Reuse allocations for decompression

src/BlockDecoder.cpp

@@ -56,13 +56,12 @@ void BlockDecoder::decode(const ustring &datastr)
 	}
 	m_version = version;
 
-	ustring datastr2;
 	if (version >= 29) {
 		// decompress whole block at once
 		m_zstd_decompressor.setData(data, length, 1);
-		datastr2 = m_zstd_decompressor.decompress();
-		data = datastr2.c_str();
-		length = datastr2.size();
+		m_zstd_decompressor.decompress(m_scratch);
+		data = m_scratch.c_str();
+		length = m_scratch.size();
 	}
 
 	size_t dataOffset = 0;
@@ -115,8 +114,8 @@ void BlockDecoder::decode(const ustring &datastr)
 	// version < 29
 	ZlibDecompressor decompressor(data, length);
 	decompressor.setSeekPos(dataOffset);
-	m_mapData = decompressor.decompress();
-	decompressor.decompress(); // unused metadata
+	decompressor.decompress(m_mapData);
+	decompressor.decompress(m_scratch); // unused metadata
 	dataOffset = decompressor.seekPos();
 
 	// Skip unused node timers

src/BlockDecoder.h

@@ -23,6 +23,7 @@ private:
 	u8 m_version, m_contentWidth;
 	ustring m_mapData;
 
-	// one instance for performance
+	// cached allocations/instances for performance
 	ZstdDecompressor m_zstd_decompressor;
+	ustring m_scratch;
 };

src/ZlibDecompressor.cpp

@@ -28,18 +28,13 @@ void ZlibDecompressor::setSeekPos(size_t seekPos)
 	m_seekPos = seekPos;
 }
 
-size_t ZlibDecompressor::seekPos() const
-{
-	return m_seekPos;
-}
-
-ustring ZlibDecompressor::decompress()
+void ZlibDecompressor::decompress(ustring &buffer)
 {
 	const unsigned char *data = m_data + m_seekPos;
 	const size_t size = m_size - m_seekPos;
 
-	ustring buffer;
-	constexpr size_t BUFSIZE = 32 * 1024;
+	// output space is extended in chunks of this size
+	constexpr size_t BUFSIZE = 8 * 1024;
 
 	z_stream strm;
 	strm.zalloc = Z_NULL;
@@ -73,7 +68,5 @@ ustring ZlibDecompressor::decompress()
 	m_seekPos += strm.next_in - data;
 	buffer.resize(buffer.size() - strm.avail_out);
 	(void) Z(inflateEnd)(&strm);
-
-	return buffer;
 }
 

src/ZlibDecompressor.h

@@ -11,8 +11,10 @@ public:
 	ZlibDecompressor(const u8 *data, size_t size);
 	~ZlibDecompressor();
 	void setSeekPos(size_t seekPos);
-	size_t seekPos() const;
-	ustring decompress();
+	size_t seekPos() const { return m_seekPos; }
+	// Decompress and return one zlib stream from the buffer
+	// Advances seekPos as appropriate.
+	void decompress(ustring &dst);
 
 private:
 	const u8 *m_data;

src/ZstdDecompressor.cpp

@@ -21,18 +21,13 @@ void ZstdDecompressor::setData(const u8 *data, size_t size, size_t seekPos)
 	m_size = size;
 }
 
-std::size_t ZstdDecompressor::seekPos() const
-{
-	return m_seekPos;
-}
-
-ustring ZstdDecompressor::decompress()
+void ZstdDecompressor::decompress(ustring &buffer)
 {
 	ZSTD_DStream *stream = reinterpret_cast<ZSTD_DStream*>(m_stream);
 	ZSTD_inBuffer inbuf = { m_data, m_size, m_seekPos };
 
-	ustring buffer;
-	constexpr size_t BUFSIZE = 32 * 1024;
+	// output space is extended in chunks of this size
+	constexpr size_t BUFSIZE = 8 * 1024;
 
 	buffer.resize(BUFSIZE);
 	ZSTD_outBuffer outbuf = { &buffer[0], buffer.size(), 0 };
@@ -42,17 +37,15 @@ ustring ZstdDecompressor::decompress()
 	size_t ret;
 	do {
 		ret = ZSTD_decompressStream(stream, &outbuf, &inbuf);
+		if (ret && ZSTD_isError(ret))
+			throw DecompressError();
 		if (outbuf.size == outbuf.pos) {
 			outbuf.size += BUFSIZE;
 			buffer.resize(outbuf.size);
 			outbuf.dst = &buffer[0];
 		}
-		if (ret && ZSTD_isError(ret))
-			throw DecompressError();
 	} while (ret != 0);
 
 	m_seekPos = inbuf.pos;
 	buffer.resize(outbuf.pos);
-
-	return buffer;
 }

src/ZstdDecompressor.h

@@ -11,8 +11,10 @@ public:
 	ZstdDecompressor();
 	~ZstdDecompressor();
 	void setData(const u8 *data, size_t size, size_t seekPos);
-	size_t seekPos() const;
-	ustring decompress();
+	size_t seekPos() const { return m_seekPos; }
+	// Decompress and return one zstd stream from the buffer
+	// Advances seekPos as appropriate.
+	void decompress(ustring &dst);
 
 private:
 	void *m_stream; // ZSTD_DStream