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+<?php
+///////////////////////////////////////////////////////////////////////////
+//                                                                       //
+// NOTICE OF COPYRIGHT                                                   //
+//                                                                       //
+// TeXH - convert TeX code in <tex> tags into images, using HeVeA        //
+//                                                                       //
+// Copyright (C) 2001-2003  Bruno Vernier  bruno@vsbeducation.ca         //
+//                                                                       //
+// This program 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 2 of the License, or     //
+// (at your option) any later version.                                   //
+//                                                                       //
+// This program 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:                          //
+//                                                                       //
+//          http://www.gnu.org/copyleft/gpl.html                         //
+//                                                                       //
+///////////////////////////////////////////////////////////////////////////
+
+//-------------------------------------------------------------------------
+//
+// README PREREQUISITES:
+//
+// 1: install HeVeA on your server http://pauillac.inria.fr/~maranget/hevea/
+//     (debian and rpm available) (it is a TeX to HTML/MathML filter)
+//
+// 2: must include this file "texh.php" in the filters in config.php
+//
+// 3: must include TEX in the legal tags list in lib/weblib.php
+//    CAVEAT: if above not yet done, <TEX> gets converted to &lt;TEX&gt;
+//    and can only be fixed by editing in HTML conversion mode
+//
+// 4: if you are using Windows then the hevea.bat file that comes with the
+//    windows distribution needs to have the line
+//        if "%1"=="" goto syn
+//    removed.
+//
+//-------------------------------------------------------------------------
+
+/// These lines are important - the variable must match the name 
+/// of the actual function below
+
+    $textfilter_function='texh_filter';
+
+    if (function_exists($textfilter_function)) {
+        return;
+    }
+
+
+function texh_pipe_cmd($cmd, $text) {
+    // general function for external system calls to $cmd,  piping in $text, retrieving output
+    // taken mostly from a php tutorial on php.net website (modified slightly)    
+    $spec = array(   
+            0 => array("pipe", "r"),  // stdin 
+            1 => array("pipe", "w"),  // stdout 
+            );
+    $process = proc_open($cmd , $spec, $pipes);
+    $output="";
+    if (is_resource($process)) {
+        fwrite($pipes[0], $text);
+        fclose($pipes[0]);
+        while (!feof($pipes[1])) { 
+            $buffer = fgets($pipes[1], 1024);
+            $output .= $buffer;
+        }
+        fclose($pipes[1]);
+        proc_close($process);
+    }
+    return $output;     
+}
+
+function texh_filter ($courseid, $text) {  
+    // TeX conversion with $$ tex code $$ 
+    // written by Bruno Vernier (c) 2004 GPL 
+
+    /// Do a quick check using stripos to avoid unnecessary work
+    if (!(stripos($text, '<tex') or stripos($text, '$$'))) {
+        return $text;
+    }
+
+    if (isadmin()) { error_reporting (E_ALL); }; //for debugging
+
+    // <TEX> some general TeX expression </TEX> 
+    preg_match_all('/<tex>(.+?)<\/tex>/is', $text, $matches);  
+    for ($i=0; $i<count($matches[0]); $i++) { 
+        $pipe = texh_pipe_cmd("hevea",$matches[1][$i]);  // take the content only (non-math TeX expected)
+        $text = str_replace( $matches[0][$i], $pipe, $text);
+    }
+
+    // $$ some MATHEMATICAL TeX expression $$
+    preg_match_all('/\$\$(.+?)\$\$/', $text, $matches);  
+    for ($i=0; $i<count($matches[0]); $i++) { 
+        $pipe = texh_pipe_cmd("hevea",$matches[0][$i]);  // take everything since the $$ is part of the TeX syntax for math
+        $text = str_replace( $matches[0][$i], $pipe, $text);
+    }
+
+    return $text; 
+
+}
+
+
+
+//  Various Notes and Possibilities for Developers only:    
+    
+//   Note to self:  the following did not work consistently:    
+//   $text = preg_replace('/\$\$(.*?)\$\$/', texh_pipe_cmd('hevea',"\\$0"), $text);
+//   whereas this seems to work consistently:
+//   $text = str_replace( $matches[0][$i], texh_pipe_cmd("hevea",$matches[0][$i]), $text);
+
+
+// TODO:
+//
+// - user-friendly guide to inputting TeX (at least enough for up to high school level math)
+// - caveat regarding square-roots and roots being converted to fractional exponents 
+// - integrate memaid http://memaid.sourceforge.net
+// - compile a list of useful shortcuts and shorthands worth doing here
+
+
+// future possibilities
+// $text = str_replace("my_marks", "my marks are ...%", $text);                   // insert SQL/php_function results here
+// $text = str_replace("my_uptime", "uptime: ". shell_exec("/usr/bin/uptime") , $text);  // a shortcut to a safe system call
+// $text = preg_replace("/vernier/", "Mr. Bruno Vernier" , $text);                // a shortcut expanded
+
+// Version 0.1  Jan 24,2004
+
+?>
+