Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

1 files changed, 76 insertions, 0 deletions

diff --git a/hugolib/page__position.go b/hugolib/page__position.go
new file mode 100644
index 000000000..458b3e423
--- /dev/null
+++ b/hugolib/page__position.go
@@ -0,0 +1,76 @@
+// 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 hugolib
+
+import (
+	"github.com/gohugoio/hugo/lazy"
+	"github.com/gohugoio/hugo/resources/page"
+)
+
+func newPagePosition(n *nextPrev) pagePosition {
+	return pagePosition{nextPrev: n}
+}
+
+func newPagePositionInSection(n *nextPrev) pagePositionInSection {
+	return pagePositionInSection{nextPrev: n}
+
+}
+
+type nextPrev struct {
+	init     *lazy.Init
+	prevPage page.Page
+	nextPage page.Page
+}
+
+func (n *nextPrev) next() page.Page {
+	n.init.Do()
+	return n.nextPage
+}
+
+func (n *nextPrev) prev() page.Page {
+	n.init.Do()
+	return n.prevPage
+}
+
+type pagePosition struct {
+	*nextPrev
+}
+
+func (p pagePosition) Next() page.Page {
+	return p.next()
+}
+
+func (p pagePosition) NextPage() page.Page {
+	return p.Next()
+}
+
+func (p pagePosition) Prev() page.Page {
+	return p.prev()
+}
+
+func (p pagePosition) PrevPage() page.Page {
+	return p.Prev()
+}
+
+type pagePositionInSection struct {
+	*nextPrev
+}
+
+func (p pagePositionInSection) NextInSection() page.Page {
+	return p.next()
+}
+
+func (p pagePositionInSection) PrevInSection() page.Page {
+	return p.prev()
+}