Add computeSerializedSizeInWords() utility function.

c++/src/capnp/serialize.c++

@@ -91,15 +91,7 @@ kj::ArrayPtr<const word> FlatArrayMessageReader::getSegment(uint id) {
 }
 
 kj::Array<word> messageToFlatArray(kj::ArrayPtr<const kj::ArrayPtr<const word>> segments) {
-  KJ_REQUIRE(segments.size() > 0, "Tried to serialize uninitialized message.");
-
-  size_t totalSize = segments.size() / 2 + 1;
-
-  for (auto& segment: segments) {
-    totalSize += segment.size();
-  }
-
-  kj::Array<word> result = kj::heapArray<word>(totalSize);
+  kj::Array<word> result = kj::heapArray<word>(computeSerializedSizeInWords(segments));
 
   _::WireValue<uint32_t>* table =
       reinterpret_cast<_::WireValue<uint32_t>*>(result.begin());
@@ -130,6 +122,18 @@ kj::Array<word> messageToFlatArray(kj::ArrayPtr<const kj::ArrayPtr<const word>>
   return kj::mv(result);
 }
 
+size_t computeSerializedSizeInWords(kj::ArrayPtr<const kj::ArrayPtr<const word>> segments) {
+  KJ_REQUIRE(segments.size() > 0, "Tried to serialize uninitialized message.");
+
+  size_t totalSize = segments.size() / 2 + 1;
+
+  for (auto& segment: segments) {
+    totalSize += segment.size();
+  }
+
+  return totalSize;
+}
+
 // =======================================================================================
 
 InputStreamMessageReader::InputStreamMessageReader(

c++/src/capnp/serialize.h

@@ -77,6 +77,12 @@ kj::Array<word> messageToFlatArray(MessageBuilder& builder);
 kj::Array<word> messageToFlatArray(kj::ArrayPtr<const kj::ArrayPtr<const word>> segments);
 // Version of messageToFlatArray that takes a raw segment array.
 
+size_t computeSerializedSizeInWords(MessageBuilder& builder);
+// Returns the size, in words, that will be needed to serialize the message, including the header.
+
+size_t computeSerializedSizeInWords(kj::ArrayPtr<const kj::ArrayPtr<const word>> segments);
+// Version of computeSerializedSizeInWords that takes a raw segment array.
+
 // =======================================================================================
 
 class InputStreamMessageReader: public MessageReader {
@@ -151,6 +157,10 @@ inline kj::Array<word> messageToFlatArray(MessageBuilder& builder) {
   return messageToFlatArray(builder.getSegmentsForOutput());
 }
 
+inline size_t computeSerializedSizeInWords(MessageBuilder& builder) {
+  return computeSerializedSizeInWords(builder.getSegmentsForOutput());
+}
+
 inline void writeMessage(kj::OutputStream& output, MessageBuilder& builder) {
   writeMessage(output, builder.getSegmentsForOutput());
 }