Invert function: base 38 conversion

Question

Hello!

Can you tell me how the invert function for the following PHP function is?

<?php
function id2secure($old_number) {
 $alphabet_en = '1357902468acegikmoqsuwybdfhjlnprtvxz-_';
 $new_number = '';
 while ($old_number > 0) {
  $rest = $old_number%38;
  if ($rest >= 38) { return FALSE; }
  $new_number .= $alphabet_en[$rest];
  $old_number = floor($old_number/38);
 }
 $new_number = strrev($new_number);
 return $new_number;
}
echo id2secure(172293);
?>

Thank you very much in advance!

Answer 1

Haven't tested this code, but it might work:

<?php function secure2id($sr)
{
  $s = strrev($sr);
  $alpha = '1357902468acegikmoqsuwybdfhjlnprtvxz';
  $alpha2num = array();
  $n = strlen($alpha);
  for($i = 0; $i < $n; $i++)
  {
    $alpha2num[$alpha[$i]] = $i;
  }
  $rez = 0;
  $n = strlen($s);
  $b = 1;
  for($i = 0; $i < $n; $i++)
  {
    $rez += $b * $alpha2num[$s[$i]];
    $b *= 38;
  }
  return $rez;
} ?>

`

Answer 2

Are you asking how to convert base 38 to base 10? The numerical algorithm is this:

1. Let N be the new number in base 10. Set N to zero to start with.
2. Let X be the original number in base 38.
3. Multiply N by 38.
4. Let D be the most significant (leftmost) digit of X.
5. Let T be the value of D in base 10.
6. Add T to N.
7. Remove D from X (so the number X is now 1 digit shorter).
8. If X is empty, goto 10.
9. Goto 3.
10. Return N, which is now the fully converted base 10 number.

Now that you understand the math, it should be fairly straightforward to convert these steps to a PHP function.

Answer 3

This is secure :) Took me a few minutes to crack it. Here you go,

function secure2id($new_number) {
        $alphabet_en = '1357902468acegikmoqsuwybdfhjlnprtvxz';
        $old_number = 0;
        $new_number = strrev($new_number);
        $len=strlen($new_number);
        $n=0;
        $base=1;
        while($n<$len){
            $c=$new_number[$n];
            $index = strpos($alphabet_en, $c);
            if ($index === false)
                 break;
            $old_number += $base * $index;
            $base *= 38;
            $n++;
        }
        return $old_number;
}