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, 113 insertions, 0 deletions

diff --git a/hugolib/page__tree.go b/hugolib/page__tree.go
new file mode 100644
index 000000000..a99e6f16c
--- /dev/null
+++ b/hugolib/page__tree.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 hugolib
+
+import (
+	"github.com/gohugoio/hugo/common/types"
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/resources/page"
+)
+
+type pageTree struct {
+	p *pageState
+}
+
+func (pt pageTree) IsAncestor(other interface{}) (bool, error) {
+	if pt.p == nil {
+		return false, nil
+	}
+
+	pp, err := unwrapPage(other)
+	if err != nil || pp == nil {
+		return false, err
+	}
+
+	if pt.p.Kind() == page.KindPage && len(pt.p.SectionsEntries()) == len(pp.SectionsEntries()) {
+		// A regular page is never its section's ancestor.
+		return false, nil
+	}
+
+	return helpers.HasStringsPrefix(pp.SectionsEntries(), pt.p.SectionsEntries()), nil
+}
+
+func (pt pageTree) CurrentSection() page.Page {
+	p := pt.p
+
+	if p.IsHome() || p.IsSection() {
+		return p
+	}
+
+	return p.Parent()
+}
+
+func (pt pageTree) IsDescendant(other interface{}) (bool, error) {
+	if pt.p == nil {
+		return false, nil
+	}
+	pp, err := unwrapPage(other)
+	if err != nil || pp == nil {
+		return false, err
+	}
+
+	if pp.Kind() == page.KindPage && len(pt.p.SectionsEntries()) == len(pp.SectionsEntries()) {
+		// A regular page is never its section's descendant.
+		return false, nil
+	}
+	return helpers.HasStringsPrefix(pt.p.SectionsEntries(), pp.SectionsEntries()), nil
+}
+
+func (pt pageTree) FirstSection() page.Page {
+	p := pt.p
+
+	parent := p.Parent()
+
+	if types.IsNil(parent) || parent.IsHome() {
+		return p
+	}
+
+	for {
+		current := parent
+		parent = parent.Parent()
+		if types.IsNil(parent) || parent.IsHome() {
+			return current
+		}
+	}
+
+}
+
+func (pt pageTree) InSection(other interface{}) (bool, error) {
+	if pt.p == nil || types.IsNil(other) {
+		return false, nil
+	}
+
+	pp, err := unwrapPage(other)
+	if err != nil {
+		return false, err
+	}
+
+	if pp == nil {
+		return false, nil
+	}
+
+	return pp.CurrentSection().Eq(pt.p.CurrentSection()), nil
+
+}
+
+func (pt pageTree) Parent() page.Page {
+	return pt.p.parent
+}
+
+func (pt pageTree) Sections() page.Pages {
+	return pt.p.subSections
+}