txscript: Convert to use non-parsed opcode disasm.

This converts the engine's current program counter disasembly to make
use of the standalone disassembly function to remove the dependency on
the parsed opcode struct.

It also updates the tests accordingly.

txscript/engine.go

@@ -10,6 +10,7 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"math/big"
+	"strings"
 
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/wire"
@@ -331,8 +332,10 @@ func (vm *Engine) executeOpcode(pop *parsedOpcode) error {
 // provided position in the script.  It does no error checking and leaves that
 // to the caller to provide a valid offset.
 func (vm *Engine) disasm(scriptIdx int, scriptOff int) string {
-	return fmt.Sprintf("%02x:%04x: %s", scriptIdx, scriptOff,
-		vm.scripts[scriptIdx][scriptOff].print(false))
+	var buf strings.Builder
+	pop := vm.scripts[scriptIdx][scriptOff]
+	disasmOpcode(&buf, pop.opcode, pop.data, false)
+	return fmt.Sprintf("%02x:%04x: %s", scriptIdx, scriptOff, buf.String())
 }
 
 // validPC returns an error if the current script position is valid for

txscript/opcode.go

@@ -740,14 +740,6 @@ func disasmOpcode(buf *strings.Builder, op *opcode, data []byte, compact bool) {
 	buf.WriteString(fmt.Sprintf(" 0x%02x", data))
 }
 
-// print returns a human-readable string representation of the opcode for use
-// in script disassembly.
-func (pop *parsedOpcode) print(compact bool) string {
-	var buf strings.Builder
-	disasmOpcode(&buf, pop.opcode, pop.data, compact)
-	return buf.String()
-}
-
 // bytes returns any data associated with the opcode encoded as it would be in
 // a script.  This is used for unparsing scripts from parsed opcodes.
 func (pop *parsedOpcode) bytes() ([]byte, error) {

txscript/opcode_test.go

@@ -127,8 +127,9 @@ func TestOpcodeDisasm(t *testing.T) {
 			expectedStr = "OP_UNKNOWN" + strconv.Itoa(opcodeVal)
 		}
 
-		pop := parsedOpcode{opcode: &opcodeArray[opcodeVal], data: data}
-		gotStr := pop.print(true)
+		var buf strings.Builder
+		disasmOpcode(&buf, &opcodeArray[opcodeVal], data, true)
+		gotStr := buf.String()
 		if gotStr != expectedStr {
 			t.Errorf("pop.print (opcode %x): Unexpected disasm "+
 				"string - got %v, want %v", opcodeVal, gotStr,
@@ -193,8 +194,9 @@ func TestOpcodeDisasm(t *testing.T) {
 			expectedStr = "OP_UNKNOWN" + strconv.Itoa(opcodeVal)
 		}
 
-		pop := parsedOpcode{opcode: &opcodeArray[opcodeVal], data: data}
-		gotStr := pop.print(false)
+		var buf strings.Builder
+		disasmOpcode(&buf, &opcodeArray[opcodeVal], data, false)
+		gotStr := buf.String()
 		if gotStr != expectedStr {
 			t.Errorf("pop.print (opcode %x): Unexpected disasm "+
 				"string - got %v, want %v", opcodeVal, gotStr,