wire: Add func to get pkscript locs from a tx.

This commit provides a new function named PkScriptLocs on the MsgTx type
which can be used to efficiently retrieve a list of offsets for the public
key scripts for the serialized form of the transaction.

This is useful for certain applications which store fully serialized
transactions and want to be able to quickly index into the serialized
transaction to extract a give public key script directly thereby avoiding
the need to deserialize the entire transaction.

wire/msgblock.go

@@ -108,8 +108,9 @@ func (msg *MsgBlock) Deserialize(r io.Reader) error {
 }
 
 // DeserializeTxLoc decodes r in the same manner Deserialize does, but it takes
-// a byte buffer instead of a generic reader and returns a slice containing the start and length of
-// each transaction within the raw data that is being deserialized.
+// a byte buffer instead of a generic reader and returns a slice containing the
+// start and length of each transaction within the raw data that is being
+// deserialized.
 func (msg *MsgBlock) DeserializeTxLoc(r *bytes.Buffer) ([]TxLoc, error) {
 	fullLen := r.Len()
 

wire/msgtx.go

@@ -430,6 +430,43 @@ func (msg *MsgTx) MaxPayloadLength(pver uint32) uint32 {
 	return MaxBlockPayload
 }
 
+// PkScriptLocs returns a slice containing the start of each public key script
+// within the raw serialized transaction.  The caller can easily obtain the
+// length of each script by using len on the script available via the
+// appropriate transaction output entry.
+func (msg *MsgTx) PkScriptLocs() []int {
+	numTxOut := len(msg.TxOut)
+	if numTxOut == 0 {
+		return nil
+	}
+
+	// The starting offset in the serialized transaction of the first
+	// transaction output is:
+	//
+	// Version 4 bytes + serialized varint size for the number of
+	// transaction inputs and outputs + serialized size of each transaction
+	// input.
+	n := 4 + VarIntSerializeSize(uint64(len(msg.TxIn))) +
+		VarIntSerializeSize(uint64(numTxOut))
+	for _, txIn := range msg.TxIn {
+		n += txIn.SerializeSize()
+	}
+
+	// Calculate and set the appropriate offset for each public key script.
+	pkScriptLocs := make([]int, numTxOut)
+	for i, txOut := range msg.TxOut {
+		// The offset of the script in the transaction output is:
+		//
+		// Value 8 bytes + serialized varint size for the length of
+		// PkScript.
+		n += 8 + VarIntSerializeSize(uint64(len(txOut.PkScript)))
+		pkScriptLocs[i] = n
+		n += len(txOut.PkScript)
+	}
+
+	return pkScriptLocs
+}
+
 // NewMsgTx returns a new bitcoin tx message that conforms to the Message
 // interface.  The return instance has a default version of TxVersion and there
 // are no transaction inputs or outputs.  Also, the lock time is set to zero

wire/msgtx_test.go

@@ -389,15 +389,17 @@ func TestTxSerialize(t *testing.T) {
 	}
 
 	tests := []struct {
-		in  *wire.MsgTx // Message to encode
-		out *wire.MsgTx // Expected decoded message
-		buf []byte      // Serialized data
+		in           *wire.MsgTx // Message to encode
+		out          *wire.MsgTx // Expected decoded message
+		buf          []byte      // Serialized data
+		pkScriptLocs []int       // Expected output script locations
 	}{
 		// No transactions.
 		{
 			noTx,
 			noTx,
 			noTxEncoded,
+			nil,
 		},
 
 		// Multiple transactions.
@@ -405,6 +407,7 @@ func TestTxSerialize(t *testing.T) {
 			multiTx,
 			multiTx,
 			multiTxEncoded,
+			multiTxPkScriptLocs,
 		},
 	}
 
@@ -436,6 +439,25 @@ func TestTxSerialize(t *testing.T) {
 				spew.Sdump(&tx), spew.Sdump(test.out))
 			continue
 		}
+
+		// Ensure the public key script locations are accurate.
+		pkScriptLocs := test.in.PkScriptLocs()
+		if !reflect.DeepEqual(pkScriptLocs, test.pkScriptLocs) {
+			t.Errorf("PkScriptLocs #%d\n got: %s want: %s", i,
+				spew.Sdump(pkScriptLocs),
+				spew.Sdump(test.pkScriptLocs))
+			continue
+		}
+		for j, loc := range pkScriptLocs {
+			wantPkScript := test.in.TxOut[j].PkScript
+			gotPkScript := test.buf[loc : loc+len(wantPkScript)]
+			if !bytes.Equal(gotPkScript, wantPkScript) {
+				t.Errorf("PkScriptLocs #%d:%d\n unexpected "+
+					"script got: %s want: %s", i, j,
+					spew.Sdump(gotPkScript),
+					spew.Sdump(wantPkScript))
+			}
+		}
 	}
 }
 
@@ -611,7 +633,7 @@ func TestTxSerializeSize(t *testing.T) {
 		{noTx, 10},
 
 		// Transcaction with an input and an output.
-		{multiTx, 134},
+		{multiTx, 210},
 	}
 
 	t.Logf("Running %d tests", len(tests))
@@ -657,6 +679,22 @@ var multiTx = &wire.MsgTx{
 				0xac, // OP_CHECKSIG
 			},
 		},
+		{
+			Value: 0x5f5e100,
+			PkScript: []byte{
+				0x41, // OP_DATA_65
+				0x04, 0xd6, 0x4b, 0xdf, 0xd0, 0x9e, 0xb1, 0xc5,
+				0xfe, 0x29, 0x5a, 0xbd, 0xeb, 0x1d, 0xca, 0x42,
+				0x81, 0xbe, 0x98, 0x8e, 0x2d, 0xa0, 0xb6, 0xc1,
+				0xc6, 0xa5, 0x9d, 0xc2, 0x26, 0xc2, 0x86, 0x24,
+				0xe1, 0x81, 0x75, 0xe8, 0x51, 0xc9, 0x6b, 0x97,
+				0x3d, 0x81, 0xb0, 0x1c, 0xc3, 0x1f, 0x04, 0x78,
+				0x34, 0xbc, 0x06, 0xd6, 0xd6, 0xed, 0xf6, 0x20,
+				0xd1, 0x84, 0x24, 0x1a, 0x6a, 0xed, 0x8b, 0x63,
+				0xa6, // 65-byte signature
+				0xac, // OP_CHECKSIG
+			},
+		},
 	},
 	LockTime: 0,
 }
@@ -674,7 +712,7 @@ var multiTxEncoded = []byte{
 	0x07,                                     // Varint for length of signature script
 	0x04, 0x31, 0xdc, 0x00, 0x1b, 0x01, 0x62, // Signature script
 	0xff, 0xff, 0xff, 0xff, // Sequence
-	0x01,                                           // Varint for number of output transactions
+	0x02,                                           // Varint for number of output transactions
 	0x00, 0xf2, 0x05, 0x2a, 0x01, 0x00, 0x00, 0x00, // Transaction amount
 	0x43, // Varint for length of pk script
 	0x41, // OP_DATA_65
@@ -686,7 +724,24 @@ var multiTxEncoded = []byte{
 	0x3d, 0x81, 0xb0, 0x1c, 0xc3, 0x1f, 0x04, 0x78,
 	0x34, 0xbc, 0x06, 0xd6, 0xd6, 0xed, 0xf6, 0x20,
 	0xd1, 0x84, 0x24, 0x1a, 0x6a, 0xed, 0x8b, 0x63,
+	0xa6,                                           // 65-byte signature
+	0xac,                                           // OP_CHECKSIG
+	0x00, 0xe1, 0xf5, 0x05, 0x00, 0x00, 0x00, 0x00, // Transaction amount
+	0x43, // Varint for length of pk script
+	0x41, // OP_DATA_65
+	0x04, 0xd6, 0x4b, 0xdf, 0xd0, 0x9e, 0xb1, 0xc5,
+	0xfe, 0x29, 0x5a, 0xbd, 0xeb, 0x1d, 0xca, 0x42,
+	0x81, 0xbe, 0x98, 0x8e, 0x2d, 0xa0, 0xb6, 0xc1,
+	0xc6, 0xa5, 0x9d, 0xc2, 0x26, 0xc2, 0x86, 0x24,
+	0xe1, 0x81, 0x75, 0xe8, 0x51, 0xc9, 0x6b, 0x97,
+	0x3d, 0x81, 0xb0, 0x1c, 0xc3, 0x1f, 0x04, 0x78,
+	0x34, 0xbc, 0x06, 0xd6, 0xd6, 0xed, 0xf6, 0x20,
+	0xd1, 0x84, 0x24, 0x1a, 0x6a, 0xed, 0x8b, 0x63,
 	0xa6,                   // 65-byte signature
 	0xac,                   // OP_CHECKSIG
 	0x00, 0x00, 0x00, 0x00, // Lock time
 }
+
+// multiTxPkScriptLocs is the location information for the public key scripts
+// located in multiTx.
+var multiTxPkScriptLocs = []int{63, 139}