vm: provide writer in PrintOps()

Make it more flexible and testable. Fallback to using
stdout if no writer is provided.

cli/smartcontract/smart_contract.go

@@ -678,7 +678,7 @@ func inspect(ctx *cli.Context) error {
 	}
 	v := vm.New()
 	v.LoadScript(b)
-	v.PrintOps()
+	v.PrintOps(ctx.App.Writer)
 
 	return nil
 }

pkg/vm/cli/cli.go

@@ -429,7 +429,9 @@ func handleOps(c *ishell.Context) {
 		return
 	}
 	v := getVMFromContext(c)
-	v.PrintOps()
+	out := bytes.NewBuffer(nil)
+	v.PrintOps(out)
+	c.Println(out.String())
 }
 
 func changePrompt(c ishell.Actions, v *vm.VM) {

pkg/vm/vm.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"math"
 	"math/big"
@@ -151,8 +152,11 @@ func (v *VM) LoadArgs(method []byte, args []stackitem.Item) {
 }
 
 // PrintOps prints the opcodes of the current loaded program to stdout.
-func (v *VM) PrintOps() {
-	w := tabwriter.NewWriter(os.Stdout, 0, 0, 4, ' ', 0)
+func (v *VM) PrintOps(out io.Writer) {
+	if out == nil {
+		out = os.Stdout
+	}
+	w := tabwriter.NewWriter(out, 0, 0, 4, ' ', 0)
 	fmt.Fprintln(w, "INDEX\tOPCODE\tPARAMETER\t")
 	realctx := v.Context()
 	ctx := realctx.Copy()

pkg/vm/vm_test.go

@@ -225,8 +225,6 @@ func TestISTYPE(t *testing.T) {
 func testCONVERT(to stackitem.Type, item, res stackitem.Item) func(t *testing.T) {
 	return func(t *testing.T) {
 		prog := []byte{byte(opcode.CONVERT), byte(to)}
-		v := load(prog)
-		v.PrintOps()
 		runWithArgs(t, prog, res, item)
 	}
 }