txscript: export calcSignatureHash

This is a useful function for users of this library, and deserves to
be public.

txscript/script.go

@@ -582,6 +582,17 @@ func shallowCopyTx(tx *wire.MsgTx) wire.MsgTx {
 	return txCopy
 }
 
+// CalcSignatureHash will, given a script and hash type for the current script
+// engine instance, calculate the signature hash to be used for signing and
+// verification.
+func CalcSignatureHash(script []byte, hashType SigHashType, tx *wire.MsgTx, idx int) ([]byte, error) {
+	parsedScript, err := parseScript(script)
+	if err != nil {
+		return nil, fmt.Errorf("cannot parse output script: %v", err)
+	}
+	return calcSignatureHash(parsedScript, hashType, tx, idx), nil
+}
+
 // calcSignatureHash will, given a script and hash type for the current script
 // engine instance, calculate the signature hash to be used for signing and
 // verification.

txscript/sign.go

@@ -74,11 +74,10 @@ func WitnessSignature(tx *wire.MsgTx, sigHashes *TxSigHashes, idx int, amt int64
 func RawTxInSignature(tx *wire.MsgTx, idx int, subScript []byte,
 	hashType SigHashType, key *btcec.PrivateKey) ([]byte, error) {
 
-	parsedScript, err := parseScript(subScript)
+	hash, err := CalcSignatureHash(subScript, hashType, tx, idx)
 	if err != nil {
-		return nil, fmt.Errorf("cannot parse output script: %v", err)
+		return nil, err
 	}
-	hash := calcSignatureHash(parsedScript, hashType, tx, idx)
 	signature, err := key.Sign(hash)
 	if err != nil {
 		return nil, fmt.Errorf("cannot sign tx input: %s", err)