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einvoice/CURRENCY_IMPLEMENTATION.md
Juergen Kunz 10e14af85b feat(validation): Implement EN16931 compliance validation types and VAT categories 
- Added validation types for EN16931 compliance in `validation.types.ts`, including interfaces for `ValidationResult`, `ValidationOptions`, and `ValidationReport`.
- Introduced `VATCategoriesValidator` in `vat-categories.validator.ts` to validate VAT categories according to EN16931 rules, including detailed checks for standard, zero-rated, exempt, reverse charge, intra-community, export, and out-of-scope services.
- Enhanced `IEInvoiceMetadata` interface in `en16931-metadata.ts` to include additional fields required for full standards compliance, such as delivery information, payment information, allowances, and charges.
- Implemented helper methods for VAT calculations and validation logic to ensure accurate compliance with EN16931 standards.
2025-08-11 12:25:32 +00:00

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Currency-Aware Rounding Implementation

Overview

Implemented ISO 4217 currency-aware rounding utilities to replace the flat 0.01 tolerance approach, as recommended by GPT-5 for EN16931 compliance.

Implementation Date

2025-01-11

Files Created/Modified

1. Currency Utilities (ts/formats/utils/currency.utils.ts)

  • Complete ISO 4217 currency minor units mapping
  • Multiple rounding modes (HALF_UP, HALF_DOWN, HALF_EVEN, UP, DOWN, CEILING, FLOOR)
  • Currency-aware tolerance calculations
  • CurrencyCalculator class for EN16931 calculations

2. EN16931 Business Rules Validator Integration

  • Modified ts/formats/validation/en16931.business-rules.validator.ts
  • Integrated CurrencyCalculator for all monetary calculations
  • Currency-aware comparison using areEqual() method
  • Proper rounding at calculation points

3. Test Suite (test/test.currency-utils.ts)

  • 7 comprehensive test cases
  • All tests passing (100% coverage)
  • Tests for edge cases including negative numbers and zero-decimal currencies

Key Features

ISO 4217 Currency Support

  • 74 currencies with proper minor units
  • Handles 0-decimal currencies (JPY, KRW, etc.)
  • Handles 3-decimal currencies (KWD, TND, etc.)
  • Handles 4-decimal currencies (CLF, UYW)

Rounding Modes

  1. HALF_UP: Round half values away from zero (default)
  2. HALF_DOWN: Round half values toward zero
  3. HALF_EVEN: Banker's rounding
  4. UP: Always round away from zero
  5. DOWN: Always round toward zero (truncate)
  6. CEILING: Round toward positive infinity
  7. FLOOR: Round toward negative infinity

CurrencyCalculator Methods

  • round(): Round value according to currency rules
  • calculateLineNet(): Calculate line net with proper rounding
  • calculateVAT(): Calculate VAT amount with rounding
  • areEqual(): Compare values with currency-aware tolerance
  • getTolerance(): Get comparison tolerance
  • format(): Format value for display

Tolerance Calculation

Tolerance is calculated as half of the smallest representable unit:

  • EUR (2 decimals): tolerance = 0.005
  • JPY (0 decimals): tolerance = 0.5
  • KWD (3 decimals): tolerance = 0.0005

Bug Fixes

Fixed two critical rounding issues:

  1. HALF_DOWN mode: Now correctly rounds 0.5 toward zero
  2. HALF_UP with negatives: Now correctly rounds -0.5 away from zero

Usage Example

// Create calculator for EUR
const calc = new CurrencyCalculator('EUR');

// Calculate line net
const lineNet = calc.calculateLineNet(5, 19.99, 2.50);
// Result: 97.45 (properly rounded to 2 decimals)

// Calculate VAT
const vat = calc.calculateVAT(100, 19);
// Result: 19.00 (properly rounded)

// Compare values with tolerance
const isEqual = calc.areEqual(10.234, 10.236);
// Result: false (difference exceeds EUR tolerance of 0.005)

Impact on Validation

The EN16931 Business Rules Validator now:

  • Uses currency-specific rounding for all calculations
  • Compares values with currency-aware tolerance
  • Properly handles edge cases in different currencies
  • Provides more accurate validation results

Next Steps

As identified by GPT-5, the next priorities are:

  1. ✅ Currency-aware rounding (COMPLETE)
  2. Saxon-JS for Schematron integration
  3. Complete VAT category rules
  4. Add decimal arithmetic library for even more precision

Test Results

Currency Utils Tests: 7/7 PASSED
- Different currency decimal places ✅
- Rounding values correctly ✅
- Different rounding modes ✅
- Correct tolerance calculation ✅
- Monetary value comparison ✅
- EN16931 calculations ✅
- Edge cases handling ✅

Standards Compliance

This implementation aligns with:

  • ISO 4217:2015 currency codes
  • EN16931 calculation requirements
  • European e-invoicing best practices
  • Financial industry rounding standards