diff of removed page, require handling right rev=0

When page is removed, and diff can be requested between a revision and
current situation. This results in right revision is 0.

Similar case just after creating a page. A diff between the first
version and nothing before, result in left revision is 0.

In these cases a empty dummy revision is placed as selected value in
dropdown. Otherwise user got distracted by the revisions details shown
in select field,  which are not related to the diff below.

2 files changed, 35 insertions, 12 deletions

diff --git a/inc/changelog.php b/inc/changelog.php
index d2ad23c08..28e53e77a 100644
--- a/inc/changelog.php
+++ b/inc/changelog.php
@@ -642,7 +642,7 @@ abstract class ChangeLog {
      * When available it returns $max entries for each revision
      *
      * @param int $rev1 oldest revision timestamp
-     * @param int $rev2 newest revision timestamp
+     * @param int $rev2 newest revision timestamp (0 looks up last revision)
      * @param int $max maximum number of revisions returned
      * @return array with two arrays with revisions surrounding rev1 respectively rev2
      */
@@ -651,10 +651,16 @@ abstract class ChangeLog {
         $rev1 = max($rev1, 0);
         $rev2 = max($rev2, 0);
 
-        if($rev2 < $rev1) {
-            $rev = $rev2;
-            $rev2 = $rev1;
-            $rev1 = $rev;
+        if($rev2) {
+            if($rev2 < $rev1) {
+                $rev = $rev2;
+                $rev2 = $rev1;
+                $rev1 = $rev;
+            }
+        } else {
+            //empty right side means a removed page. Look up last revision.
+            $revs = $this->getRevisions(-1, 1);
+            $rev2 = $revs[0];
         }
         //collect revisions around rev2
         list($revs2, $allrevs, $fp, $lines, $head, $tail) = $this->retrieveRevisionsAround($rev2, $max);
diff --git a/inc/html.php b/inc/html.php
index 1a549824e..39d0eeada 100644
--- a/inc/html.php
+++ b/inc/html.php
@@ -1308,21 +1308,27 @@ function html_diff($text = '', $intro = true, $type = null) {
 function html_diff_navigation($pagelog, $type, $l_rev, $r_rev) {
     global $INFO, $ID;
 
-    //last timestamp is not in changelog
+    //last timestamp is not in changelog (note: when page is removed, the metadata timestamp is zero as well)
     $r_rev = $r_rev ? $r_rev : $INFO['meta']['last_change']['date'];
 
     //retrieve revisions with additional info
     list($l_revs, $r_revs) = $pagelog->getRevisionsAround($l_rev, $r_rev);
     $l_revisions = array();
+    if(!$l_rev) {
+        $l_revisions[0] = array(0, "", false); //no left revision given, add dummy
+    }
     foreach($l_revs as $rev) {
         $info = $pagelog->getRevisionInfo($rev);
         $l_revisions[$rev] = array(
             $rev,
             dformat($info['date']) . ' ' . editorinfo($info['user']) . ' ' . $info['sum'],
-            $rev >= $r_rev //disable?
+            $r_rev ? $rev >= $r_rev : false //disable?
         );
     }
     $r_revisions = array();
+    if(!$r_rev) {
+        $r_revisions[0] = array(0, "", false); //no right revision given, add dummy
+    }
     foreach($r_revs as $rev) {
         $info = $pagelog->getRevisionInfo($rev);
         $r_revisions[$rev] = array(
@@ -1331,13 +1337,24 @@ function html_diff_navigation($pagelog, $type, $l_rev, $r_rev) {
             $rev <= $l_rev //disable?
         );
     }
+
     //determine previous/next revisions
     $l_index = array_search($l_rev, $l_revs);
     $l_prev = $l_revs[$l_index + 1];
     $l_next = $l_revs[$l_index - 1];
-    $r_index = array_search($r_rev, $r_revs);
-    $r_prev = $r_revs[$r_index + 1];
-    $r_next = $r_revs[$r_index - 1];
+    if($r_rev) {
+        $r_index = array_search($r_rev, $r_revs);
+        $r_prev = $r_revs[$r_index + 1];
+        $r_next = $r_revs[$r_index - 1];
+    } else {
+        //removed page
+        if($l_next) {
+            $r_prev = $r_revs[0];
+        } else {
+            $r_prev = null;
+        }
+        $r_next = null;
+    }
 
     /*
      * Left side:
@@ -1366,7 +1383,7 @@ function html_diff_navigation($pagelog, $type, $l_rev, $r_rev) {
     $form->addElement(form_makeButton('submit', 'diff', 'Go'));
     $l_nav .= $form->getForm();
     //move forward
-    if($l_next && $l_next < $r_rev) {
+    if($l_next && ($l_next < $r_rev || !$r_rev)) {
         $l_nav .= html_diff_navigationlink($type, 'diffnextrev', $l_next, $r_rev);
     }
 
@@ -1418,7 +1435,7 @@ function html_diff_navigation($pagelog, $type, $l_rev, $r_rev) {
  */
 function html_diff_navigationlink($difftype, $linktype, $lrev, $rrev = null) {
     global $ID, $lang;
-    if($rrev === null) {
+    if(!$rrev) {
         $urlparam = array(
             'do' => 'diff',
             'rev' => $lrev,