encoder: Fix dict wrt protocol version

- we can use EMPTY_DICT only if protocol >= 1

Also: similarly to list (33d1926f), since we are now using EMPTY_DICT
only optionally, it is logical to swit to

	MARK + ... + DICT

from

	EMPTY_DICT (or MARK + DICT @proto=0) + MARK + ... + SETITEMS

which is at least 1 byte longer.

For the reference - SETITEMS is also from protocol 1, while DICT is from
protocol 0.

encode.go

@@ -376,37 +376,33 @@ func (e *Encoder) encodeMap(m reflect.Value) error {
 
 	l := len(keys)
 
-	err := e.emit(opEmptyDict)
+	// protocol >= 1: ø dict -> EMPTY_DICT
+	if e.config.Protocol >= 1 && l == 0 {
+		return e.emit(opEmptyDict)
+	}
+
+	// MARK + ... + DICT
+	// TODO detect cycles and double references to the same object
+	// XXX sort keys, so the output is stable?
+	err := e.emit(opMark)
 	if err != nil {
 		return err
 	}
 
-	if l > 0 {
-		err := e.emit(opMark)
+	for _, k := range keys {
+		err = e.encode(k)
 		if err != nil {
 			return err
 		}
+		v := m.MapIndex(k)
 
-		for _, k := range keys {
-			err = e.encode(k)
-			if err != nil {
-				return err
-			}
-			v := m.MapIndex(k)
-
-			err = e.encode(v)
-			if err != nil {
-				return err
-			}
-		}
-
-		err = e.emit(opSetitems)
+		err = e.encode(v)
 		if err != nil {
 			return err
 		}
 	}
 
-	return nil
+	return e.emit(opDict)
 }
 
 func (e *Encoder) encodeCall(v *Call) error {

ogorek_test.go

@@ -263,9 +263,27 @@ var tests = []TestEntry{
 
 
 	X("dict({})", make(map[interface{}]interface{}),
+		P0("(d."), // MARK + DICT
+		P1_("}."), // EMPTY_DICT
 		I("(dp0\n.")),
 
+	X("dict({'a': '1'})", map[interface{}]interface{}{"a": "1"},
+		P0("(S\"a\"\nS\"1\"\nd."),                     // MARK + STRING + DICT
+		P12("(U\x01aU\x011d."),                        // MARK + SHORT_BINSTRING + DICT
+		P3("(X\x01\x00\x00\x00aX\x01\x00\x00\x001d."), // MARK + BINUNICODE + DICT
+		P4_("(\x8c\x01a\x8c\x011d.")),                 // MARK + SHORT_BINUNICODE + DICT
+
 	X("dict({'a': '1', 'b': '2'})", map[interface{}]interface{}{"a": "1", "b": "2"},
+		// map iteration order is not stable - test only decoding
+		I("(S\"a\"\nS\"1\"\nS\"b\"\nS\"2\"\nd."), // P0: MARK + STRING + DICT
+		I("(U\x01aU\x011U\x01bU\x012d."),         // P12: MARK + SHORT_BINSTRING + DICT
+
+		// P3: MARK + BINUNICODE + DICT
+		I("(X\x01\x00\x00\x00aX\x01\x00\x00\x001X\x01\x00\x00\x00bX\x01\x00\x00\x002d."),
+
+		I("(\x8c\x01a\x8c\x011\x8c\x01b\x8c\x012d."), // P4_: MARK + SHORT_BINUNICODE + DICT
+		I("(dS'a'\nS'1'\nsS'b'\nS'2'\ns."),           // MARK + DICT + STRING + SETITEM
+		I("}(U\x01aU\x011U\x01bU\x012u."),            // EMPTY_DICT + MARK + SHORT_BINSTRING + SETITEMS
 		I("(dp0\nS'a'\np1\nS'1'\np2\nsS'b'\np3\nS'2'\np4\ns.")),
 
 	X("foo.bar  # global", Class{Module: "foo", Name: "bar"},