name: Financial Integrity description: Patterns for provable financial accuracy and invariant testing

Financial Integrity

In the "Action Era", LivestockAI agents may autonomously propose sales or analyze profit. Trust is our currency. Calculations must be mathematically provable.

Core Principle: The "Penny Perfect" Rule

All financial storage and calculation must treat money as an integer (minor units) or high-precision decimal. NEVER use standard generic number floating point math for money.

1. Storage Standards

Database: DECIMAL(19, 4) for unit prices (allows for precise small-unit costs like feed per gram). DECIMAL(19, 2) for final transaction amounts (invoices, payments).

TypeScript: Always use decimal.js via the project's currency utilities.

// ❌ Dangerous
const total = 0.1 + 0.2 // 0.30000000000000004

// ✅ Safe
import { add } from '~/features/settings/currency'
const total = add(0.1, 0.2) // Decimal(0.3)

2. Invariant Testing (Property Tests)

Use fast-check to prove financial laws hold true for all inputs.

Profit Law

Profit = Revenue - (COGS + Expenses)

test('Profit Invariant', () => {
  fc.assert(
    fc.property(fc.integer(), fc.integer(), (revenue, cost) => {
      const profit = calculateProfit(revenue, cost)
      // Profit + Cost must always exactly equal Revenue
      return profit.add(cost).equals(revenue)
    }),
  )
})

Allocation Law

When splitting a cost across N batches, the sum of parts must equal the total. Watch out for rounding leftovers.

Pattern: The "Penny Allocate" Algorithm If splitting $100 among 3 batches: Batch 1: $33.33 Batch 2: $33.33 Batch 3: $33.34 (Takes the remainder)

3. Immutability

Financial records (invoices, sales, expenses) should be "Append-Only" to the user. If a mistake is made:

1. Create a Reversal Transaction (negative amount) to void the error.
2. Create a new Correct Transaction. This preserves the audit trail.

4. Multi-Currency Safety

Always store exchangeRate and currencyCode at the time of transaction. Never calculate historical value using current exchange rates.

Related Skills

• financial-calculations - The specific library calls implementation
• property-testing - The value verification method