incrementing a string through allowed characters

[schwim]

Hi there everyone!

I'm trying to solve an issue that I have and it's definitely gotten the better of me. Here's what I'd like to do.

Allowed chars are a-z and 0-9.

entering items into a database, I would like to increment an id to them in the following way.

First entry gets "a". then it will go to "b", "c", etc, and through "0"-"9". Once it gets to "9", the first position reverts to "a" and an "a" gets appended to that, so the id at 37 would be "aa" 36 entries later and you would have an id of "ba". Only once it got to "00" would it add an additional space, giving it an id of "aaa". and so on and so forth, forever and ever.

I thought at first I had it figured out, but my attempt would get the last character to "0" and then append an "a", giving me an id of "0a".

Could I get some help on how to handle this? It would be greatly appreciated!

thanks,
json

[laserlight]

From what I see, using to mean exponentiation:
'a' = 1 1
'ab' = 36 1 + 1 2
'abc' = 362 1 + 36 2 + 1 3
'abcd' = 363 1 + 362 2 + 36 3 + 1 4

'9' = 1 36
'99' = 36 36 + 1 36
'999' = 362 36 + 36 36 + 1 36
'9999' = 363 36 + 36**2 36 + 36 36 + 1 36

This suggests an iterative solution for converting from a natural number $n:

<?php

function compute_id($n) {
    $result = '';
    $divisor = 1;
    while ($divisor <= $n) {
        $m = ($n % ($divisor * 36)) / $divisor;
        if ($m == 0) {
            $m = 36;
        }
        $result = 'abcdefghijklmnopqrstuvwxyz0123456789'[$m - 1] . $result;
        $n -= $m * $divisor;
        $divisor *= 36;
    }
    return $result;
}

echo compute_id(49360) . ' ' . compute_id(1727604);

EDIT:
You probably want to use an autoincrementing integer primary key, then convert that key using this algorithm.

[Weedpacket]

Apart from the rearrangement of having the characters 0-9 following a-z instead of vice versa, this is just writing an integer in conventional base-36 numerals. [man]base_convert[/man]. Afterwards, the reallocation of which character is used to represent which base-36 digit can be done with [man]strtr[/man].

[schwim]

laserlight;11042335 wrote:

function compute_id($n) {
$result = '';
$divisor = 1;
while ($divisor <= $n) {
$m = ($n % ($divisor 36)) / $divisor;
if ($m == 0) {
$m = 36;
}
$result = 'abcdefghijklmnopqrstuvwxyz0123456789'[$m - 1] . $result;
$n -= $m $divisor;
$divisor *= 36;
}
return $result;
}

echo compute_id(49360) . ' ' . compute_id(1727604);[/code]

EDIT:
You probably want to use an autoincrementing integer primary key, then convert that key using this algorithm.

Thanks very much for your help, laserlight. I'm getting an "Unexpected [" on the following line:

$result = 'abcdefghijklmnopqrstuvwxyz0123456789'[$m - 1] . $result; 

and am not sure how to fix it.

[laserlight]

Weedpacket wrote:

Apart from the rearrangement of having the characters 0-9 following a-z instead of vice versa, this is just writing an integer in conventional base-36 numerals. base_convert. Afterwards, the reallocation of which character is used to represent which base-36 digit can be done with strtr.

I considered then discarded that idea since there does not appear to be a concept of zero used here, e.g., z_36 + 1 would be 10_36, but in this case '9' + 1 would be 'aa'. Is there a way to get around this?

schwim wrote:

I'm getting an "Unexpected [" on the following line:

I am using syntax that was introduced in PHP5.5. You can change this:

$result = 'abcdefghijklmnopqrstuvwxyz0123456789'[$m - 1] . $result;

to:

$characters = 'abcdefghijklmnopqrstuvwxyz0123456789';
$result = $characters[$m - 1] . $result;

[schwim]

Thank you again laserlight for your help. I have one more question, if I could.

Is there a snippet of script that I could use to find the id number that correlates to the alphanumeric string that was created with your function?

[laserlight]

schwim wrote:

Is there a snippet of script that I could use to find the id number that correlates to the alphanumeric string that was created with your function?

Write it. This is should be quite easy since it is just addition and multiplication in a loop, no fancy modulo.

[schwim]

I will do my best, but am so far over my head, I'm currently at a loss regarding how to start. Should I first assign a number to each letter, like:

a=1;
b=2;
c=3;

followed by counting the length of the string, then breaking it up into separate characters? I know there's much to it than this, but I'll have to try it in chunks.

Thanks!

[laserlight]

Yes.