Aangifte Optimaal
I'm working on an advanced calculator for Dutch income taxes: https://aangifteoptimaal.nl It shows all intermediate steps, dependencies, references to laws and yearly parameters, and allows you to compare scenarios and optimize how deductions are divided between you and your partner using mixed integer linear programming (MILP). It started out as a spreadsheet which I used to do my taxes when I was freelancing, and over time grew to a ClojureScript webapp. The tax rules are writen as regular code, which is converted to a graph using lisp macros. This calculation graph is used to calculate the taxes, show the intermediates steps, but can also be rendered to a visual form to show dependencies, converted to batch or reactive functions, and even to a MILP form that a solver can use to optimize some deductions. You can also load scenarios from PDFs of tax returns to avoid manual data entry. The app runs completely locally in your browser, no data is shared with a backend at all.