1 files changed, 64 insertions, 0 deletions
diff --git a/resources/page/pages_language_merge.go b/resources/page/pages_language_merge.go
new file mode 100644
index 000000000..11393a754
--- /dev/null
+++ b/resources/page/pages_language_merge.go
@@ -0,0 +1,64 @@
+// 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 page
+
+import (
+	"fmt"
+)
+
+var (
+	_ pagesLanguageMerger = (*Pages)(nil)
+)
+
+type pagesLanguageMerger interface {
+	MergeByLanguage(other Pages) Pages
+	// Needed for integration with the tpl package.
+	MergeByLanguageInterface(other interface{}) (interface{}, error)
+}
+
+// MergeByLanguage supplies missing translations in p1 with values from p2.
+// The result is sorted by the default sort order for pages.
+func (p1 Pages) MergeByLanguage(p2 Pages) Pages {
+	merge := func(pages *Pages) {
+		m := make(map[string]bool)
+		for _, p := range *pages {
+			m[p.TranslationKey()] = true
+		}
+
+		for _, p := range p2 {
+			if _, found := m[p.TranslationKey()]; !found {
+				*pages = append(*pages, p)
+			}
+		}
+
+		SortByDefault(*pages)
+	}
+
+	out, _ := spc.getP("pages.MergeByLanguage", merge, p1, p2)
+
+	return out
+}
+
+// MergeByLanguageInterface is the generic version of MergeByLanguage. It
+// is here just so it can be called from the tpl package.
+func (p1 Pages) MergeByLanguageInterface(in interface{}) (interface{}, error) {
+	if in == nil {
+		return p1, nil
+	}
+	p2, ok := in.(Pages)
+	if !ok {
+		return nil, fmt.Errorf("%T cannot be merged by language", in)
+	}
+	return p1.MergeByLanguage(p2), nil
+}