Convertions between decimal and base 36

Question

I want to convert numbers in base 36 using PHP. The function base_convert is not working because I want to convert large numbers: I don't get my initial number if I convert it again from base 36 to decimal.

I tried some function given on multiple websites, but I never get the same result. Also, these two websites (in Javascript) give the same result:

• http://www.unitconversion.org/numbers/base-10-to-base-36-conversion.html
• http://www.translatorscafe.com/cafe/units-converter/numbers/calculator/decimal-to-base-36/

For example 1010701001118000000000000000 has to be converted to 3IZS0ZE1RQ68W8SSW4.

Here are the functions I tried (and which don't work):

• http://www.php.net/manual/fr/function.base-convert.php#34510
• http://www.php.net/manual/fr/function.base-convert.php#106546
• http://www.tonymarston.net/php-mysql/showsource.php?file=converter.php
• http://www.geoffray.be/blog/php/convertir-des-nombres-en-base-62

Answer 1

Here are two simple functions using an algorithm found on Wikipedia, while using bcmath to get the calculations even for very large numbers right:

function fromDecimalToBase($in, $to) {
    $in = (string) $in;
    $out = '';

    for ($i = strlen($in) - 1; $i >= 0; $i--) {
        $out = base_convert(bcmod($in, $to), 10, $to) . $out;
        $in = bcdiv($in, $to);
    }

    return preg_replace('/^0+/', '', $out);
}

function fromBaseToDecimal($in, $from) {
    $in = (string) $in;
    $out = '';

    for ($i = 0, $l = strlen($in); $i < $l; $i++) {
        $x = base_convert(substr($in, $i, 1), $from, 10);
        $out = bcadd(bcmul($out, $from), $x);
    }

    return preg_replace('/^0+/', '', $out);
}

However, I get 3izs0ze1rq66tifrpc for the number you've provided - maybe your conversion was wrong there?

Answer 2

http://www.pgregg.com/projects/php/base_conversion/base_conversion.php

This page show you how to convert arbitrary length numbers between different bases. I tried your example and it looks like it works in both directions. Source code written in PHP is available.