MDL-31244, MDL-25063 algebra filter: fix common false positives.

There are two well-known cases where the algebra filter messes up input
that is obviously not meant for the algebra filter:

1. Copy and paste of unified diffs.

2. @@PLUGINFILE@@ tokens in the HTML that are due to be replaced by the
files API.

This fix detects these two cases, and just stops the algebra filter from
replacing them.

filter/algebra/filter.php

@@ -89,7 +89,7 @@ function filter_algebra_image($imagefile, $tex= "", $height="", $width="", $alig
 }
 
 class filter_algebra extends moodle_text_filter {
-    function filter($text, array $options = array()){
+    public function filter($text, array $options = array()){
         global $CFG, $DB;
 
         /// Do a quick check using stripos to avoid unnecessary wor
@@ -114,9 +114,6 @@ class filter_algebra extends moodle_text_filter {
 #        return $text;
 #    }
 
-
-        $text .= ' ';
-
         preg_match_all('/@(@@+)([^@])/',$text,$matches);
         for ($i=0;$i<count($matches[0]);$i++) {
             $replacement = str_replace('@','&#x00040;',$matches[1][$i]).$matches[2][$i];
@@ -129,6 +126,17 @@ class filter_algebra extends moodle_text_filter {
         preg_match_all('/<algebra>(.+?)<\/algebra>|@@(.+?)@@/is', $text, $matches);
         for ($i=0; $i<count($matches[0]); $i++) {
             $algebra = $matches[1][$i] . $matches[2][$i];
+
+            // Look for some common false positives, and skip processing them.
+            if ($algebra == 'PLUGINFILE' || $algebra == 'DRAFTFILE') {
+                // Raw pluginfile URL.
+                continue;
+            }
+            if (preg_match('/^ -\d+(,\d+)? \+\d+(,\d+)? $/', $algebra)) {
+                // Part of a unified diff.
+                continue;
+            }
+
             $algebra = str_replace('<nolink>','',$algebra);
             $algebra = str_replace('</nolink>','',$algebra);
             $algebra = str_replace('<span class="nolink">','',$algebra);
@@ -159,7 +167,7 @@ class filter_algebra extends moodle_text_filter {
                $algebra = str_replace('upsilon','zupslon',$algebra);
                $algebra = preg_replace('!\r\n?!',' ',$algebra);
                $algebra = escapeshellarg($algebra);
-               if ( (PHP_OS == "WINNT") || (PHP_OS == "WIN32") || (PHP_OS == "Windows") ) {
+               if ( (PHP_OS == "WINNT") || (PHP_OS == "WIN32") || (PHP_OS == "Windows")) {
                   $cmd  = "cd $CFG->dirroot\\filter\\algebra & algebra2tex.pl $algebra";
                } else {
                   $cmd  = "cd $CFG->dirroot/filter/algebra; ./algebra2tex.pl $algebra";
@@ -220,6 +228,7 @@ class filter_algebra extends moodle_text_filter {
                   $texexp = preg_replace('/\\\int\\\left\((.+?d[a-z])\\\right\)/s','\int '. "\$1 ",$texexp);
                   $texexp = preg_replace('/\\\lim\\\left\((.+?),(.+?),(.+?)\\\right\)/s','\lim_'. "{\$2\\to \$3}\$1 ",$texexp);
                   $texexp = str_replace('\mbox', '', $texexp); // now blacklisted in tex, sorry
+                  $texcache = new stdClass();
                   $texcache->filter = 'algebra';
                   $texcache->version = 1;
                   $texcache->md5key = $md5;

filter/algebra/tests/filter_test.php

@@ -0,0 +1,90 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Unit test for the filter_algebra
+ *
+ * @package    filter_algebra
+ * @category   phpunit
+ * @copyright  2012 Tim Hunt
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+global $CFG;
+require_once($CFG->dirroot . '/filter/algebra/filter.php');
+
+
+/**
+ * Unit tests for filter_algebra.
+ *
+ * Note that this only tests some of the filter logic. It does not acutally test
+ * the normal case of the filter working, because I cannot make it work on my
+ * test server, and if it does not work here, it probably does not also work
+ * for other people. A failing test will be irritating noise.
+ *
+ * @copyright  2012 Tim Hunt
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class filter_algebra_testcase extends basic_testcase {
+
+    protected $filter;
+
+    protected function setUp() {
+        parent::setUp();
+        $this->filter = new filter_algebra(context_system::instance(), array());
+    }
+
+    function test_algebra_filter_no_algebra() {
+        $this->assertEquals('<p>Look no algebra!</p>',
+                $this->filter->filter('<p>Look no algebra!</p>'));
+    }
+
+
+    function test_algebra_filter_pluginfile() {
+        $this->assertEquals('<img src="@@PLUGINFILE@@/photo.jpg">',
+                $this->filter->filter('<img src="@@PLUGINFILE@@/photo.jpg">'));
+    }
+
+    function test_algebra_filter_draftfile() {
+        $this->assertEquals('<img src="@@DRAFTFILE@@/photo.jpg">',
+                $this->filter->filter('<img src="@@DRAFTFILE@@/photo.jpg">'));
+    }
+
+    function test_algebra_filter_unified_diff() {
+        $diff = '
+diff -u -r1.1 Worksheet.php
+--- Worksheet.php   26 Sep 2003 04:18:02 -0000  1.1
++++ Worksheet.php   18 Nov 2009 03:58:50 -0000
+@@ -1264,10 +1264,10 @@
+         }
+
+         // Strip the = or @ sign at the beginning of the formula string
+-        if (ereg("^=",$formula)) {
++        if (preg_match("/^=/",$formula)) {
+             $formula = preg_replace("/(^=)/","",$formula);
+         }
+-        elseif(ereg("^@",$formula)) {
++        elseif(preg_match("/^@/",$formula)) {
+             $formula = preg_replace("/(^@)/","",$formula);
+         }
+         else {
+';
+        $this->assertEquals('<pre>' . $diff . '</pre>',
+                $this->filter->filter('<pre>' . $diff . '</pre>'));
+    }
+}