internal/encoding/defval: unify logic for handling default values

Logic for serializing the default value in textual form exists in
multiple places in trivially similar forms. Centralize that logic.

Change-Id: I4408ddfeef2c0dfa5c7468e01a4d4df5654ae57f
Reviewed-on: https://go-review.googlesource.com/c/153022
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/encoding/defval/default_test.go b/internal/encoding/defval/default_test.go
new file mode 100644
index 0000000..e91eb7e
--- /dev/null
+++ b/internal/encoding/defval/default_test.go
@@ -0,0 +1,54 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package defval
+
+import (
+	"math"
+	"reflect"
+	"testing"
+
+	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+func Test(t *testing.T) {
+	V := pref.ValueOf
+	tests := []struct {
+		val   pref.Value
+		kind  pref.Kind
+		strPB string
+		strGo string
+	}{
+		{V(bool(true)), pref.BoolKind, "true", "1"},
+		{V(int32(-0x1234)), pref.Int32Kind, "-4660", "-4660"},
+		{V(float32(math.Pi)), pref.FloatKind, "3.1415927", "3.1415927"},
+		{V(float64(math.Pi)), pref.DoubleKind, "3.141592653589793", "3.141592653589793"},
+		{V(string("hello, \xde\xad\xbe\xef\n")), pref.StringKind, "hello, \xde\xad\xbe\xef\n", "hello, \xde\xad\xbe\xef\n"},
+		{V([]byte("hello, \xde\xad\xbe\xef\n")), pref.BytesKind, "hello, \\336\\255\\276\\357\\n", "hello, \\336\\255\\276\\357\\n"},
+	}
+
+	for _, tt := range tests {
+		t.Run("", func(t *testing.T) {
+			gotStrPB, _ := Marshal(tt.val, tt.kind, Descriptor)
+			if gotStrPB != tt.strPB {
+				t.Errorf("Marshal(%v, %v, Descriptor) = %q, want %q", tt.val, tt.kind, gotStrPB, tt.strPB)
+			}
+
+			gotStrGo, _ := Marshal(tt.val, tt.kind, GoTag)
+			if gotStrGo != tt.strGo {
+				t.Errorf("Marshal(%v, %v, GoTag) = %q, want %q", tt.val, tt.kind, gotStrGo, tt.strGo)
+			}
+
+			gotValPB, _ := Unmarshal(tt.strPB, tt.kind, Descriptor)
+			if !reflect.DeepEqual(gotValPB.Interface(), tt.val.Interface()) {
+				t.Errorf("Unmarshal(%v, %v, Descriptor) = %q, want %q", tt.strPB, tt.kind, gotValPB, tt.val)
+			}
+
+			gotValGo, _ := Unmarshal(tt.strGo, tt.kind, GoTag)
+			if !reflect.DeepEqual(gotValGo.Interface(), tt.val.Interface()) {
+				t.Errorf("Unmarshal(%v, %v, GoTag) = %q, want %q", tt.strGo, tt.kind, gotValGo, tt.val)
+			}
+		})
+	}
+}