badges/lib/bakerlib.php

   1 <?php
   2 // This file is part of Moodle - http://moodle.org/
   3 //
   4 // Moodle is free software: you can redistribute it and/or modify
   5 // it under the terms of the GNU General Public License as published by
   6 // the Free Software Foundation, either version 3 of the License, or
   7 // (at your option) any later version.
   8 //
   9 // Moodle is distributed in the hope that it will be useful,
  10 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12 // GNU General Public License for more details.
  13 //
  14 // You should have received a copy of the GNU General Public License
  15 // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
  16
  17 /**
  18  * Baking badges library.
  19  *
  20  * @package    core
  21  * @subpackage badges
  22  * @copyright  2012 onwards Totara Learning Solutions Ltd {@link http://www.totaralms.com/}
  23  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  24  * @author     Yuliya Bozhko <yuliya.bozhko@totaralms.com>
  25  */
  26
  27 defined('MOODLE_INTERNAL') || die();
  28
  29 /**
  30  * Information on PNG file chunks can be found at http://www.w3.org/TR/PNG/#11Chunks
  31  * Some other info on PNG that I used http://garethrees.org/2007/11/14/pngcrush/
  32  *
  33  * Example of use:
  34  * $png = new PNG_MetaDataHandler('file.png');
  35  *
  36  * if ($png->check_chunks("tEXt", "openbadge")) {
  37  *     $newcontents = $png->add_chunks("tEXt", "openbadge", 'http://some.public.url/to.your.assertion.file');
  38  * }
  39  *
  40  * file_put_contents('file.png', $newcontents);
  41  */
  42
  43 class PNG_MetaDataHandler
  44 {
  45     /** @var string File content as a string */
  46     private $_contents;
  47     /** @var int Length of the image file */
  48     private $_size;
  49     /** @var array Variable for storing parsed chunks */
  50     private $_chunks;
  51
  52     /**
  53      * Prepares file for handling metadata.
  54      * Verifies that this file is a valid PNG file.
  55      * Unpacks file chunks and reads them into an array.
  56      *
  57      * @param string $contents File content as a string
  58      */
  59     public function __construct($contents) {
  60         $this->_contents = $contents;
  61         $png_signature = pack("C8", 137, 80, 78, 71, 13, 10, 26, 10);
  62
  63         // Read 8 bytes of PNG header and verify.
  64         $header = substr($this->_contents, 0, 8);
  65
  66         if ($header != $png_signature) {
  67             debugging('This is not a valid PNG image');
  68         }
  69
  70         $this->_size = strlen($this->_contents);
  71
  72         $this->_chunks = array();
  73
  74         // Skip 8 bytes of IHDR image header.
  75         $position = 8;
  76         do {
  77             $chunk = @unpack('Nsize/a4type', substr($this->_contents, $position, 8));
  78             $this->_chunks[$chunk['type']][] = substr($this->_contents, $position + 8, $chunk['size']);
  79
  80             // Skip 12 bytes chunk overhead.
  81             $position += $chunk['size'] + 12;
  82         } while ($position < $this->_size);
  83     }
  84
  85     /**
  86      * Checks if a key already exists in the chunk of said type.
  87      * We need to avoid writing same keyword into file chunks.
  88      *
  89      * @param string $type Chunk type, like iTXt, tEXt, etc.
  90      * @param string $check Keyword that needs to be checked.
  91      *
  92      * @return boolean (true|false) True if file is safe to write this keyword, false otherwise.
  93      */
  94     public function check_chunks($type, $check) {
  95         if (array_key_exists($type, $this->_chunks)) {
  96             foreach (array_keys($this->_chunks[$type]) as $typekey) {
  97                 list($key, $data) = explode("\0", $this->_chunks[$type][$typekey]);
  98
  99                 if (strcmp($key, $check) == 0) {
 100                     debugging('Key "' . $check . '" already exists in "' . $type . '" chunk.');
 101                     return false;
 102                 }
 103             }
 104         }
 105         return true;
 106     }
 107
 108     /**
 109      * Adds a chunk with keyword and data to the file content.
 110      * Chunk is added to the end of the file, before IEND image trailer.
 111      *
 112      * @param string $type Chunk type, like iTXt, tEXt, etc.
 113      * @param string $key Keyword that needs to be added.
 114      * @param string $value Currently an assertion URL that is added to an image metadata.
 115      *
 116      * @return string $result File content with a new chunk as a string. Can be used in file_put_contents() to write to a file.
 117      */
 118     public function add_chunks($type, $key, $value) {
 119         if (strlen($key) > 79) {
 120             debugging('Key is too big');
 121         }
 122
 123         // tEXt Textual data.
 124         // Keyword:        1-79 bytes (character string)
 125         // Null separator: 1 byte
 126         // Text:           n bytes (character string)
 127         $data = $key . "\0" . $value;
 128         $crc = pack("N", crc32($type . $data));
 129         $len = pack("N", strlen($data));
 130
 131         // Chunk format: length + type + data + CRC.
 132         // CRC is a CRC-32 computed over the chunk type and chunk data.
 133         $newchunk = $len . $type . $data . $crc;
 134
 135         $result = substr($this->_contents, 0, $this->_size - 12)
 136                 . $newchunk
 137                 . substr($this->_contents, $this->_size - 12, 12);
 138
 139         return $result;
 140     }
 141 }