Add head/draft/autosave layer comparison at three UI entry points

Node#resolve_page_reference and #available_layer_pairs let
Page#diff_against compare named layers (head/draft/autosave), not
just numbered revisions -- autosave was never part of Node#pages, so
this was the missing piece.

Wired into nodes#show's Status section, nodes#edit right after an
autosave gets resurrected ("What changed?"), and the admin wizard's
current-drafts table, which now also lists autosave-only nodes it
previously never showed.

revisions#diff hides the numbered-revision picker when comparing
named layers (it can't represent them), shows a plain label instead,
and offers buttons to switch between whichever other pairs make
sense for the node's current state. Destroying the topmost layer is
available directly from the diff view, reusing the existing revert!
path.

"Discard changes" is renamed "Discard Autosave" everywhere it
appears, to match "Destroy Draft".

13 files changed, 179 insertions, 20 deletions

diff --git a/app/controllers/admin_controller.rb b/app/controllers/admin_controller.rb
index 3fa0519..6ab2135 100644
--- a/app/controllers/admin_controller.rb
+++ b/app/controllers/admin_controller.rb
@@ -5,10 +5,10 @@ class AdminController < ApplicationController
   before_action :login_required
 
   def index
-    @drafts = Node.where("draft_id IS NOT NULL")
+    @drafts = Node.where("draft_id IS NOT NULL OR autosave_id IS NOT NULL")
       .limit(50).order("updated_at desc")
 
-    @drafts_count = Node.where("draft_id IS NOT NULL").count
+    @drafts_count = Node.where("draft_id IS NOT NULL OR autosave_id IS NOT NULL").count
 
     @recent_changes = Node.where(
       "updated_at < ? AND updated_at > ? AND parent_id IS NOT NULL",
diff --git a/app/controllers/nodes_controller.rb b/app/controllers/nodes_controller.rb
index 38d42d9..d1538e1 100644
--- a/app/controllers/nodes_controller.rb
+++ b/app/controllers/nodes_controller.rb
@@ -72,7 +72,7 @@ class NodesController < ApplicationController
     if @node.autosave
       flash.now[:notice] =
         "This page has unsaved changes from a previous session, shown below. " \
-        "Save to keep them, or use \"Discard changes\" below to go back to the last saved version."
+        "Save to keep them, or use \"Discard Autosave\" below to go back to the last saved version."
     elsif freshly_locked
       flash.now[:notice] = "Node locked and ready to edit"
     end
diff --git a/app/controllers/revisions_controller.rb b/app/controllers/revisions_controller.rb
index 9acb26f..4b0c549 100644
--- a/app/controllers/revisions_controller.rb
+++ b/app/controllers/revisions_controller.rb
@@ -21,10 +21,18 @@ class RevisionsController < ApplicationController
       params[:start_revision], params[:end_revision] = 1, 1
     end
 
-    @start      = @node.pages.find_by_revision( params[:start_revision] )
-    @end        = @node.pages.find_by_revision( params[:end_revision] )
-    @diff_view  = params[:view] == "side_by_side" ? :side_by_side : :inline
-    @diff       = @end.diff_against( @start, view: @diff_view )
+    @start = @node.resolve_page_reference(params[:start_revision])
+    @end   = @node.resolve_page_reference(params[:end_revision])
+
+    if @start.nil? || @end.nil?
+      flash[:error] = "That comparison is no longer available."
+      redirect_to(node_path(@node)) and return
+    end
+
+    @diff_view              = params[:view] == "side_by_side" ? :side_by_side : :inline
+    @diff                   = @end.diff_against(@start, view: @diff_view)
+    @available_layer_pairs  = @node.available_layer_pairs
+    @locked_by_other        = @node.locked? && @node.lock_owner != current_user
   end
 
   def show
diff --git a/app/helpers/revisions_helper.rb b/app/helpers/revisions_helper.rb
index fdb51f8..a629013 100644
--- a/app/helpers/revisions_helper.rb
+++ b/app/helpers/revisions_helper.rb
@@ -1,2 +1,7 @@
 module RevisionsHelper
+  # Human-readable label for a diff endpoint -- "head"/"draft"/"autosave"
+  # get their name; anything else is a revision number.
+  def describe_page_reference(ref)
+    %w[head draft autosave].include?(ref.to_s) ? ref.to_s.capitalize : "revision #{ref}"
+  end
 end
diff --git a/app/models/node.rb b/app/models/node.rb
index 7675ab6..82d9954 100644
--- a/app/models/node.rb
+++ b/app/models/node.rb
@@ -123,6 +123,27 @@ class Node < ApplicationRecord
     self.draft.reload
   end
 
+  def resolve_page_reference ref
+    case ref.to_s
+    when "head" then head
+    when "draft" then draft
+    when "autosave" then autosave
+    else pages.find_by_revision(ref)
+    end
+  end
+
+  # Which layer-pairs are meaningful to compare right now, given this
+  # node's actual state. Head vs autosave only shows up when no draft
+  # sits between them -- with a draft present, autosave is compared
+  # against the draft, never past it straight to head.
+  def available_layer_pairs
+    pairs = []
+    pairs << [:head, :draft]     if head && draft
+    pairs << [:draft, :autosave] if draft && autosave
+    pairs << [:head, :autosave]  if head && autosave && !draft
+    pairs
+  end
+
   def find_or_create_draft current_user
     self.wipe_draft!
     if draft && self.lock_owner == current_user
diff --git a/app/views/admin/index.html.erb b/app/views/admin/index.html.erb
index 77b45f4..c67ccb3 100644
--- a/app/views/admin/index.html.erb
+++ b/app/views/admin/index.html.erb
@@ -82,9 +82,9 @@
 </div>
 
 <div id="current_drafts_table">
-  
-  <h2>Current Drafts (<%= @drafts_count %>)</h2>
-  
+
+  <h2>Current Drafts & Autosaves (<%= @drafts_count %>)</h2>
+
   <table class="node_table">
     <tr class="header">
       <th class="node_id">ID</th>
@@ -92,6 +92,7 @@
       <th class="actions">Actions</th>
       <th class="editor">Locked by</th>
       <th class="revision">Rev.</th>
+      <th class="autosave">Autosave</th>
     </tr>
     <% @drafts.each do |node| %>
     <tr class="<%= cycle("even", "odd") %>">
@@ -103,6 +104,9 @@
       <td class="actions">
         <%= link_to 'show',      node_path(node) %>
         <%= link_to 'Revisions', node_revisions_path(node) %>
+        <% if pair = node.available_layer_pairs.last %>
+          <%= link_to 'diff', diff_node_revisions_path(node, start_revision: pair.first, end_revision: pair.last) %>
+        <% end %>
       </td>
       <td>
         <%= node.lock_owner.login if node.lock_owner %>
@@ -110,6 +114,9 @@
       <td>
         <%= link_to ( node.draft ? node.draft.revision : (node.head ? node.head.revision : "EMPTY" ) ), node_revisions_path(node) %>
       </td>
+      <td class="autosave">
+        <%= node.autosave ? "unsaved changes" : "" %>
+      </td>
     </tr>
     <% end %>
   </table>
diff --git a/app/views/nodes/edit.html.erb b/app/views/nodes/edit.html.erb
index cdc9b36..1c19410 100644
--- a/app/views/nodes/edit.html.erb
+++ b/app/views/nodes/edit.html.erb
@@ -6,9 +6,16 @@
         disabled: @node.autosave.present? %>
 
   <% if @node.autosave || (@node.draft && @node.head) %>
-    <%= button_to (@node.draft && !@node.autosave ? 'Destroy Draft' : 'Discard changes'),
+    <%= button_to (@node.draft && !@node.autosave ? 'Destroy Draft' : 'Discard Autosave'),
           revert_node_path(@node), method: :put,
           form: { data: { confirm: "This cannot be undone. Continue?" }, class: 'button_to destructive' } %>
+    <% if pair = @node.available_layer_pairs.find { |p| p.include?(:autosave) } %>
+      <%= button_to 'What changed?',
+            diff_node_revisions_path(@node),
+            method: :get,
+            params: { start_revision: pair.first, end_revision: pair.last },
+            form: { class: 'button_to computation' } %>
+    <% end %>
   <% end %>
 
   <%= submit_tag "Save Draft", form: dom_id(@node, :edit) %>
diff --git a/app/views/nodes/show.html.erb b/app/views/nodes/show.html.erb
index 036caf2..2469310 100644
--- a/app/views/nodes/show.html.erb
+++ b/app/views/nodes/show.html.erb
@@ -32,6 +32,16 @@
           <% end %>
         </div>
 
+        <% @node.available_layer_pairs.each do |pair| %>
+          <div class="node_info_item">
+            <%= button_to "Diff #{pair.first.to_s.capitalize} vs. #{pair.last.to_s.capitalize}",
+                  diff_node_revisions_path(@node),
+                  method: :get,
+                  params: { start_revision: pair.first, end_revision: pair.last },
+                  form: { class: 'button_to computation' } %>
+          </div>
+        <% end %>
+
         <% unless locked_by_other %>
           <% if @node.draft && !@node.autosave %>
             <div class="node_info_item">
@@ -41,7 +51,7 @@
           <% end %>
           <% if @node.draft || @node.autosave %>
             <div class="node_info_item">
-              <%= button_to (@node.draft && !@node.autosave ? 'Destroy Draft' : 'Discard changes'),
+              <%= button_to (@node.draft && !@node.autosave ? 'Destroy Draft' : 'Discard Autosave'),
                     revert_node_path(@node), method: :put,
                     form: { data: { confirm: "This cannot be undone. Continue?" }, class: 'button_to destructive' } %>
             </div>
diff --git a/app/views/revisions/diff.html.erb b/app/views/revisions/diff.html.erb
index d7bb528..3157dca 100644
--- a/app/views/revisions/diff.html.erb
+++ b/app/views/revisions/diff.html.erb
@@ -4,11 +4,41 @@
   <%= link_to 'Revisions', node_revisions_path(@node) %>
 </p>
 
-<%= form_tag diff_node_revisions_path do %>
-  <%= select_tag :start_revision, options_for_select(@node.pages.map{|x| x.revision}, params[:start_revision].to_i) %>
-  <%= select_tag :end_revision, options_for_select(@node.pages.map{|x| x.revision}, params[:end_revision].to_i) %>
-  <%= select_tag :view, options_for_select([['Inline', 'inline'], ['Side by side', 'side_by_side']], @diff_view) %>
-  <%= submit_tag 'Diff' %>
+<p class="diff_comparison_label">
+  Comparing <strong><%= describe_page_reference(params[:start_revision]) %></strong>
+  against <strong><%= describe_page_reference(params[:end_revision]) %></strong>
+</p>
+
+<% numeric_comparison = params[:start_revision].to_s =~ /\A\d+\z/ && params[:end_revision].to_s =~ /\A\d+\z/ %>
+
+<% if numeric_comparison %>
+  <%= form_tag diff_node_revisions_path do %>
+    <%= select_tag :start_revision, options_for_select(@node.pages.map{|x| x.revision}, params[:start_revision].to_i) %>
+    <%= select_tag :end_revision, options_for_select(@node.pages.map{|x| x.revision}, params[:end_revision].to_i) %>
+    <%= select_tag :view, options_for_select([['Inline', 'inline'], ['Side by side', 'side_by_side']], @diff_view) %>
+    <%= submit_tag 'Diff' %>
+  <% end %>
+<% else %>
+  <p class="node_action_bar standalone_action_bar"><%= link_to 'Compare two numbered revisions instead', node_revisions_path(@node) %></p>
+<% end %>
+
+<% if @available_layer_pairs.present? %>
+  <p class="node_action_bar standalone_action_bar">
+    <% @available_layer_pairs.each do |pair| %>
+      <% next if [params[:start_revision].to_s, params[:end_revision].to_s].sort == pair.map(&:to_s).sort %>
+      <%= button_to "Diff #{pair.first.to_s.capitalize} vs. #{pair.last.to_s.capitalize}",
+            diff_node_revisions_path(@node),
+            method: :get,
+            params: { start_revision: pair.first, end_revision: pair.last, view: @diff_view },
+            form: { class: 'button_to computation' } %>
+    <% end %>
+
+    <% if !@locked_by_other && (@node.autosave || @node.draft) %>
+      <%= button_to (@node.draft && !@node.autosave ? 'Destroy Draft' : 'Discard Autosave'),
+            revert_node_path(@node), method: :put,
+            form: { data: { confirm: "This cannot be undone. Continue?" }, class: 'button_to destructive' } %>
+    <% end %>
+  </p>
 <% end %>
 
 <div id="diffview">
diff --git a/config/routes.rb b/config/routes.rb
index c0aef2f..da6b626 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -47,6 +47,7 @@ Cccms::Application.routes.draw do
       resources :revisions do
         collection do
           post :diff
+          get  :diff
         end
         member do
           put :restore
diff --git a/test/controllers/admin_controller_test.rb b/test/controllers/admin_controller_test.rb
index 9bbf29b..d6005ba 100644
--- a/test/controllers/admin_controller_test.rb
+++ b/test/controllers/admin_controller_test.rb
@@ -1,8 +1,17 @@
 require 'test_helper'
 
 class AdminControllerTest < ActionController::TestCase
-  # Replace this with your real tests.
-  test "the truth" do
-    assert true
+  test "current drafts includes nodes with only an autosave" do
+    node = Node.root.children.create!(:slug => "admin_autosave_only")
+    node.lock_for_editing!(User.find_by_login("aaron"))
+    node.autosave!({title: "in progress"}, User.find_by_login("aaron"))
+    node.save_draft!(User.find_by_login("aaron"))
+    node.publish_draft!
+    node.lock_for_editing!(User.find_by_login("aaron"))
+    node.autosave!({title: "editing again"}, User.find_by_login("aaron"))
+
+    login_as :quentin
+    get :index
+    assert_includes assigns(:drafts), node
   end
 end
diff --git a/test/controllers/revisions_controller_test.rb b/test/controllers/revisions_controller_test.rb
index caca6bf..162e6f1 100644
--- a/test/controllers/revisions_controller_test.rb
+++ b/test/controllers/revisions_controller_test.rb
@@ -100,4 +100,43 @@ class RevisionsControllerTest < ActionController::TestCase
     assert_response :success
     assert_select ".diff_column", 2
   end
+
+  test "diffing head against draft by name" do
+    login_as :quentin
+    @node.find_or_create_draft(@user)
+    @node.draft.update(:body => "draft body")
+
+    post(:diff, params: { :node_id => @node.id, :start_revision => "head", :end_revision => "draft" })
+    assert_response :success
+  end
+
+  test "diffing a layer pair that no longer exists redirects with a flash" do
+    login_as :quentin
+    post(:diff, params: { :node_id => @node.id, :start_revision => "draft", :end_revision => "autosave" })
+    assert_redirected_to node_path(@node)
+    assert flash[:error].present?
+  end
+
+  test "diffing by name shows a clear comparison label instead of a misleading revision picker" do
+    login_as :quentin
+    @node.find_or_create_draft(@user)
+
+    post(:diff, params: { :node_id => @node.id, :start_revision => "head", :end_revision => "draft" })
+    assert_response :success
+    assert_select "strong", "Head"
+    assert_select "strong", "Draft"
+    assert_select "select[name=?]", "start_revision", :count => 0
+  end
+
+  test "pair-switcher buttons carry their params as real hidden fields, not a query string" do
+    login_as :quentin
+    @node.find_or_create_draft(@user)
+    @node.lock_for_editing!(@user)
+    @node.autosave!({ :body => "unsaved" }, @user)
+
+    post(:diff, params: { :node_id => @node.id, :start_revision => "head", :end_revision => "draft" })
+    assert_response :success
+    assert_select "form.computation input[type=hidden][name=start_revision]"
+    assert_select "form.computation input[type=hidden][name=end_revision]"
+  end
 end
diff --git a/test/models/node_test.rb b/test/models/node_test.rb
index 2138c19..9e71dec 100644
--- a/test/models/node_test.rb
+++ b/test/models/node_test.rb
@@ -473,4 +473,26 @@ class NodeTest < ActiveSupport::TestCase
       node.publish_draft!
     end
   end
+
+  test "available_layer_pairs matches the six-state table" do
+    node = Node.root.children.create!(:slug => "layer_pairs_test")
+    user = @user1 || User.find_by_login("aaron")
+
+    assert_equal [[:draft, :autosave]], (node.lock_for_editing!(user); node.autosave!({title: "v1"}, user); node.available_layer_pairs) # state F
+
+    node.save_draft!(user)
+    node.publish_draft!
+    assert_equal [], node.available_layer_pairs # state A
+
+    node.lock_for_editing!(user)
+    node.autosave!({title: "v2"}, user)
+    assert_equal [[:head, :autosave]], node.available_layer_pairs # state B
+
+    node.save_draft!(user)
+    assert_equal [[:head, :draft]], node.available_layer_pairs # state C
+
+    node.lock_for_editing!(user)
+    node.autosave!({title: "v3"}, user)
+    assert_equal [[:head, :draft], [:draft, :autosave]], node.available_layer_pairs # state D
+  end
 end