TextFormat Performance: Speed up encoding of submessages

The previous logic created a completely	new encoder, encoded the
subobject, and then copied the result to append	it.   The new
logic avoids that by traversing the subobject with the current
encoder.  (This	is the same strategy that was already used by
the binary encoder.)

Author: tbkka

Sources/SwiftProtobuf/TextFormatEncoder.swift

@@ -29,6 +29,7 @@ private let asciiNewline = UInt8(ascii: "\n")
 private let asciiUpperA = UInt8(ascii: "A")
 
 private let tabSize = 2
+private let tab = [UInt8](repeating: asciiSpace, count: tabSize)
 
 /// TextFormatEncoder has no public members.
 internal struct TextFormatEncoder {
@@ -49,6 +50,10 @@ internal struct TextFormatEncoder {
         data.append(contentsOf: name.utf8Buffer)
     }
 
+    internal mutating func append(bytes: [UInt8]) {
+        data.append(contentsOf: bytes)
+    }
+
     private mutating func append(text: String) {
         data.append(contentsOf: text.utf8)
     }
@@ -69,6 +74,11 @@ internal struct TextFormatEncoder {
         emitFieldName(name: buff)
     }
 
+    mutating func emitFieldName(name: [UInt8]) {
+        indent()
+        data.append(contentsOf: name)
+    }
+
     mutating func emitExtensionFieldName(name: String) {
         indent()
         data.append(asciiOpenSquareBracket)
@@ -93,15 +103,11 @@ internal struct TextFormatEncoder {
     //    name_of_field {key: value key2: value2}
     mutating func startMessageField() {
         append(staticText: " {\n")
-        for _ in 1...tabSize {
-            indentString.append(asciiSpace)
-        }
+        indentString.append(contentsOf: tab)
     }
 
     mutating func endMessageField() {
-        for _ in 1...tabSize {
-            indentString.remove(at: indentString.count - 1)
-        }
+        indentString.removeLast(tabSize)
         indent()
         append(staticText: "}\n")
     }

Sources/SwiftProtobuf/TextFormatEncodingVisitor.swift

@@ -63,6 +63,23 @@ internal struct TextFormatEncodingVisitor: Visitor {
     self.options = options
   }
 
+  private func formatFieldName(lookingUp fieldNumber: Int) -> [UInt8] {
+      var bytes = [UInt8]()
+      if let protoName = nameMap?.names(for: fieldNumber)?.proto {
+          bytes.append(contentsOf: protoName.utf8Buffer)
+      } else if let protoName = nameResolver[fieldNumber] {
+          let buff = UnsafeBufferPointer(start: protoName.utf8Start, count: protoName.utf8CodeUnitCount)
+          bytes.append(contentsOf: buff)
+      } else if let extensionName = extensions?[fieldNumber]?.protobufExtension.fieldName {
+          bytes.append(UInt8(ascii: "["))
+          bytes.append(contentsOf: extensionName.utf8)
+          bytes.append(UInt8(ascii: "]"))
+      } else {
+          bytes.append(contentsOf: fieldNumber.description.utf8)
+      }
+      return bytes
+  }
+
   private mutating func emitFieldName(lookingUp fieldNumber: Int) {
       if let protoName = nameMap?.names(for: fieldNumber)?.proto {
           encoder.emitFieldName(name: protoName.utf8Buffer)
@@ -220,15 +237,27 @@ internal struct TextFormatEncodingVisitor: Visitor {
   mutating func visitSingularMessageField<M: Message>(value: M,
                                              fieldNumber: Int) throws {
       emitFieldName(lookingUp: fieldNumber)
+
+      // Cache old encoder state
+      let oldNameMap = self.nameMap
+      let oldNameResolver = self.nameResolver
+      let oldExtensions = self.extensions
+      // Update encoding state for new message
+      self.nameMap = (M.self as? _ProtoNameProviding.Type)?._protobuf_nameMap
+      self.nameResolver = [:]
+      self.extensions = (value as? ExtensibleMessage)?._protobuf_extensionFieldValues
+      // Encode submessage
       encoder.startMessageField()
-      var visitor = TextFormatEncodingVisitor(message: value, encoder: encoder, options: options)
       if let any = value as? Google_Protobuf_Any {
-          any.textTraverse(visitor: &visitor)
+          any.textTraverse(visitor: &self)
       } else {
-          try! value.traverse(visitor: &visitor)
+          try! value.traverse(visitor: &self)
       }
-      encoder = visitor.encoder
       encoder.endMessageField()
+      // Restore state
+      self.extensions = oldExtensions
+      self.nameResolver = oldNameResolver
+      self.nameMap = oldNameMap
   }
 
   // Emit the full "verbose" form of an Any.  This writes the typeURL
@@ -372,18 +401,31 @@ internal struct TextFormatEncodingVisitor: Visitor {
   // Messages and groups
   mutating func visitRepeatedMessageField<M: Message>(value: [M],
                                              fieldNumber: Int) throws {
+      // Look up field name against outer message encoding state
+      let fieldName = formatFieldName(lookingUp: fieldNumber)
+      // Cache old encoder state
+      let oldNameMap = self.nameMap
+      let oldNameResolver = self.nameResolver
+      let oldExtensions = self.extensions
+      // Update encoding state for new message type
+      self.nameMap = (M.self as? _ProtoNameProviding.Type)?._protobuf_nameMap
+      self.nameResolver = [:]
+      self.extensions = (value as? ExtensibleMessage)?._protobuf_extensionFieldValues
+      // Iterate and encode each message
       for v in value {
-          emitFieldName(lookingUp: fieldNumber)
+          encoder.emitFieldName(name: fieldName)
           encoder.startMessageField()
-          var visitor = TextFormatEncodingVisitor(message: v, encoder: encoder, options: options)
           if let any = v as? Google_Protobuf_Any {
-              any.textTraverse(visitor: &visitor)
+              any.textTraverse(visitor: &self)
           } else {
-              try! v.traverse(visitor: &visitor)
+              try! v.traverse(visitor: &self)
           }
-          encoder = visitor.encoder
           encoder.endMessageField()
       }
+      // Restore state
+      self.extensions = oldExtensions
+      self.nameResolver = oldNameResolver
+      self.nameMap = oldNameMap
   }
 
   // Google's C++ implementation of Text format supports two formats