<?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/>.

/**
 * Baking badges library.
 *
 * @package    core
 * @subpackage badges
 * @copyright  2012 onwards Totara Learning Solutions Ltd {@link http://www.totaralms.com/}
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @author     Yuliya Bozhko <yuliya.bozhko@totaralms.com>
 */

defined('MOODLE_INTERNAL') || die();

/**
 * Information on PNG file chunks can be found at http://www.w3.org/TR/PNG/#11Chunks
 * Some other info on PNG that I used http://garethrees.org/2007/11/14/pngcrush/
 *
 * Example of use:
 * $png = new PNG_MetaDataHandler('file.png');
 *
 * if ($png->check_chunks("tEXt", "openbadge")) {
 *     $newcontents = $png->add_chunks("tEXt", "openbadge", 'http://some.public.url/to.your.assertion.file');
 * }
 *
 * file_put_contents('file.png', $newcontents);
 */

class PNG_MetaDataHandler
{
    /** @var string File content as a string */
    private $_contents;
    /** @var int Length of the image file */
    private $_size;
    /** @var array Variable for storing parsed chunks */
    private $_chunks;

    /**
     * Prepares file for handling metadata.
     * Verifies that this file is a valid PNG file.
     * Unpacks file chunks and reads them into an array.
     *
     * @param string $contents File content as a string
     */
    public function __construct($contents) {
        $this->_contents = $contents;
        $png_signature = pack("C8", 137, 80, 78, 71, 13, 10, 26, 10);

        // Read 8 bytes of PNG header and verify.
        $header = substr($this->_contents, 0, 8);

        if ($header != $png_signature) {
            debugging('This is not a valid PNG image');
        }

        $this->_size = strlen($this->_contents);

        $this->_chunks = array();

        // Skip 8 bytes of IHDR image header.
        $position = 8;
        do {
            $chunk = @unpack('Nsize/a4type', substr($this->_contents, $position, 8));
            $this->_chunks[$chunk['type']][] = substr($this->_contents, $position + 8, $chunk['size']);

            // Skip 12 bytes chunk overhead.
            $position += $chunk['size'] + 12;
        } while ($position < $this->_size);
    }

    /**
     * Checks if a key already exists in the chunk of said type.
     * We need to avoid writing same keyword into file chunks.
     *
     * @param string $type Chunk type, like iTXt, tEXt, etc.
     * @param string $check Keyword that needs to be checked.
     *
     * @return boolean (true|false) True if file is safe to write this keyword, false otherwise.
     */
    public function check_chunks($type, $check) {
        if (array_key_exists($type, $this->_chunks)) {
            foreach (array_keys($this->_chunks[$type]) as $typekey) {
                list($key, $data) = explode("\0", $this->_chunks[$type][$typekey]);

                if (strcmp($key, $check) == 0) {
                    debugging('Key "' . $check . '" already exists in "' . $type . '" chunk.');
                    return false;
                }
            }
        }
        return true;
    }

    /**
     * Adds a chunk with keyword and data to the file content.
     * Chunk is added to the end of the file, before IEND image trailer.
     *
     * @param string $type Chunk type, like iTXt, tEXt, etc.
     * @param string $key Keyword that needs to be added.
     * @param string $value Currently an assertion URL that is added to an image metadata.
     *
     * @return string $result File content with a new chunk as a string. Can be used in file_put_contents() to write to a file.
     */
    public function add_chunks($type, $key, $value) {
        if (strlen($key) > 79) {
            debugging('Key is too big');
        }

        // tEXt Textual data.
        // Keyword:        1-79 bytes (character string)
        // Null separator: 1 byte
        // Text:           n bytes (character string)
        $data = $key . "\0" . $value;
        $crc = pack("N", crc32($type . $data));
        $len = pack("N", strlen($data));

        // Chunk format: length + type + data + CRC.
        // CRC is a CRC-32 computed over the chunk type and chunk data.
        $newchunk = $len . $type . $data . $crc;

        $result = substr($this->_contents, 0, $this->_size - 12)
                . $newchunk
                . substr($this->_contents, $this->_size - 12, 12);

        return $result;
    }
}