wire: add method TxID to MsgTx

This commit adds a method to MsgTx called TxID. This method returns the
transaction ID (txid) of the subject transaction. This commit also adds
a function for deriving the transaction ID from the double hash of the
transaction.

wire/msgtx.go

@@ -6,6 +6,7 @@ package wire
 
 import (
 	"bytes"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -366,6 +367,29 @@ func (msg *MsgTx) TxHash() chainhash.Hash {
 	return chainhash.DoubleHashH(buf.Bytes())
 }
 
+// TxIdFromMsgTxHash generates the transaction ID (txid) of the transaction from
+// the double hash of the transaction.
+func TxIdFromMsgTxHash(txHash chainhash.Hash) string {
+	// Reverse the bytes of the hash.
+	revBytes := txHash
+	size := chainhash.HashSize
+	for i := 0; i < size/2; i++ {
+		revBytes[i], revBytes[size-1-i] = revBytes[size-1-i], revBytes[i]
+	}
+
+	// Hex encode the reversed bytes.
+	return hex.EncodeToString(revBytes[:])
+}
+
+// TxID generates the transaction ID of the transaction.
+func (msg *MsgTx) TxID() string {
+	// Encode the transaction and calculate the double sha256 hash of the
+	// result.
+	txHash := msg.TxHash()
+
+	return TxIdFromMsgTxHash(txHash)
+}
+
 // WitnessHash generates the hash of the transaction serialized according to
 // the new witness serialization defined in BIP0141 and BIP0144. The final
 // output is used within the Segregated Witness commitment of all the witnesses

wire/msgtx_test.go

@@ -756,6 +756,38 @@ func TestTxSerializeSizeStripped(t *testing.T) {
 	}
 }
 
+// TestTxID performs tests to ensure the serialize size for
+// various transactions is accurate.
+func TestTxID(t *testing.T) {
+	// Empty tx message.
+	noTx := NewMsgTx(1)
+	noTx.Version = 1
+
+	tests := []struct {
+		in   *MsgTx // Tx to encode.
+		txid string // Expected transaction ID.
+	}{
+		// No inputs or outputs.
+		{noTx, "d21633ba23f70118185227be58a63527675641ad37967e2aa461559f577aec43"},
+
+		// Transaction with an input and an output.
+		{multiTx, "0100d15a522ff38de05c164ca0a56379a1b77dd1e4805a6534dc9b3d88290e9d"},
+
+		// Transaction with an input which includes witness data, and
+		// one output.
+		{multiWitnessTx, "0f167d1385a84d1518cfee208b653fc9163b605ccf1b75347e2850b3e2eb19f3"},
+	}
+
+	t.Logf("Running %d tests", len(tests))
+	for i, test := range tests {
+		txid := test.in.TxID()
+		if txid != test.txid {
+			t.Errorf("MsgTx.TxID: #%d got: %s, want: %s", i, txid, test.txid)
+			continue
+		}
+	}
+}
+
 // TestTxWitnessSize performs tests to ensure that the serialized size for
 // various types of transactions that include witness data is accurate.
 func TestTxWitnessSize(t *testing.T) {