Does anyone know how id's are given out for a custom entity Salesforce?

I presumed that the last 15 characters (case sensitive) followed a sequence "0-9" then "A-Z" and then "abcdefghijklmnopqrstuvwxyz". It seems to be sequential for a while and then jumps forward.

e.g. 000A, 000B, 000C, 000D and then 021C

The reason for this is that I am developing an sControl to generate a sequential number such as an order number.

I do this by inserting a record into a custom enitity which returns a unique id each time, I then convert it to a number using the following function:

calculate(salesforceId.substring(4,15))

function convert(input) {
var convertBase="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
var  origin;
var  dest;
var origin=61;
var dest=10;
var b=0;
var Result="";
if (Number(origin)>convertBase.length || Number(dest)>convertBase.length || Number(origin)<2 || Number(dest)<2) return "Invalid numbering system"
for (var c=1;c<=input.length;c++) { b+=convertBase.indexOf(input.substring(c-1,c))*(Math.pow(origin,input.length-c)); if
(convertBase.indexOf(input.substring(c-1,c)) > Number(origin)) return 'Character "'+input.substring(c-1,c)+'" not present in origin system'; }
var a=Math.floor(Math.log(b)/Math.log(dest))
while (a>-1) {
  var e=Math.pow(dest,a)
  a--;
  var d=(b-b%e)/e+1;
  b%=e;
  Ciffer=convertBase.substring(d-1,d);
  Result+=Ciffer;
  }
return Result
}

The problem is that every so often the sequence breaks.