Slightly optimize writeVarInt for the normal case.

Most variable length integers are smaller numbers, so this commit reverses
the order of the if checks in the writeVarInt to assume smaller numbers
are more common.

This is part of the ongoing effort to optimize serialization as noted in
conformal/btcd#27

common.go

@@ -96,30 +96,21 @@ func readVarInt(r io.Reader, pver uint32) (uint64, error) {
 // writeVarInt serializes val to w using a variable number of bytes depending
 // on its value.
 func writeVarInt(w io.Writer, pver uint32, val uint64) error {
-	if val > math.MaxUint32 {
+	if val < 0xfd {
-		err := writeElements(w, []byte{0xff}, uint64(val))
-		if err != nil {
-			return err
-		}
-		return nil
-	}
-	if val > math.MaxUint16 {
-		err := writeElements(w, []byte{0xfe}, uint32(val))
-		if err != nil {
-			return err
-		}
-		return nil
-	}
-	if val >= 0xfd {
-		err := writeElements(w, []byte{0xfd}, uint16(val))
-		if err != nil {
-			return err
-		}
-		return nil
-	}
 		return writeElement(w, uint8(val))
 	}
 
+	if val <= math.MaxUint16 {
+		return writeElements(w, []byte{0xfd}, uint16(val))
+	}
+
+	if val <= math.MaxUint32 {
+		return writeElements(w, []byte{0xfe}, uint32(val))
+	}
+
+	return writeElements(w, []byte{0xff}, uint64(val))
+}
+
 // readVarString reads a variable length string from r and returns it as a Go
 // string.  A varString is encoded as a varInt containing the length of the
 // string, and the bytes that represent the string itself.  An error is returned

test_coverage.txt

@@ -16,15 +16,14 @@ github.com/conformal/btcwire/msgblock.go	 MsgBlock.DeserializeTxLoc		 100.00% (1
 github.com/conformal/btcwire/msggetheaders.go	 MsgGetHeaders.BtcEncode		 100.00% (18/18)
 github.com/conformal/btcwire/msggetblocks.go	 MsgGetBlocks.BtcEncode			 100.00% (18/18)
 github.com/conformal/btcwire/msgheaders.go	 MsgHeaders.BtcDecode			 100.00% (17/17)
-github.com/conformal/btcwire/common.go		 writeVarInt				 100.00% (16/16)
+github.com/conformal/btcwire/msgblock.go	 MsgBlock.BtcDecode			 100.00% (15/15)
+github.com/conformal/btcwire/msgtx.go		 readTxOut				 100.00% (15/15)
 github.com/conformal/btcwire/msgaddr.go		 MsgAddr.BtcEncode			 100.00% (15/15)
 github.com/conformal/btcwire/shahash.go		 NewShaHashFromStr			 100.00% (15/15)
 github.com/conformal/btcwire/msgheaders.go	 MsgHeaders.BtcEncode			 100.00% (15/15)
-github.com/conformal/btcwire/msgblock.go	 MsgBlock.BtcDecode			 100.00% (15/15)
 github.com/conformal/btcwire/msgtx.go		 writeTxIn				 100.00% (15/15)
-github.com/conformal/btcwire/msgtx.go		 readTxOut				 100.00% (15/15)
-github.com/conformal/btcwire/msgaddr.go		 MsgAddr.BtcDecode			 100.00% (14/14)
 github.com/conformal/btcwire/msginv.go		 MsgInv.BtcDecode			 100.00% (14/14)
+github.com/conformal/btcwire/msgaddr.go		 MsgAddr.BtcDecode			 100.00% (14/14)
 github.com/conformal/btcwire/netaddress.go	 writeNetAddress			 100.00% (14/14)
 github.com/conformal/btcwire/msggetdata.go	 MsgGetData.BtcDecode			 100.00% (14/14)
 github.com/conformal/btcwire/msgnotfound.go	 MsgNotFound.BtcDecode			 100.00% (14/14)
@@ -32,39 +31,40 @@ github.com/conformal/btcwire/msginv.go		 MsgInv.BtcEncode			 100.00% (12/12)
 github.com/conformal/btcwire/msgnotfound.go	 MsgNotFound.BtcEncode			 100.00% (12/12)
 github.com/conformal/btcwire/msggetdata.go	 MsgGetData.BtcEncode			 100.00% (12/12)
 github.com/conformal/btcwire/protocol.go	 ServiceFlag.String			 100.00% (12/12)
-github.com/conformal/btcwire/msgtx.go		 writeTxOut				 100.00% (11/11)
 github.com/conformal/btcwire/common.go		 readVarString				 100.00% (11/11)
-github.com/conformal/btcwire/message.go		 discardInput				 100.00% (10/10)
+github.com/conformal/btcwire/msgtx.go		 writeTxOut				 100.00% (11/11)
 github.com/conformal/btcwire/blockheader.go	 readBlockHeader			 100.00% (10/10)
+github.com/conformal/btcwire/message.go		 discardInput				 100.00% (10/10)
 github.com/conformal/btcwire/msgblock.go	 MsgBlock.BtcEncode			 100.00% (9/9)
 github.com/conformal/btcwire/msgalert.go	 MsgAlert.BtcEncode			 100.00% (8/8)
 github.com/conformal/btcwire/msgalert.go	 MsgAlert.BtcDecode			 100.00% (8/8)
 github.com/conformal/btcwire/blockheader.go	 writeBlockHeader			 100.00% (8/8)
-github.com/conformal/btcwire/common.go		 randomUint64				 100.00% (7/7)
-github.com/conformal/btcwire/msgpong.go		 MsgPong.BtcEncode			 100.00% (7/7)
-github.com/conformal/btcwire/msgpong.go		 MsgPong.BtcDecode			 100.00% (7/7)
 github.com/conformal/btcwire/message.go		 readMessageHeader			 100.00% (7/7)
-github.com/conformal/btcwire/msgversion.go	 NewMsgVersionFromConn			 100.00% (7/7)
+github.com/conformal/btcwire/common.go		 randomUint64				 100.00% (7/7)
+github.com/conformal/btcwire/common.go		 writeVarInt				 100.00% (7/7)
+github.com/conformal/btcwire/msgpong.go		 MsgPong.BtcEncode			 100.00% (7/7)
 github.com/conformal/btcwire/common.go		 writeVarString				 100.00% (7/7)
+github.com/conformal/btcwire/msgpong.go		 MsgPong.BtcDecode			 100.00% (7/7)
+github.com/conformal/btcwire/msgversion.go	 NewMsgVersionFromConn			 100.00% (7/7)
 github.com/conformal/btcwire/common.go		 DoubleSha256				 100.00% (6/6)
 github.com/conformal/btcwire/msgnotfound.go	 MsgNotFound.AddInvVect			 100.00% (5/5)
-github.com/conformal/btcwire/msgaddr.go		 MsgAddr.AddAddresses			 100.00% (5/5)
-github.com/conformal/btcwire/msginv.go		 MsgInv.AddInvVect			 100.00% (5/5)
 github.com/conformal/btcwire/msgaddr.go		 MsgAddr.AddAddress			 100.00% (5/5)
+github.com/conformal/btcwire/msginv.go		 MsgInv.AddInvVect			 100.00% (5/5)
 github.com/conformal/btcwire/msggetblocks.go	 MsgGetBlocks.AddBlockLocatorHash	 100.00% (5/5)
 github.com/conformal/btcwire/msggetheaders.go	 MsgGetHeaders.AddBlockLocatorHash	 100.00% (5/5)
 github.com/conformal/btcwire/shahash.go		 NewShaHash				 100.00% (5/5)
 github.com/conformal/btcwire/msgheaders.go	 MsgHeaders.AddBlockHeader		 100.00% (5/5)
 github.com/conformal/btcwire/msggetdata.go	 MsgGetData.AddInvVect			 100.00% (5/5)
 github.com/conformal/btcwire/blockheader.go	 BlockHeader.BlockSha			 100.00% (5/5)
-github.com/conformal/btcwire/msgtx.go		 MsgTx.TxSha				 100.00% (5/5)
 github.com/conformal/btcwire/shahash.go		 ShaHash.SetBytes			 100.00% (5/5)
+github.com/conformal/btcwire/msgtx.go		 MsgTx.TxSha				 100.00% (5/5)
 github.com/conformal/btcwire/common.go		 readElements				 100.00% (5/5)
+github.com/conformal/btcwire/common.go		 writeElements				 100.00% (5/5)
 github.com/conformal/btcwire/msgping.go		 MsgPing.BtcEncode			 100.00% (5/5)
 github.com/conformal/btcwire/msgping.go		 MsgPing.BtcDecode			 100.00% (5/5)
-github.com/conformal/btcwire/common.go		 writeElements				 100.00% (5/5)
-github.com/conformal/btcwire/msgblock.go	 MsgBlock.TxShas			 100.00% (5/5)
 github.com/conformal/btcwire/netaddress.go	 NewNetAddress				 100.00% (5/5)
+github.com/conformal/btcwire/msgblock.go	 MsgBlock.TxShas			 100.00% (5/5)
+github.com/conformal/btcwire/msgaddr.go		 MsgAddr.AddAddresses			 100.00% (5/5)
 github.com/conformal/btcwire/invvect.go		 readInvVect				 100.00% (4/4)
 github.com/conformal/btcwire/invvect.go		 writeInvVect				 100.00% (4/4)
 github.com/conformal/btcwire/msgmempool.go	 MsgMemPool.BtcDecode			 100.00% (4/4)
@@ -154,5 +154,5 @@ github.com/conformal/btcwire/msggetaddr.go	 MsgGetAddr.BtcEncode			 100.00% (1/1
 github.com/conformal/btcwire/msginv.go		 MsgInv.Command				 100.00% (1/1)
 github.com/conformal/btcwire/msginv.go		 MsgInv.MaxPayloadLength		 100.00% (1/1)
 github.com/conformal/btcwire/msginv.go		 NewMsgInv				 100.00% (1/1)
-github.com/conformal/btcwire			 ---------------------------------	 100.00% (964/964)
+github.com/conformal/btcwire			 ---------------------------------	 100.00% (955/955)