Add WriteMeasuredVarUInt to avoid measuring ints twice before writing (#665)

Avoid double resize of buffer in CbWriter::SetName and CbWriter::AddBinary
Add WriteMeasuredVarUInt to avoid measuring ints twice before writing

2 files changed, 53 insertions, 26 deletions

diff --git a/src/zencore/compactbinarybuilder.cpp b/src/zencore/compactbinarybuilder.cpp
index 5c08d2e6e..3331291ce 100644
--- a/src/zencore/compactbinarybuilder.cpp
+++ b/src/zencore/compactbinarybuilder.cpp
@@ -221,12 +221,11 @@ CbWriter::SetName(const std::string_view Name)
 	State.Flags |= StateFlags::Name;
 	const uint32_t NameLenByteCount = MeasureVarUInt(uint32_t(Name.size()));
 	const int64_t  NameLenOffset	= Data.size();
-	Data.resize(NameLenOffset + NameLenByteCount);
+	Data.resize(NameLenOffset + NameLenByteCount + Name.size());
 
-	WriteVarUInt(uint64_t(Name.size()), Data.data() + NameLenOffset);
+	WriteMeasuredVarUInt(uint64_t(Name.size()), NameLenByteCount, Data.data() + NameLenOffset);
 
-	const uint8_t* NamePtr = reinterpret_cast<const uint8_t*>(Name.data());
-	Data.insert(Data.end(), NamePtr, NamePtr + Name.size());
+	memcpy(Data.data() + NameLenOffset + NameLenByteCount, Name.data(), Name.size());
 	return *this;
 }
 
@@ -341,7 +340,7 @@ CbWriter::EndObject()
 	const uint64_t Size			 = uint64_t(Data.size() - PayloadOffset);
 	const uint32_t SizeByteCount = MeasureVarUInt(Size);
 	Data.insert(Data.begin() + PayloadOffset, SizeByteCount, 0);
-	WriteVarUInt(Size, Data.data() + PayloadOffset);
+	WriteMeasuredVarUInt(Size, SizeByteCount, Data.data() + PayloadOffset);
 
 	EndField(bUniform ? CbFieldType::UniformObject : CbFieldType::Object);
 }
@@ -400,8 +399,8 @@ CbWriter::EndArray()
 	const uint64_t Size			  = uint64_t(Data.size() - PayloadOffset) + CountByteCount;
 	const uint32_t SizeByteCount  = MeasureVarUInt(Size);
 	Data.insert(Data.begin() + PayloadOffset, SizeByteCount + CountByteCount, 0);
-	WriteVarUInt(Size, Data.data() + PayloadOffset);
-	WriteVarUInt(Count, Data.data() + PayloadOffset + SizeByteCount);
+	WriteMeasuredVarUInt(Size, SizeByteCount, Data.data() + PayloadOffset);
+	WriteMeasuredVarUInt(Count, CountByteCount, Data.data() + PayloadOffset + SizeByteCount);
 
 	EndField(bUniform ? CbFieldType::UniformArray : CbFieldType::Array);
 }
@@ -429,13 +428,13 @@ CbWriter::AddNull()
 void
 CbWriter::AddBinary(const void* const Value, const uint64_t Size)
 {
-	const size_t SizeByteCount = MeasureVarUInt(Size);
+	const uint32_t SizeByteCount = MeasureVarUInt(Size);
 	Data.reserve(Data.size() + 1 + SizeByteCount + Size);
 	BeginField();
 	const size_t SizeOffset = Data.size();
-	Data.resize(Data.size() + SizeByteCount);
-	WriteVarUInt(Size, Data.data() + SizeOffset);
-	Data.insert(Data.end(), static_cast<const uint8_t*>(Value), static_cast<const uint8_t*>(Value) + Size);
+	Data.resize(Data.size() + SizeByteCount + Size);
+	WriteMeasuredVarUInt(Size, SizeByteCount, Data.data() + SizeOffset);
+	memcpy(Data.data() + SizeOffset + SizeByteCount, Value, Size);
 	EndField(CbFieldType::Binary);
 }
 
@@ -468,7 +467,7 @@ CbWriter::AddString(const std::string_view Value)
 	Data.resize(Offset + SizeByteCount + Size);
 
 	uint8_t* StringData = Data.data() + Offset;
-	WriteVarUInt(Size, StringData);
+	WriteMeasuredVarUInt(Size, SizeByteCount, StringData);
 	StringData += SizeByteCount;
 	if (Size > 0)
 	{
@@ -489,7 +488,7 @@ CbWriter::AddString(const std::wstring_view Value)
 	const int64_t  Offset		 = Data.size();
 	Data.resize(Offset + SizeByteCount + Size);
 	uint8_t* StringData = Data.data() + Offset;
-	WriteVarUInt(Size, StringData);
+	WriteMeasuredVarUInt(Size, SizeByteCount, StringData);
 	StringData += SizeByteCount;
 	if (Size > 0)
 	{
@@ -511,7 +510,7 @@ CbWriter::AddInteger(const int32_t Value)
 	const uint32_t MagnitudeByteCount = MeasureVarUInt(Magnitude);
 	const int64_t  Offset			  = Data.size();
 	Data.resize(Offset + MagnitudeByteCount);
-	WriteVarUInt(Magnitude, Data.data() + Offset);
+	WriteMeasuredVarUInt(Magnitude, MagnitudeByteCount, Data.data() + Offset);
 	EndField(CbFieldType::IntegerNegative);
 }
 
@@ -526,7 +525,7 @@ CbWriter::AddInteger(const int64_t Value)
 	const uint64_t Magnitude		  = ~uint64_t(Value);
 	const uint32_t MagnitudeByteCount = MeasureVarUInt(Magnitude);
 	const uint64_t Offset			  = AddUninitialized(Data, MagnitudeByteCount);
-	WriteVarUInt(Magnitude, Data.data() + Offset);
+	WriteMeasuredVarUInt(Magnitude, MagnitudeByteCount, Data.data() + Offset);
 	EndField(CbFieldType::IntegerNegative);
 }
 
@@ -537,7 +536,7 @@ CbWriter::AddInteger(const uint32_t Value)
 	BeginField();
 	const uint32_t ValueByteCount = MeasureVarUInt(Value);
 	const uint64_t Offset		  = AddUninitialized(Data, ValueByteCount);
-	WriteVarUInt(Value, Data.data() + Offset);
+	WriteMeasuredVarUInt(Value, ValueByteCount, Data.data() + Offset);
 	EndField(CbFieldType::IntegerPositive);
 }
 
@@ -548,7 +547,7 @@ CbWriter::AddInteger(const uint64_t Value)
 	BeginField();
 	const uint32_t ValueByteCount = MeasureVarUInt(Value);
 	const uint64_t Offset		  = AddUninitialized(Data, ValueByteCount);
-	WriteVarUInt(Value, Data.data() + Offset);
+	WriteMeasuredVarUInt(Value, ValueByteCount, Data.data() + Offset);
 	EndField(CbFieldType::IntegerPositive);
 }
 
diff --git a/src/zencore/include/zencore/varint.h b/src/zencore/include/zencore/varint.h
index c0d63d814..ae9aceed6 100644
--- a/src/zencore/include/zencore/varint.h
+++ b/src/zencore/include/zencore/varint.h
@@ -181,17 +181,17 @@ ReadVarInt(const void* InData, uint32_t& OutByteCount)
 }
 
 /**
- * Write a variable-length unsigned integer.
+ * Write a pre-measured variable-length unsigned integer.
  *
  * @param InValue An unsigned integer to encode.
+ * @param ByteCount The number of bytes the integer requires to be encoded
  * @param OutData A buffer of at least 5 bytes to write the output to.
  * @return The number of bytes used in the output.
  */
-inline uint32_t
-WriteVarUInt(uint32_t InValue, void* OutData)
+inline void
+WriteMeasuredVarUInt(uint32_t InValue, uint32_t ByteCount, void* OutData)
 {
-	const uint32_t ByteCount = MeasureVarUInt(InValue);
-	uint8_t*	   OutBytes	 = static_cast<uint8_t*>(OutData) + ByteCount - 1;
+	uint8_t* OutBytes = static_cast<uint8_t*>(OutData) + ByteCount - 1;
 	switch (ByteCount - 1)
 	{
 		case 4:
@@ -214,21 +214,35 @@ WriteVarUInt(uint32_t InValue, void* OutData)
 			break;
 	}
 	*OutBytes = uint8_t(0xff << (9 - ByteCount)) | uint8_t(InValue);
-	return ByteCount;
 }
 
 /**
  * Write a variable-length unsigned integer.
  *
  * @param InValue An unsigned integer to encode.
- * @param OutData A buffer of at least 9 bytes to write the output to.
+ * @param OutData A buffer of at least 5 bytes to write the output to.
  * @return The number of bytes used in the output.
  */
 inline uint32_t
-WriteVarUInt(uint64_t InValue, void* OutData)
+WriteVarUInt(uint32_t InValue, void* OutData)
 {
 	const uint32_t ByteCount = MeasureVarUInt(InValue);
-	uint8_t*	   OutBytes	 = static_cast<uint8_t*>(OutData) + ByteCount - 1;
+	WriteMeasuredVarUInt(InValue, MeasureVarUInt(InValue), OutData);
+	return ByteCount;
+}
+
+/**
+ * Write a pre-measured variable-length unsigned integer.
+ *
+ * @param InValue An unsigned integer to encode.
+ * @param ByteCount The number of bytes the integer requires to be encoded
+ * @param OutData A buffer of at least 9 bytes to write the output to.
+ * @return The number of bytes used in the output.
+ */
+inline void
+WriteMeasuredVarUInt(uint64_t InValue, uint32_t ByteCount, void* OutData)
+{
+	uint8_t* OutBytes = static_cast<uint8_t*>(OutData) + ByteCount - 1;
 	switch (ByteCount - 1)
 	{
 		case 8:
@@ -267,6 +281,20 @@ WriteVarUInt(uint64_t InValue, void* OutData)
 			break;
 	}
 	*OutBytes = uint8_t(0xff << (9 - ByteCount)) | uint8_t(InValue);
+}
+
+/**
+ * Write a variable-length unsigned integer.
+ *
+ * @param InValue An unsigned integer to encode.
+ * @param OutData A buffer of at least 9 bytes to write the output to.
+ * @return The number of bytes used in the output.
+ */
+inline uint32_t
+WriteVarUInt(uint64_t InValue, void* OutData)
+{
+	const uint32_t ByteCount = MeasureVarUInt(InValue);
+	WriteMeasuredVarUInt(InValue, ByteCount, OutData);
 	return ByteCount;
 }