txscript: Make asSmallInt accept raw opcode.

This converts the asSmallInt function to accept an opcode as a byte
instead of the internal opcode data struct in order to make it more
flexible for raw script analysis.

It also updates all callers accordingly.

txscript/script.go

@@ -159,7 +159,7 @@ func ExtractWitnessProgramInfo(script []byte) (int, []byte, error) {
 			"unable to extract version or witness program")
 	}
 
-	witnessVersion := asSmallInt(pops[0].opcode)
+	witnessVersion := asSmallInt(pops[0].opcode.value)
 	witnessProgram := pops[1].data
 
 	return witnessVersion, witnessProgram, nil
@@ -700,12 +700,12 @@ func calcSignatureHash(prevOutScript []parsedOpcode, hashType SigHashType,
 
 // asSmallInt returns the passed opcode, which must be true according to
 // isSmallInt(), as an integer.
-func asSmallInt(op *opcode) int {
-	if op.value == OP_0 {
+func asSmallInt(op byte) int {
+	if op == OP_0 {
 		return 0
 	}
 
-	return int(op.value - (OP_1 - 1))
+	return int(op - (OP_1 - 1))
 }
 
 // getSigOpCount is the implementation function for counting the number of
@@ -730,7 +730,7 @@ func getSigOpCount(pops []parsedOpcode, precise bool) int {
 			if precise && i > 0 &&
 				pops[i-1].opcode.value >= OP_1 &&
 				pops[i-1].opcode.value <= OP_16 {
-				nSigs += asSmallInt(pops[i-1].opcode)
+				nSigs += asSmallInt(pops[i-1].opcode.value)
 			} else {
 				nSigs += MaxPubKeysPerMultiSig
 			}

txscript/standard.go

@@ -127,7 +127,7 @@ func isMultiSig(pops []parsedOpcode) bool {
 
 	// Verify the number of pubkeys specified matches the actual number
 	// of pubkeys provided.
-	if l-2-1 != asSmallInt(pops[l-2].opcode) {
+	if l-2-1 != asSmallInt(pops[l-2].opcode.value) {
 		return false
 	}
 
@@ -238,7 +238,7 @@ func expectedInputs(pops []parsedOpcode, class ScriptClass) int {
 		// the original bitcoind bug where OP_CHECKMULTISIG pops an
 		// additional item from the stack, add an extra expected input
 		// for the extra push that is required to compensate.
-		return asSmallInt(pops[0].opcode) + 1
+		return asSmallInt(pops[0].opcode.value) + 1
 
 	case NullDataTy:
 		fallthrough
@@ -395,8 +395,8 @@ func CalcMultiSigStats(script []byte) (int, int, error) {
 		return 0, 0, scriptError(ErrNotMultisigScript, str)
 	}
 
-	numSigs := asSmallInt(pops[0].opcode)
-	numPubKeys := asSmallInt(pops[len(pops)-2].opcode)
+	numSigs := asSmallInt(pops[0].opcode.value)
+	numPubKeys := asSmallInt(pops[len(pops)-2].opcode.value)
 	return numPubKeys, numSigs, nil
 }
 
@@ -617,8 +617,8 @@ func ExtractPkScriptAddrs(pkScript []byte, chainParams *chaincfg.Params) (Script
 		// Therefore the number of required signatures is the 1st item
 		// on the stack and the number of public keys is the 2nd to last
 		// item on the stack.
-		requiredSigs = asSmallInt(pops[0].opcode)
-		numPubKeys := asSmallInt(pops[len(pops)-2].opcode)
+		requiredSigs = asSmallInt(pops[0].opcode.value)
+		numPubKeys := asSmallInt(pops[len(pops)-2].opcode.value)
 
 		// Extract the public keys while skipping any that are invalid.
 		addrs = make([]btcutil.Address, 0, numPubKeys)
@@ -706,7 +706,7 @@ func ExtractAtomicSwapDataPushes(version uint16, pkScript []byte) (*AtomicSwapDa
 		}
 		pushes.SecretSize = int64(locktime)
 	} else if op := pops[2].opcode; isSmallInt(op.value) {
-		pushes.SecretSize = int64(asSmallInt(op))
+		pushes.SecretSize = int64(asSmallInt(op.value))
 	} else {
 		return nil, nil
 	}
@@ -717,7 +717,7 @@ func ExtractAtomicSwapDataPushes(version uint16, pkScript []byte) (*AtomicSwapDa
 		}
 		pushes.LockTime = int64(locktime)
 	} else if op := pops[11].opcode; isSmallInt(op.value) {
-		pushes.LockTime = int64(asSmallInt(op))
+		pushes.LockTime = int64(asSmallInt(op.value))
 	} else {
 		return nil, nil
 	}