From 2fd1dcbbff5ac0c47fdaf7538e167dc3ab659fef Mon Sep 17 00:00:00 2001
From: Dave Collins <davec@conformal.com>
Date: Wed, 13 Mar 2019 01:12:40 -0500
Subject: [PATCH] txscript: Optimize ExtractPkScriptAltSigType.

This converts the ExtractPkScriptAltSigType function to use the
optimized extraction functions recently introduced as part of the
typeOfScript conversion.

It is important to note that this new implementation intentionally has
the same semantic differences from the existing implementation as
discussed in the relevant commits that introduced the extraction
functions.

The following is a before and after comparison of analyzing a typical
script:

benchmark                    old ns/op    new ns/op    delta
---------------------------------------------------------------
BenchmarkExtractAltSigType   497          12.8         -97.42%

benchmark                    old allocs   new allocs   delta
---------------------------------------------------------------
BenchmarkExtractAltSigType   1            0            -100.00%

benchmark                    old bytes    new bytes    delta
---------------------------------------------------------------
BenchmarkExtractAltSigType   896          0            -100.00%
---
 txscript/standard.go | 37 ++++++++++---------------------------
 1 file changed, 10 insertions(+), 27 deletions(-)

diff --git a/txscript/standard.go b/txscript/standard.go
index cdad8931..d594cecf 100644
--- a/txscript/standard.go
+++ b/txscript/standard.go
@@ -1572,38 +1572,21 @@ func extractOneBytePush(po parsedOpcode) int {
 
 // ExtractPkScriptAltSigType returns the signature scheme to use for an
 // alternative check signature script.
+//
+// NOTE: This function only attempts to identify version 0 scripts.  Since the
+// function does not accept a script version, the results are undefined for
+// other script versions.
 func ExtractPkScriptAltSigType(pkScript []byte) (dcrec.SignatureType, error) {
-	pops, err := parseScript(pkScript)
-	if err != nil {
-		return 0, err
+	if pk, sigType := extractPubKeyAltDetails(pkScript); pk != nil {
+		return sigType, nil
 	}
 
-	isPKA := isPubkeyAlt(pops)
-	isPKHA := isPubkeyHashAlt(pops)
-	if !(isPKA || isPKHA) {
-		return -1, fmt.Errorf("wrong script type")
+	if pk, sigType := extractPubKeyHashAltDetails(pkScript); pk != nil {
+		return sigType, nil
 	}
 
-	sigTypeLoc := 1
-	if isPKHA {
-		sigTypeLoc = 4
-	}
-
-	valInt := extractOneBytePush(pops[sigTypeLoc])
-	if valInt < 0 {
-		return 0, fmt.Errorf("bad type push")
-	}
-	val := dcrec.SignatureType(valInt)
-	switch val {
-	case dcrec.STEd25519:
-		return val, nil
-	case dcrec.STSchnorrSecp256k1:
-		return val, nil
-	default:
-		break
-	}
-
-	return -1, fmt.Errorf("bad signature scheme type")
+	return -1, fmt.Errorf("not a standard pay-to-alt-pubkey or " +
+		"pay-to-alt-pubkey-hash script")
 }
 
 // AtomicSwapDataPushes houses the data pushes found in atomic swap contracts.