txscript: Optimize IsPayToPubKey

This converts the IsPayToScriptHash function to analyze the raw script
instead of using the far less efficient parseScript, thereby
significantly optimizing the function.

In order to accomplish this, it introduces four new functions:
extractCompressedPubKey, extractUncompressedPubKey, extractPubKey, and
isPubKeyScript.  The extractPubKey function makes use of
extractCompressedPubKey and extractUncompressedPubKey to combine their
functionality as a convenience and isPubKeyScript is defined in terms of
extractPubKey.

The extractCompressedPubKey works with the raw script bytes to
simultaneously determine if the script is a pay-to-compressed-pubkey
script, and in the case it is, extract and return the raw compressed
pubkey bytes.

Similarly, the extractUncompressedPubKey works in the same way except it
determines if the script is a pay-to-uncompressed-pubkey script and
returns the raw uncompressed pubkey bytes in the case it is.

The extract function approach was chosen because it is common for
callers to want to only extract relevant details from a script if the
script is of the specific type.  Extracting those details requires
performing the exact same checks to ensure the script is of the correct
type, so it is more efficient to combine the two into one and define the
type determination in terms of the result so long as the extraction does
not require allocations.

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

benchmark                     old ns/op     new ns/op     delta
BenchmarkIsPubKeyScript-8     62323         2.97          -100.00%

benchmark                     old allocs     new allocs     delta
BenchmarkIsPubKeyScript-8     1              0              -100.00%

benchmark                     old bytes     new bytes     delta
BenchmarkIsPubKeyScript-8     311299        0             -100.00%

txscript/script.go

@@ -66,11 +66,7 @@ func isScriptHash(pops []parsedOpcode) bool {
 // IsPayToPubKey returns true if the script is in the standard pay-to-pubkey
 // (P2PK) format, false otherwise.
 func IsPayToPubKey(script []byte) bool {
-	pops, err := parseScript(script)
-	if err != nil {
-		return false
-	}
-	return isPubkey(pops)
+	return isPubKeyScript(script)
 }
 
 // IsPayToScriptHash returns true if the script is in the standard

txscript/standard.go

@@ -85,6 +85,60 @@ func (t ScriptClass) String() string {
 	return scriptClassToName[t]
 }
 
+// extractCompressedPubKey extracts a compressed public key from the passed
+// script if it is a standard pay-to-compressed-secp256k1-pubkey script.  It
+// will return nil otherwise.
+func extractCompressedPubKey(script []byte) []byte {
+	// A pay-to-compressed-pubkey script is of the form:
+	//  OP_DATA_33 <33-byte compresed pubkey> OP_CHECKSIG
+
+	// All compressed secp256k1 public keys must start with 0x02 or 0x03.
+	if len(script) == 35 &&
+		script[34] == OP_CHECKSIG &&
+		script[0] == OP_DATA_33 &&
+		(script[1] == 0x02 || script[1] == 0x03) {
+
+		return script[1:34]
+	}
+
+	return nil
+}
+
+// extractUncompressedPubKey extracts an uncompressed public key from the
+// passed script if it is a standard pay-to-uncompressed-secp256k1-pubkey
+// script.  It will return nil otherwise.
+func extractUncompressedPubKey(script []byte) []byte {
+	// A pay-to-compressed-pubkey script is of the form:
+	//   OP_DATA_65 <65-byte uncompressed pubkey> OP_CHECKSIG
+	// All non-hybrid uncompressed secp256k1 public keys must start with 0x04.
+	if len(script) == 67 &&
+		script[66] == OP_CHECKSIG &&
+		script[0] == OP_DATA_65 &&
+		(script[1] == 0x04 || script[1] == 0x06 || script[1] == 0x07) {
+
+		return script[1:66]
+	}
+	return nil
+}
+
+// extractPubKey extracts either compressed or uncompressed public key from the
+// passed script if it is a either a standard pay-to-compressed-secp256k1-pubkey
+// or pay-to-uncompressed-secp256k1-pubkey script, respectively.  It will return
+// nil otherwise.
+func extractPubKey(script []byte) []byte {
+	if pubKey := extractCompressedPubKey(script); pubKey != nil {
+		return pubKey
+	}
+	return extractUncompressedPubKey(script)
+}
+
+// isPubKeyScript returns whether or not the passed script is either a standard
+// pay-to-compressed-secp256k1-pubkey or pay-to-uncompressed-secp256k1-pubkey
+// script.
+func isPubKeyScript(script []byte) bool {
+	return extractPubKey(script) != nil
+}
+
 // isPubkey returns true if the script passed is a pay-to-pubkey transaction,
 // false otherwise.
 func isPubkey(pops []parsedOpcode) bool {