com/deepmind/objecthash-proto>.  That repo is now archived and
 was never updated for [protobuf-apiv2](https://go.dev/blog/protobuf-apiv2).
 
-This package is currently experimental; the hash values for messages will likely
-change without warning until v1.
+This package is currently experimental; the hash values for messages may change
+without warning until v1.
 
 # Usage
 
@@ -36,11 +36,22 @@ import (
 func main() {
     msg := mustGetProtoMessageSomewhere()
 
-    hex, err := protoreflecthash.String(msg)
+    hasher := protoreflect.NewHasher() // see options
+    hex, err := protoreflecthash.String(msg.ProtoReflect())
     if err != nil {
         panic(err.Error())
     }
 
     println(hex)
 }
 ```
+
+# Background
+
+`protoreflecthash` computes the hash value for a protobuf message by taking a
+sha256 of the sum the individual component hashes of the message.
+
+This implementation passes all functional unit tests from the original library
+[deepmind/objecthash-proto](https://github.com/deepmind/objecthash-proto)
+(excluding badness detection).
+

hasher.go

@@ -11,17 +11,37 @@ import (
 
 const valueName = protoreflect.Name("value")
 
-type ProtoHasherOption func(*hasher)
+// Option modifies how hashes for protobufs is calculated.
+type Option func(*hasher)
 
+// ProtoHasher is an interface for hashers that are capable of returning an
+// ObjectHash for protobufs.
 type ProtoHasher interface {
+	// HashProto returns the object hash of a given protocol buffer message.
 	HashProto(msg protoreflect.Message) ([]byte, error)
 }
 
-func NewHasher(options ...ProtoHasherOption) ProtoHasher {
-	return &hasher{}
+// NewHasher creates a new ProtoHasher with the options specified in the
+// argument.
+func NewHasher(options ...Option) ProtoHasher {
+	h := &hasher{}
+	for _, opt := range options {
+		opt(h)
+	}
+	return h
 }
 
-func FieldNamesAsKeys() ProtoHasherOption {
+// MessageFullnameIdentifier is an option that uses the message descriptor
+// fullname rather than a generic map identifier when hashing messages.
+func MessageFullnameIdentifier() Option {
+	return func(h *hasher) {
+		h.messageFullnameIdentifier = true
+	}
+}
+
+// FieldNamesAsKeys is an option that uses field names for key hashing rather
+// than the field number.
+func FieldNamesAsKeys() Option {
 	return func(h *hasher) {
 		h.fieldNamesAsKeys = true
 	}
@@ -31,6 +51,9 @@ type hasher struct {
 	// Whether to use the proto field name as its key, as opposed to using the
 	// tag number as the key.
 	fieldNamesAsKeys bool
+	// Whether to use the fullname of the message descriptor rather than 'm'
+	// (mapIdentifier) for proto messages.
+	messageFullnameIdentifier bool
 }
 
 type fieldHashEntry struct {
@@ -91,9 +114,10 @@ func (h *hasher) hashMessage(msg protoreflect.Message) ([]byte, error) {
 	}
 
 	identifier := mapIdentifier
-	// if hasher.messageIdentifier != "" {
-	// 	identifier = hasher.messageIdentifier
-	// }
+	if h.messageFullnameIdentifier {
+		identifier = string(md.FullName())
+	}
+
 	return hash(identifier, buf.Bytes())
 }
 
@@ -328,7 +352,7 @@ func (h *hasher) hashGoogleProtobufAny(md protoreflect.MessageDescriptor, msg pr
 	// files := protoregistry.GlobalFiles - create option to set files explictly?
 	typeUrl := msg.Get(md.Fields().ByName("type_url")).String()
 	// TODO: lookup at a type server?
-	return nil, fmt.Errorf("unsupported type: " + typeUrl)
+	return nil, fmt.Errorf("protoreflecthash does not support hashing of Any type: " + typeUrl)
 }
 
 func (h *hasher) hashGoogleProtobufDuration(md protoreflect.MessageDescriptor, msg protoreflect.Message) ([]byte, error) {