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Proforma payslip simulation

HR admins repeatedly need the answer to a hypothetical — "if I offer €4.100 bruto wit, what is the net?", "what does an AOW-age hire net?", "how does a 0,8 part-time factor land?" — without creating throw-away Employee/EmploymentContract/PayrollRun/Payslip records. The proforma simulator is a persist-nothing front door to the same engine documented on The payroll engine: it builds a CalculationInput from hypothetical inputs, runs the existing PayrollCalculator against the real nl-<YYYY> tables, and returns the full gross-to-net breakdown.

Nothing is written​

The simulator creates no Employee, EmploymentContract, PayrollRun or Payslip, and performs no OpenRegister write of any kind — the register's object count is unchanged before and after a simulation. It adds no new tax logic: the calculator, its input/result value objects, and the tables are consumed exactly as they are for a real run. Given the anchor input (€3.800,00 wit, below AOW, period 2026-02), the simulator reproduces the engine's own anchor figures exactly — netto €3.081,17 — proving the reuse is faithful rather than a reimplementation that happens to agree today.

What it returns​

The full breakdown: grossPay, loonheffing, arbeidskorting, volksverzekeringen, zvw (mode + rate), appliedTaxRate, vakantiegeldReserved, werknemersverzekeringen, employerCharges, and nettoPay.

A one-off bijzondere beloning is folded into the period gross as a combined-loon estimate — explicitly not the statutory bijzonder tarief (a named engine fast-follow) — so the response is honest about what shortcut it took.

Running a simulation​

occ hrmq:payroll:proforma --gross 3800 --table wit --period 2026-02

This is the support-facing surface: reproduce a net figure with no browser and no database access. Flags: --gross, --table (wit/groen), --date-of-birth, --parttime, --bijzonder, --period, --aof, --whk.

In the UI, the "Simuleer loonstrook" page renders the hypothetical-input form and the breakdown by calling POST /api/payroll/proforma. This endpoint is RBAC-gated the same way the rest of payroll is: a caller whose RBAC cannot resolve the payroll register — i.e. anyone who could not see a real Payslip — gets HTTP 404, so the simulator surface itself is never leaked to an unauthorized caller.