What tools do you use to describe complex algorithms?

Question

block diagram

I got a complex project, in the algorithm of which there are many little things and nuances. It is very difficult to keep everything in your head, some things are forgotten, and new changes may logically conflict with the current functionality.
What is the best way to display a schematic algorithm of the application (in my case, the site)?
So that later, looking at this diagram, it would be clear how the site works, and so that it would be easy to discuss the details
with the customer. I'm currently trying to do this in mind maps and the Axure prototyping program.
Or is it better to do everything with pen and paper? How do you solve this problem?

Answer 1

umlet is quite suitable for describing domain models. Algorithms can also be described if desired.

Answer 2

www.xmind.net
logicnet.dk/DiagramDesigner
www.yworks.com/en/products_yed_about.html

Answer 3

online service:
www.lucidchart.com
There are all kinds of charts and export to different formats.
But there are some restrictions in the free account.