1 files changed, 113 insertions, 0 deletions
diff --git a/common/collections/append.go b/common/collections/append.go
new file mode 100644
index 000000000..b56455bc9
--- /dev/null
+++ b/common/collections/append.go
@@ -0,0 +1,113 @@
+// Copyright 2019 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"
+)
+
+// Append appends from to a slice to and returns the resulting slice.
+// If length of from is one and the only element is a slice of same type as to,
+// it will be appended.
+func Append(to interface{}, from ...interface{}) (interface{}, error) {
+	tov, toIsNil := indirect(reflect.ValueOf(to))
+
+	toIsNil = toIsNil || to == nil
+	var tot reflect.Type
+
+	if !toIsNil {
+		if tov.Kind() != reflect.Slice {
+			return nil, fmt.Errorf("expected a slice, got %T", to)
+		}
+
+		tot = tov.Type().Elem()
+		toIsNil = tov.Len() == 0
+
+		if len(from) == 1 {
+			fromv := reflect.ValueOf(from[0])
+			if fromv.Kind() == reflect.Slice {
+				if toIsNil {
+					// If we get nil []string, we just return the []string
+					return from[0], nil
+				}
+
+				fromt := reflect.TypeOf(from[0]).Elem()
+
+				// If we get []string []string, we append the from slice to to
+				if tot == fromt {
+					return reflect.AppendSlice(tov, fromv).Interface(), nil
+				} else if !fromt.AssignableTo(tot) {
+					// Fall back to a []interface{} slice.
+					return appendToInterfaceSliceFromValues(tov, fromv)
+
+				}
+			}
+		}
+	}
+
+	if toIsNil {
+		return Slice(from...), nil
+	}
+
+	for _, f := range from {
+		fv := reflect.ValueOf(f)
+		if !fv.Type().AssignableTo(tot) {
+			// Fall back to a []interface{} slice.
+			tov, _ := indirect(reflect.ValueOf(to))
+			return appendToInterfaceSlice(tov, from...)
+		}
+		tov = reflect.Append(tov, fv)
+	}
+
+	return tov.Interface(), nil
+}
+
+func appendToInterfaceSliceFromValues(slice1, slice2 reflect.Value) ([]interface{}, error) {
+	var tos []interface{}
+
+	for _, slice := range []reflect.Value{slice1, slice2} {
+		for i := 0; i < slice.Len(); i++ {
+			tos = append(tos, slice.Index(i).Interface())
+		}
+	}
+
+	return tos, nil
+}
+
+func appendToInterfaceSlice(tov reflect.Value, from ...interface{}) ([]interface{}, error) {
+	var tos []interface{}
+
+	for i := 0; i < tov.Len(); i++ {
+		tos = append(tos, tov.Index(i).Interface())
+	}
+
+	tos = append(tos, from...)
+
+	return tos, nil
+}
+
+// indirect is borrowed from the Go stdlib: 'text/template/exec.go'
+// TODO(bep) consolidate
+func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
+	for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
+		if v.IsNil() {
+			return v, true
+		}
+		if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
+			break
+		}
+	}
+	return v, false
+}