Spain-TaxSim

IRPF simulator handling Spanish tax bands, deductions, and the regional variations that catch everyone off-guard.

Problem

• The Spanish IRPF system has progressive bands, a state-vs-regional split, deductions that vary by autonomous community, and special rules for things like investment income and rentals.
• Exactly the kind of branching rule-set where calculators quietly go wrong.

Approach

• Encode all of it and make every assumption explicit rather than buried.

Outcome

• I built it for myself and a few friends.
• A useful exercise in turning a regulation document into a deterministic program — and in respecting how much correctness work hides in the edge cases.