Universal Credit: port full programme structure (43 Python files → ~1 Rust fn)

[vahid-ahmadi]

Gap

The Python policyengine-uk model has 43 files under policyengine_uk/variables/gov/dwp/universal_credit/ covering the childcare element, work allowance, taper, capital limits, transitional protection, managed migration, housing element, standard allowance, child element, carer element, and limited-capability-for-work element.

The Rust port currently has one function: compute_universal_credit in src/variables/benefits.rs.

Why it matters

UC is the largest UK welfare programme by caseload and spend. Until UC is fully modelled, Rust outputs cannot be used for any policy analysis touching working-age means-tested support — which covers the majority of distributional/revenue work the team does (transitional protection alone moves billions in scoring). Every reform notebook in the wider PolicyEngine catalogue that touches UC will diverge from Python.

What to port

Source of truth (Python): policyengine_uk/variables/gov/dwp/universal_credit/

Subdirectories to mirror in Rust:

• elements/ — standard allowance, child element, childcare element, housing element, carer element, LCWRA element
• work_allowance.py, earnings/ — work allowance + taper logic
• capital/ — £6k disregard, £16k cliff, tariff income
• transitional_protection/ — managed-migration top-up calculation
• entitlement/ — final UC amount, take-up adjustments

Effort

Large. Recommend landing alongside issue for the YAML test harness so each ported variable arrives with regression coverage cross-checked against Python.

References

• Python: policyengine_uk/variables/gov/dwp/universal_credit/
• Rust: src/variables/benefits.rs::compute_universal_credit
• Parameter tree: policyengine_uk/parameters/gov/dwp/universal_credit/