tpl/collections: Add collections.SymDiff

Fixes #5410

tpl/collections/symdiff.go

@@ -0,0 +1,71 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package collections
+
+import (
+	"fmt"
+	"reflect"
+
+	"github.com/pkg/errors"
+)
+
+// SymDiff returns the symmetric difference of s1 and s2.
+// Arguments must be either a slice or an array of comparable types.
+func (ns *Namespace) SymDiff(s2, s1 interface{}) (interface{}, error) {
+	ids1, err := collectIdentities(s1)
+	if err != nil {
+		return nil, err
+	}
+	ids2, err := collectIdentities(s2)
+	if err != nil {
+		return nil, err
+	}
+
+	var slice reflect.Value
+	var sliceElemType reflect.Type
+
+	for i, s := range []interface{}{s1, s2} {
+		v := reflect.ValueOf(s)
+
+		switch v.Kind() {
+		case reflect.Array, reflect.Slice:
+			if i == 0 {
+				sliceType := v.Type()
+				sliceElemType = sliceType.Elem()
+				slice = reflect.MakeSlice(sliceType, 0, 0)
+			}
+
+			for i := 0; i < v.Len(); i++ {
+				ev, _ := indirectInterface(v.Index(i))
+				if !ev.Type().Comparable() {
+					return nil, errors.New("symdiff: elements must be comparable")
+				}
+				key := normalize(ev)
+				// Append if the key is not in their intersection.
+				if ids1[key] != ids2[key] {
+					v, err := convertValue(ev, sliceElemType)
+					if err != nil {
+						return nil, errors.WithMessage(err, "symdiff: failed to convert value")
+					}
+					slice = reflect.Append(slice, v)
+				}
+			}
+		default:
+			return nil, fmt.Errorf("arguments to symdiff must be slices or arrays")
+		}
+	}
+
+	return slice.Interface(), nil
+
+}