Add support for original in marshal.Marshal (#2978)

Add support for `original` in marshal.Marshal

Now it roundtrips

go/marshal/encode.go

@@ -38,8 +38,12 @@ import (
 //
 // Struct values are encoded as Noms structs (types.Struct). Each exported Go
 // struct field becomes a member of the Noms struct unless
-//   - the field's tag is "-"
-//   - the field is empty and its tag specifies the "omitempty" option.
+//   - The field's tag is "-"
+//   - The field is empty and its tag specifies the "omitempty" option.
+//   - The field has the "original" tag, in which case the field is used as an
+//     initial value onto which the fields of the Go type are added. When
+//     combined with the corresponding support for "original" in Unmarshal(),
+//     this allows one to find and modify any values of a known subtype.
 //
 // The empty values are false, 0, any nil pointer or interface value, and any
 // array, slice, map, or string of length zero.
@@ -218,7 +222,7 @@ func structEncoder(t reflect.Type, parentStructTypes []reflect.Type) encoderFunc
 	}
 
 	parentStructTypes = append(parentStructTypes, t)
-	fields, structType := typeFields(t, parentStructTypes)
+	fields, structType, originalFieldIndex := typeFields(t, parentStructTypes)
 	if structType != nil {
 		e = func(v reflect.Value) types.Value {
 			values := make([]types.Value, len(fields))
@@ -227,8 +231,9 @@ func structEncoder(t reflect.Type, parentStructTypes []reflect.Type) encoderFunc
 			}
 			return types.NewStructWithType(structType, values)
 		}
-	} else {
-		// Cannot precompute the Noms type since there are Noms collections.
+	} else if originalFieldIndex == nil {
+		// Slower path: cannot precompute the Noms type since there are Noms collections,
+		// but at least there are a set number of fields
 		name := strings.Title(t.Name())
 		e = func(v reflect.Value) types.Value {
 			data := make(types.StructData, len(fields))
@@ -241,6 +246,20 @@ func structEncoder(t reflect.Type, parentStructTypes []reflect.Type) encoderFunc
 			}
 			return types.NewStruct(name, data)
 		}
+	} else {
+		// Slowest path - we are extending some other struct. We need to start with the
+		// type of that struct and extend.
+		e = func(v reflect.Value) types.Value {
+			ret := v.FieldByIndex(originalFieldIndex).Interface().(types.Struct)
+			for _, f := range fields {
+				fv := v.Field(f.index)
+				if !fv.IsValid() || f.omitEmpty && isEmptyValue(fv) {
+					continue
+				}
+				ret = ret.Set(f.name, f.encoder(fv))
+			}
+			return ret
+		}
 	}
 
 	encoderCache.set(t, e)
@@ -352,7 +371,7 @@ func validateField(f reflect.StructField, t reflect.Type) {
 	}
 }
 
-func typeFields(t reflect.Type, parentStructTypes []reflect.Type) (fields fieldSlice, structType *types.Type) {
+func typeFields(t reflect.Type, parentStructTypes []reflect.Type) (fields fieldSlice, structType *types.Type, originalFieldIndex []int) {
 	canComputeStructType := true
 	for i := 0; i < t.NumField(); i++ {
 		f := t.Field(i)
@@ -361,6 +380,12 @@ func typeFields(t reflect.Type, parentStructTypes []reflect.Type) (fields fieldS
 			continue
 		}
 
+		if tags.original {
+			canComputeStructType = false
+			originalFieldIndex = f.Index
+			continue
+		}
+
 		validateField(f, t)
 		nt := nomsType(f.Type, parentStructTypes)
 		if nt == nil {
@@ -441,7 +466,7 @@ func structNomsType(t reflect.Type, parentStructTypes []reflect.Type) *types.Typ
 		}
 	}
 
-	_, structType := typeFields(t, parentStructTypes)
+	_, structType, _ := typeFields(t, parentStructTypes)
 	return structType
 }
 

go/marshal/encode_test.go

@@ -647,6 +647,53 @@ func TestEncodeCanSkipUnexportedField(t *testing.T) {
 	}).Equals(v))
 }
 
+func TestEncodeOriginal(t *testing.T) {
+	assert := assert.New(t)
+
+	type S struct {
+		Foo int          `noms:",omitempty"`
+		Bar types.Struct `noms:",original"`
+	}
+
+	var s S
+	var err error
+	var orig types.Struct
+
+	// New field value clobbers old field value
+	orig = types.NewStruct("S", types.StructData{
+		"foo": types.Number(42),
+	})
+	err = Unmarshal(orig, &s)
+	assert.NoError(err)
+	s.Foo = 43
+	assert.True(MustMarshal(s).Equals(orig.Set("foo", types.Number(43))))
+
+	// New field extends old struct
+	orig = types.NewStruct("S", types.StructData{})
+	err = Unmarshal(orig, &s)
+	assert.NoError(err)
+	s.Foo = 43
+	assert.True(MustMarshal(s).Equals(orig.Set("foo", types.Number(43))))
+
+	// Old struct name always used
+	orig = types.NewStruct("Q", types.StructData{})
+	err = Unmarshal(orig, &s)
+	assert.NoError(err)
+	s.Foo = 43
+	assert.True(MustMarshal(s).Equals(orig.Set("foo", types.Number(43))))
+
+	// Field type of base are preserved
+	st := types.MakeStructType("S", []string{"foo"},
+		[]*types.Type{types.MakeUnionType(types.StringType, types.NumberType)})
+	orig = types.NewStructWithType(st, []types.Value{types.Number(42)})
+	err = Unmarshal(orig, &s)
+	assert.NoError(err)
+	s.Foo = 43
+	out := MustMarshal(s)
+	assert.True(out.Equals(orig.Set("foo", types.Number(43))))
+	assert.True(out.Type().Equals(st))
+}
+
 type TestInterface interface {
 	M()
 }