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POPHP - help fix a bug caused by two number dividing exactly
 

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  1. POPHP - help fix a bug caused by two number dividing exactly
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    <p>I have been working with a piece of code for youtube style URL's but I have found a bug and I am hoping someone can show me the most efficient way to fix it.</p> <pre><code>function alphaID($in, $to_num = false, $pad_up = false, $passKey = null) { static $passcache; if(empty($passcache)) $passcache = array(); $index = 'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'; $i = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'); if (!empty($passKey)) { // Although this function's purpose is to just make the // ID short - and not so much secure, // with this patch by Simon Franz (http://blog.snaky.org/) // you can optionally supply a password to make it harder // to calculate the corresponding numeric ID if(isset($passcache[$passKey])) $index = $passcache[$passKey]; else { if(strlen($passhash = hash('sha256',$passKey)) &lt; strlen($index)) $passhash = hash('sha512',$passKey); $p = str_split($passhash); array_multisort($p, SORT_DESC, $i); $index = implode($i); $passcache = $index; } } $base = strlen($index); if ($to_num) { // Digital number &lt;&lt;-- alphabet letter code $in = strrev($in); $out = 0; $len = strlen($in) - 1; for ($t = 0; $t &lt;= $len; $t++) { $bcpow = bcpow($base, $len - $t); $out += strpos($index, $in[$t]) * $bcpow; } if (is_numeric($pad_up)) { $pad_up--; if ($pad_up &gt; 0) { $out -= pow($base, $pad_up); } } } else { // Digital number --&gt;&gt; alphabet letter code if (is_numeric($pad_up)) { $pad_up--; if ($pad_up &gt; 0) { $in += pow($base, $pad_up); } } $out = ""; for ($t = floor(log10($in) / log10($base)); $t &gt;= 0; $t--) { $bcp = bcpow($base, $t); $a = floor($in / $bcp); $out .= $index[$a]; $in -= $a * $bcp; } $out = strrev($out); // reverse } return $out; } </code></pre> <p>The bug is only when encoding a single number 238328 as it is my base to the power of three. As a result it divides exactly and because of use of 'floor' it goes unnoticed and the script tries to add the 62nd character which does not exist and only produces a three character code rather than four... thus 'aa' is the result rather than 'aaab'.</p> <p>Here is the problem part of the code:</p> <pre><code> for ($t = floor(log10($in) / log10($base)); $t &gt;= 0; $t--) { $bcp = bcpow($base, $t); $a = floor($in / $bcp); $out .= $index[$a]; $in -= $a * $bcp; </code></pre> <p>And to make it even easier here is the call to get the error</p> <pre><code>echo alphaID(238328); </code></pre> <p>credits: Orginally written by Kevin Vanzonneveld: kevin dot vanzonneveld dot net, modified by Simon Franz: blog dot snaky dot org and optimised by Stackoverflows very own mattbasta</p>
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