BZip2 is a famous and powerful lossless data compression format. It is very good on some data, like English novels or computer source code. An outstanding aspect of BZip2 is its simplicity. No tricky hash trees or convoluted state machines. However, compression software is generally very difficult to debug, in part impossible. So it is better to do things right the first time and use a language that does its best finding errors before running your program. I show you how I built a working encoder in a few days by leveraging the precise and customisable type system in Ada, especially range types that seem to have been made for this kind of software.