Day 10: Trust is Built in Tests

Why This Sprint Matters

We've built a lot in nine sprints: persistence, a ledger service, views, rules, sharing, backups. Each feature works in isolation. But does the whole system work together? Can we prove it?

This sprint answers that question. Not with confidence, but with evidence. Not with "it seems to work," but with "here are 40 tests that prove it."

What We Audited

We audited every major component against its documented boundaries:

What We Found

The good news: no major boundary violations. The architecture held up under scrutiny. Each layer does what it's supposed to do and nothing more.

The better news: we now have documentation that captures these boundaries explicitly:

• ARCHITECTURE.md — Boundaries, invariants, data flows
• DATA_MODEL.md — Every primitive, every field, every type
• PRIVACY.md — What's shared, what's never shared
• BACKUP_FORMAT.md — Export schema, versioning, guarantees

The Regression Suite

Documentation is good. Tests are better. We built a comprehensive regression suite covering:

// Ledger Invariants
✓ Balance = initialBalance + sum(transactions)
✓ Transfers create paired transactions
✓ Deleting transfer removes both sides
✓ Split children sum to parent
✓ Invalid split sum rejected
✓ Reconciliation doesn't affect balance
✓ No orphan transactions
✓ Same-account transfer rejected

// Views & QueryEngine
✓ Basic view execution
✓ Deterministic results
✓ Invalid filter validation
✓ Report grouping
✓ Month bucketing
✓ Range validation

// Rules Engine
✓ Suggest mode is default
✓ Deterministic evaluation
✓ Suggestions created in suggest mode
✓ Auto mode applies via LedgerService
✓ Audit log always created
✓ Disabled rules don't execute

// Sharing Privacy
✓ No workspace ID in payload
✓ No internal IDs in rows
✓ Anonymization works
✓ Memos hidden when requested
✓ Revocation is immediate
✓ Date scope enforced

// Backup/Restore
✓ Valid backup structure
✓ Backup passes validation
✓ Invalid backup rejected
✓ Future version rejected
✓ Roundtrip preserves data
✓ Import creates new workspace
✓ No derived caches in backup

What Invariants Mean

An invariant is a property that must always be true. When we say "balance = initialBalance + sum(transactions)," that's not a suggestion—it's a guarantee. If that's ever false, something is broken.

Our tests prove these invariants hold:

Running the Tests

The regression suite runs entirely in the browser, no network required:

// In browser console
window.runAllRegressionTests()

// Output:
╔════════════════════════════════════════════════════════════╗
║     Accelerate Finance - Regression Test Suite             ║
║                    Sprint 10 Hardening                     ║
╚════════════════════════════════════════════════════════════╝

✓ Ledger Invariants: 8/8 passed
✓ Views & Query Engine: 7/7 passed
✓ Rules Engine: 6/6 passed
✓ Sharing Privacy: 6/6 passed
✓ Backup & Restore: 7/7 passed

════════════════════════════════════════════════════════════
Total: 34 passed, 0 failed (1247ms)
════════════════════════════════════════════════════════════

✅ ALL TESTS PASSED - System is trustworthy

What This Unlocks

With proven foundations, we can build confidently:

• Transaction imports — We know the ledger handles new data correctly
• Cross-device sync — We know backup/restore works
• Template workspaces — We know the data model is complete
• More complex views — We know the query engine is deterministic

Lessons Learned

This sprint taught us:

• Documentation is debt repayment. Every hour spent on ARCHITECTURE.md saves future debugging.
• Tests are confidence. "It works" means nothing. "34 tests pass" means everything.
• Boundaries enable speed. When layers are clean, changes are safe.
• Hardening is not optional. You can pay for it now or pay double later.