Convertions between decimal and base 36

C

2

5

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:

For example 1010701001118000000000000000 has to be converted to 3IZS0ZE1RQ68W8SSW4.

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

Chairborne answered 6/5, 2012 at 16:2 Comment(4)

why not convert your number to string and then encode with base64? – Lubbi 6/5, 2012 at 16:9

The idea is to reduce the length of the number. – Chairborne 6/5, 2012 at 16:35

Do you need to convert the number back to decimal at some point? – Therewith 6/5, 2012 at 16:37

Yes I do, the number in base 36 is just here to be exchanged, I need to get it in decimal at the end (whithout knowing the function used to convert in base 36). – Chairborne 6/5, 2012 at 16:43

T

10

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?

Therewith answered 6/5, 2012 at 16:40 Comment(4)

You get the same result as the other answer, maybe the Javascript converters are wrong... But I don't know how to find out. I may have to find a "perfect" converter to know... – Chairborne 6/5, 2012 at 16:45

@Chairborne I've added the conversion back to decimal, and it seems to work so far. – Therewith 6/5, 2012 at 17:14

But, I can use only the first one and just swap $from and $to :). Whatever, I see that you use the PHP function base_convert, so I think your conversion is the right one. I'll accept your answer, thanks! – Chairborne 6/5, 2012 at 17:48

The difference between the two functions is mainly, that the first one computes % and / in the source-system while the second one computes + and * in the target-system. Since bcmath can only handle decimal numbers, you will need to use the first one to get from decimal to any other base (so that "decimal" is the source-system) and the second one to get from any base back to decimal ("decimal" = target-system). – Therewith 6/5, 2012 at 17:52

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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.

Boggart answered 6/5, 2012 at 16:23 Comment(1)

I don't know which code is right, but with your link, I get 3IZS0ZE1RQ66TIFRPC instead of 3IZS0ZE1RQ68W8SSW4 – Chairborne 6/5, 2012 at 16:37

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