Next and previous posts

Signed-off-by: Nick Quaranto <nick@quaran.to>

features/post_data.feature

@@ -139,3 +139,17 @@ Feature: Post data
     When I run jekyll
     Then the _site directory should exist
     And I should see "Post author: Darth Vader" in "_site/2009/03/27/star-wars.html"
+
+  Scenario: Previous and next posts title
+    Given I have a _posts directory
+    And I have a _layouts directory
+    And I have the following posts:
+      | title            | date      | layout  | author      | content                 |
+      | Star Wars        | 3/27/2009 | ordered | Darth Vader | Luke, I am your father. |
+      | Some like it hot | 4/27/2009 | ordered | Osgood      | Nobody is perfect.      |
+      | Terminator       | 5/27/2009 | ordered | Arnold      | Sayonara, baby          |
+    And I have a ordered layout that contains "Previous post: {{ page.previous.title }} and next post: {{ page.next.title }}"
+    When I run jekyll
+    Then the _site directory should exist
+    And I should see "next post: Some like it hot" in "_site/2009/03/27/star-wars.html"
+    And I should see "Previous post: Some like it hot" in "_site/2009/05/27/terminator.html"

lib/jekyll/post.rb

@@ -194,12 +194,33 @@ module Jekyll
         "id" => self.id,
         "topics" => self.topics,
         "categories" => self.categories,
+        "next" => self.next,
+        "previous" => self.previous,
         "content" => self.content }.deep_merge(self.data)
     end
 
     def inspect
       "<Post: #{self.id}>"
     end
+
+    def next
+      pos = self.site.posts.index(self)
+
+      if pos && pos < self.site.posts.length-1
+        self.site.posts[pos+1]
+      else
+        nil
+      end
+    end
+
+    def previous
+      pos = self.site.posts.index(self)
+      if pos && pos > 0
+        self.site.posts[pos-1]
+      else
+        nil
+      end
+    end
   end
 
 end

test/test_post.rb

@@ -75,6 +75,32 @@ class TestPost < Test::Unit::TestCase
       end
     end
 
+    context "when in a site" do
+      setup do
+        clear_dest
+        stub(Jekyll).configuration { Jekyll::DEFAULTS }
+        @site = Site.new(Jekyll.configuration)
+        @site.posts = [setup_post('2008-02-02-published.textile'),
+                       setup_post('2009-01-27-categories.textile')]
+      end
+
+      should "have next post" do
+        assert_equal(@site.posts.last, @site.posts.first.next)
+      end
+
+      should "have previous post" do
+        assert_equal(@site.posts.first, @site.posts.last.previous)
+      end
+
+      should "not have previous post if first" do
+        assert_equal(nil, @site.posts.first.previous)
+      end
+
+      should "not have next post if last" do
+        assert_equal(nil, @site.posts.last.next)
+      end
+    end
+
     context "initializing posts" do
       should "publish when published yaml is no specified" do
         post = setup_post("2008-02-02-published.textile")