This is a documentation of source for developers.
If you want to use libfityk in your program, first see the namespace fityk, where the public API is documented.
directories
symbolic calculation of derivatives
(can be outdated) Fityk calculates derivatives of functions symbolically. It parses expression to AST based on struct OpTree, calculates recursively derivatives, tries to simplify a bit all expressions (these steps are performed in calculate_deriv()), and then produces bytecode (AnyFormula::tree_to_bytecode()) that can be executed by VM (AnyFormula::run_vm()). In case of $variables, values and derivatives are always calculated together. In case of UDFs, when using non-derivative algorithm, it's significantly more efficient to skip calculation of derivatives, that's what class AnyFormulaO was introduced for. Note, that UDF bytecode is executed n times more than $variable bytecode, where n is the number of data points, and usually n >> 1.
reusability
There are small pieces of code designed to be reusable. They can have more liberal licence than GPL.