encoder: Fix Ref wrt protocol version

- we can use BINPERSID only if protocol >= 1
- we can use text PERSID only if protocol < 4 and Ref string does not
  contain \n - else encoding have to fail.

Tests for Ref encoding at protocols 3 & 4 will follow after string
encoding is fixed.

encode.go

@@ -2,6 +2,7 @@ package ogórek
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 	"math"
@@ -383,17 +384,25 @@ func (e *Encoder) encodeClass(v *Class) error {
 	return e.emit(opStackGlobal)
 }
 
+var errP0PersIDStringLineOnly = errors.New(`protocol 0: persistent ID must be string without \n`)
+
 func (e *Encoder) encodeRef(v *Ref) error {
-	if pids, ok := v.Pid.(string); ok && !strings.Contains(pids, "\n") {
+	// protocol 0: pid must be string without \n
+	if e.config.Protocol == 0 {
+		pids, ok := v.Pid.(string)
+		if !ok || strings.Contains(pids, "\n") {
+			return errP0PersIDStringLineOnly
+		}
+
 		return e.emitf("%c%s\n", opPersid, pids)
-	} else {
-		// XXX we can use opBinpersid only if .protocol >= 1
+	}
+
+	// protocol >= 1: we can use opBinpersid which allows arbitrary object as argument
 	err := e.encode(reflectValueOf(v.Pid))
 	if err != nil {
 		return err
 	}
 	return e.emit(opBinpersid)
-	}
 }
 
 func (e *Encoder) encodeStruct(st reflect.Value) error {

ogorek_test.go

@@ -238,12 +238,17 @@ var tests = []TestEntry{
 		I("cfoo\nbar\nS'bing'\n\x85R.")), // GLOBAL + STRING + TUPLE1 + REDUCE
 
 	X(`persref("abc")`, Ref{"abc"},
-		P0("Pabc\n.")), // PERSID
+		P0("Pabc\n."),                // PERSID
+		P12("U\x03abcQ.")),           // SHORT_BINSTRING + BINPERSID
 
 	X(`persref("abc\nd")`, Ref{"abc\nd"},
+		P0(errP0PersIDStringLineOnly),   // cannot be encoded
 		P12("U\x05abc\ndQ.")),           // SHORT_BINSTRING + BINPERSID
 
 	X(`persref((1, 2))`, Ref{Tuple{int64(1), int64(2)}},
+		P0(errP0PersIDStringLineOnly), // cannot be encoded
+		P1("(K\x01K\x02tQ."),          // MARK + BININT1 + TUPLE + BINPERSID
+		P2_("K\x01K\x02\x86Q."),       // BININT1 + TUPLE2 + BINPERSID
 		I("(I1\nI2\ntQ.")),
 
 	// decode only
@@ -584,7 +589,7 @@ func TestPersistentRefs(t *testing.T) {
 	}
 
 	dconf := &DecoderConfig{PersistentLoad: loadref}
-	econf := &EncoderConfig{PersistentRef: getref}
+	econf := &EncoderConfig{PersistentRef: getref, Protocol: 1}
 
 	testv := []struct {
 		input    string