Apart from careful and diligent reading, what are the best workflow and tool to study and understand the source code of a third party application or library (mainly Ruby, for example, in order to adapt a Ruby library to Crystal)?
What would seem to me to be the most suitable would be a graph (graphviz type) representing the relationships between method calls (possibly limited to only the methods of the library or application to avoid complexity)
You have to study and understand the source code of a third party ruby application or library, at least partially.
Refactor the ruby project as much as possible make it more Crystal way. you have to use some external tools, e.g. rubocop, the key is, it must be still workable as a ruby program.
Now, if you done this, probably a graph is in your heat.