txscript: add exported sighash calc func

txscript/script.go

@@ -424,6 +424,19 @@ func calcHashOutputs(tx *wire.MsgTx) chainhash.Hash {
 	return chainhash.DoubleHashH(b.Bytes())
 }
 
+// CalcWitnessSigHash exports calcWitnessSignatureHash so that signatures
+// can be verified.
+func CalcWitnessSigHash(subScript []byte, sigHashes *TxSigHashes,
+	hashType SigHashType, tx *wire.MsgTx, idx int, amt int64) ([]byte, error) {
+	parsedScript, err := parseScript(subScript)
+	if err != nil {
+		return nil, fmt.Errorf("cannot parse output script: %v", err)
+	}
+	sighash := calcWitnessSignatureHash(
+		parsedScript, sigHashes, hashType, tx, idx, amt)
+	return sighash, nil
+}
+
 // calcWitnessSignatureHash computes the sighash digest of a transaction's
 // segwit input using the new, optimized digest calculation algorithm defined
 // in BIP0143: https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki.