all: remove protoreflect.Message.Len

Len looks like it should be O(1), but the need to check for
non-zero-length repeated fields makes it at minimum O(n) where n is
the number of repeated fields. In practice, it's O(n) where n is the
number of fields altogether.

The Len function is not especially useful, easily duplicated with Range
and a counter, and can be surprisingly inefficient. Drop it.

Change-Id: I24b27433217e131e842bd18dd58475bcdf62ef97
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/183678
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

internal/impl/legacy_test.go

@@ -342,10 +342,6 @@ func TestLegacyExtensions(t *testing.T) {
 
 	m := pimpl.Export{}.MessageOf(new(LegacyTestMessage))
 
-	if n := m.Len(); n != 0 {
-		t.Errorf("KnownFields.Len() = %v, want 0", n)
-	}
-
 	// Check that getting the zero value returns the default value for scalars,
 	// nil for singular messages, and an empty list for repeated fields.
 	defaultValues := map[int]interface{}{
@@ -444,10 +440,6 @@ func TestLegacyExtensions(t *testing.T) {
 		}
 	}
 
-	if n := m.Len(); n != 20 {
-		t.Errorf("Message.Len() = %v, want 0", n)
-	}
-
 	// Clear all singular fields and truncate all repeated fields.
 	for _, xt := range extensionTypes[:len(extensionTypes)/2] {
 		m.Clear(xt)
@@ -455,17 +447,11 @@ func TestLegacyExtensions(t *testing.T) {
 	for _, xt := range extensionTypes[len(extensionTypes)/2:] {
 		m.Get(xt).List().Truncate(0)
 	}
-	if n := m.Len(); n != 10 {
-		t.Errorf("Message.Len() = %v, want 10", n)
-	}
 
 	// Clear all repeated fields.
 	for _, xt := range extensionTypes[len(extensionTypes)/2:] {
 		m.Clear(xt)
 	}
-	if n := m.Len(); n != 0 {
-		t.Errorf("Message.Len() = %v, want 0", n)
-	}
 }
 
 func TestExtensionConvert(t *testing.T) {

internal/impl/message.go

@@ -360,15 +360,6 @@ func (m *messageReflectWrapper) ProtoUnwrap() interface{} {
 	return m.p.AsIfaceOf(m.mi.GoType.Elem())
 }
 
-func (m *messageReflectWrapper) Len() (cnt int) {
-	m.mi.init()
-	for _, fi := range m.mi.fields {
-		if fi.has(m.p) {
-			cnt++
-		}
-	}
-	return cnt + m.mi.extensionMap(m.p).Len()
-}
 func (m *messageReflectWrapper) Range(f func(pref.FieldDescriptor, pref.Value) bool) {
 	m.mi.init()
 	for _, fi := range m.mi.fields {
@@ -463,12 +454,6 @@ func (m *messageReflectWrapper) checkField(fd pref.FieldDescriptor) (*fieldInfo,
 
 type extensionMap map[int32]ExtensionField
 
-func (m *extensionMap) Len() int {
-	if m != nil {
-		return len(*m)
-	}
-	return 0
-}
 func (m *extensionMap) Range(f func(pref.FieldDescriptor, pref.Value) bool) {
 	if m != nil {
 		for _, x := range *m {

internal/testprotos/irregular/irregular.go

@@ -26,13 +26,6 @@ func (m *message) Interface() pref.ProtoMessage       { return (*IrregularMessag
 
 var fieldDescS = descriptor.Messages().Get(0).Fields().Get(0)
 
-func (m *message) Len() int {
-	if m.set {
-		return 1
-	}
-	return 0
-}
-
 func (m *message) Range(f func(pref.FieldDescriptor, pref.Value) bool) {
 	if m.set {
 		f(fieldDescS, pref.ValueOf(m.value))

proto/encode.go

@@ -145,7 +145,7 @@ func (o MarshalOptions) rangeFields(m protoreflect.Message, f func(protoreflect.
 		m.Range(f)
 		return
 	}
-	fds := make([]protoreflect.FieldDescriptor, 0, m.Len())
+	var fds []protoreflect.FieldDescriptor
 	m.Range(func(fd protoreflect.FieldDescriptor, _ protoreflect.Value) bool {
 		fds = append(fds, fd)
 		return true

proto/equal.go

@@ -35,11 +35,10 @@ func equalMessage(mx, my pref.Message) bool {
 		return false
 	}
 
-	if mx.Len() != my.Len() {
-		return false
-	}
+	nx := 0
 	equal := true
 	mx.Range(func(fd pref.FieldDescriptor, vx pref.Value) bool {
+		nx++
 		vy := my.Get(fd)
 		equal = my.Has(fd) && equalField(fd, vx, vy)
 		return equal
@@ -47,6 +46,14 @@ func equalMessage(mx, my pref.Message) bool {
 	if !equal {
 		return false
 	}
+	ny := 0
+	my.Range(func(fd pref.FieldDescriptor, vx pref.Value) bool {
+		ny++
+		return true
+	})
+	if nx != ny {
+		return false
+	}
 
 	return equalUnknown(mx.GetUnknown(), my.GetUnknown())
 }

proto/reset_test.go

@@ -49,9 +49,6 @@ func TestReset(t *testing.T) {
 		t.Errorf("m.OneofField = %p, want nil", m.OneofField)
 	}
 
-	if got := m.ProtoReflect().Len(); got != 0 {
-		t.Errorf("m.ProtoReflect().Len() = %d, want 0", got)
-	}
 	if got := m.ProtoReflect().GetUnknown(); got != nil {
 		t.Errorf("m.ProtoReflect().GetUnknown() = %d, want nil", got)
 	}

reflect/protoreflect/value.go

@@ -40,12 +40,9 @@ type Message interface {
 	// returns the underlying ProtoMessage interface.
 	Interface() ProtoMessage
 
-	// Len reports the number of populated fields (i.e., Has reports true).
-	Len() int
-
 	// Range iterates over every populated field in an undefined order,
 	// calling f for each field descriptor and value encountered.
-	// Range calls f Len times unless f returns false, which stops iteration.
+	// Range returns immediately if f returns false.
 	// While iterating, mutating operations may only be performed
 	// on the current field descriptor.
 	Range(f func(FieldDescriptor, Value) bool)

types/dynamicpb/dynamic.go

@@ -71,22 +71,6 @@ func (m *Message) Interface() pref.ProtoMessage {
 	return m
 }
 
-// Len returns the number of populated fields.
-// See protoreflect.Message for details.
-func (m *Message) Len() int {
-	count := 0
-	for num, v := range m.known {
-		if m.ext[num] != nil {
-			count++
-			continue
-		}
-		if isSet(m.desc.Fields().ByNumber(num), v) {
-			count++
-		}
-	}
-	return count
-}
-
 // Range visits every populated field in undefined order.
 // See protoreflect.Message for details.
 func (m *Message) Range(f func(pref.FieldDescriptor, pref.Value) bool) {