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

navigation/menu.go

@@ -0,0 +1,219 @@
+// 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 navigation
+
+import (
+	"html/template"
+	"sort"
+	"strings"
+
+	"github.com/spf13/cast"
+)
+
+// MenuEntry represents a menu item defined in either Page front matter
+// or in the site config.
+type MenuEntry struct {
+	URL        string
+	Page       Page
+	Name       string
+	Menu       string
+	Identifier string
+	title      string
+	Pre        template.HTML
+	Post       template.HTML
+	Weight     int
+	Parent     string
+	Children   Menu
+}
+
+// A narrow version of page.Page.
+type Page interface {
+	LinkTitle() string
+	RelPermalink() string
+	Section() string
+	Weight() int
+	IsPage() bool
+	Params() map[string]interface{}
+}
+
+// Menu is a collection of menu entries.
+type Menu []*MenuEntry
+
+// Menus is a dictionary of menus.
+type Menus map[string]Menu
+
+// PageMenus is a dictionary of menus defined in the Pages.
+type PageMenus map[string]*MenuEntry
+
+// HasChildren returns whether this menu item has any children.
+func (m *MenuEntry) HasChildren() bool {
+	return m.Children != nil
+}
+
+// KeyName returns the key used to identify this menu entry.
+func (m *MenuEntry) KeyName() string {
+	if m.Identifier != "" {
+		return m.Identifier
+	}
+	return m.Name
+}
+
+func (m *MenuEntry) hopefullyUniqueID() string {
+	if m.Identifier != "" {
+		return m.Identifier
+	} else if m.URL != "" {
+		return m.URL
+	} else {
+		return m.Name
+	}
+}
+
+// IsEqual returns whether the two menu entries represents the same menu entry.
+func (m *MenuEntry) IsEqual(inme *MenuEntry) bool {
+	return m.hopefullyUniqueID() == inme.hopefullyUniqueID() && m.Parent == inme.Parent
+}
+
+// IsSameResource returns whether the two menu entries points to the same
+// resource (URL).
+func (m *MenuEntry) IsSameResource(inme *MenuEntry) bool {
+	return m.URL != "" && inme.URL != "" && m.URL == inme.URL
+}
+
+func (m *MenuEntry) MarshallMap(ime map[string]interface{}) {
+	for k, v := range ime {
+		loki := strings.ToLower(k)
+		switch loki {
+		case "url":
+			m.URL = cast.ToString(v)
+		case "weight":
+			m.Weight = cast.ToInt(v)
+		case "name":
+			m.Name = cast.ToString(v)
+		case "title":
+			m.title = cast.ToString(v)
+		case "pre":
+			m.Pre = template.HTML(cast.ToString(v))
+		case "post":
+			m.Post = template.HTML(cast.ToString(v))
+		case "identifier":
+			m.Identifier = cast.ToString(v)
+		case "parent":
+			m.Parent = cast.ToString(v)
+		}
+	}
+}
+
+func (m Menu) Add(me *MenuEntry) Menu {
+	m = append(m, me)
+	// TODO(bep)
+	m.Sort()
+	return m
+}
+
+/*
+ * Implementation of a custom sorter for Menu
+ */
+
+// A type to implement the sort interface for Menu
+type menuSorter struct {
+	menu Menu
+	by   menuEntryBy
+}
+
+// Closure used in the Sort.Less method.
+type menuEntryBy func(m1, m2 *MenuEntry) bool
+
+func (by menuEntryBy) Sort(menu Menu) {
+	ms := &menuSorter{
+		menu: menu,
+		by:   by, // The Sort method's receiver is the function (closure) that defines the sort order.
+	}
+	sort.Stable(ms)
+}
+
+var defaultMenuEntrySort = func(m1, m2 *MenuEntry) bool {
+	if m1.Weight == m2.Weight {
+		if m1.Name == m2.Name {
+			return m1.Identifier < m2.Identifier
+		}
+		return m1.Name < m2.Name
+	}
+
+	if m2.Weight == 0 {
+		return true
+	}
+
+	if m1.Weight == 0 {
+		return false
+	}
+
+	return m1.Weight < m2.Weight
+}
+
+func (ms *menuSorter) Len() int      { return len(ms.menu) }
+func (ms *menuSorter) Swap(i, j int) { ms.menu[i], ms.menu[j] = ms.menu[j], ms.menu[i] }
+
+// Less is part of sort.Interface. It is implemented by calling the "by" closure in the sorter.
+func (ms *menuSorter) Less(i, j int) bool { return ms.by(ms.menu[i], ms.menu[j]) }
+
+// Sort sorts the menu by weight, name and then by identifier.
+func (m Menu) Sort() Menu {
+	menuEntryBy(defaultMenuEntrySort).Sort(m)
+	return m
+}
+
+// Limit limits the returned menu to n entries.
+func (m Menu) Limit(n int) Menu {
+	if len(m) > n {
+		return m[0:n]
+	}
+	return m
+}
+
+// ByWeight sorts the menu by the weight defined in the menu configuration.
+func (m Menu) ByWeight() Menu {
+	menuEntryBy(defaultMenuEntrySort).Sort(m)
+	return m
+}
+
+// ByName sorts the menu by the name defined in the menu configuration.
+func (m Menu) ByName() Menu {
+	title := func(m1, m2 *MenuEntry) bool {
+		return m1.Name < m2.Name
+	}
+
+	menuEntryBy(title).Sort(m)
+	return m
+}
+
+// Reverse reverses the order of the menu entries.
+func (m Menu) Reverse() Menu {
+	for i, j := 0, len(m)-1; i < j; i, j = i+1, j-1 {
+		m[i], m[j] = m[j], m[i]
+	}
+
+	return m
+}
+
+func (m *MenuEntry) Title() string {
+	if m.title != "" {
+		return m.title
+	}
+
+	if m.Page != nil {
+		return m.Page.LinkTitle()
+	}
+
+	return ""
+}