# E-Invoice Standards Compliance Implementation Plan

## Executive Summary
Current compliance: ~75% of required rules (up from ~70%)
Target: 100% compliance with EN16931, XRechnung, Peppol BIS 3.0, and Factur-X profiles

**Latest Update (2025-01-11)**: 
- Completed Saxon-JS Schematron integration with official EN16931 rules
- Implemented comprehensive VAT category validator (all BR-S-*, BR-Z-*, BR-E-*, BR-AE-*, BR-K-*, BR-G-*, BR-O-* rules)
- Added conformance test harness with official test samples
- Created BR coverage matrix generation

## Scale of Work
- EN16931 core: ~120-150 business rules
- XRechnung CIUS: 100-200+ format-specific constraints
- Peppol BIS 3.0: Additional Schematron layer
- Factur-X profiles: Profile-specific cardinalities
- **Total: 300-500+ validations needed**

## Implementation Roadmap

### Phase 0: Baseline Infrastructure (Week 1) ✅ COMPLETE
- [x] Create rule registry with all EN16931, XRechnung, Peppol rule IDs (partial - EN16931 done)
- [x] Build coverage tracking system (ValidationReport with coverage metrics)
- [x] Set up validation result data model (ValidationResult interface with BT/BG references)
- [x] Implement Schematron engine integration ✅ (Saxon-JS with official rules)

### Phase 1: Core EN16931 Business Rules (Weeks 1-2) ✅ PARTIALLY COMPLETE
- [x] Create EN16931BusinessRulesValidator class
- [x] Implement calculation rules (BR-CO-*)
  - BR-CO-10: Sum of invoice lines = Line extension amount ✅
  - BR-CO-13: Tax exclusive = Lines - Allowances + Charges ✅
  - BR-CO-15: Tax inclusive = Tax exclusive + VAT ✅
  - BR-CO-14: Invoice total VAT amount ✅
  - BR-CO-16: Amount due for payment ✅
- [x] Implement VAT rules (BR-S-*, BR-Z-*, partial)
  - BR-S-01 to BR-S-03: Standard rated VAT ✅
  - BR-Z-01: Zero rated VAT ✅
- [x] Add document level rules (BR-01 to BR-65) - ~25 rules implemented
  - BR-01 to BR-11: Mandatory fields ✅
  - BR-16: Invoice lines ✅
- [x] Add line level rules (BR-21 to BR-30) - All implemented ✅

### Phase 2: Calculation Engine (Weeks 2-3) ✅ PARTIALLY COMPLETE
- [ ] Build canonical semantic model (BT/BG fields)
- [ ] Create UBL/CII adapters to semantic model
- [x] Implement calculation verification:
  - Line totals (quantity × price) ✅
  - Tax base per category ✅
  - Header allowance/charge distribution ✅
  - Rounding and tolerance handling ✅ (ISO 4217 currency-aware)
- [x] Handle edge cases:
  - Mixed VAT categories ✅
  - Reverse charge (partial)
  - Multi-currency ✅ (ISO 4217 support)

### Phase 3: XRechnung CIUS (Week 4)
- [ ] Integrate XRechnung Schematron pack
- [ ] Implement Leitweg-ID validation (pattern: [0-9]{2,3}-[0-9]{1,12}-[0-9]{2,30})
- [ ] Enforce mandatory buyer reference (BT-10)
- [ ] Add German-specific payment terms validation
- [ ] IBAN/BIC validation for SEPA

### Phase 4: Peppol BIS 3.0 (Week 5)
- [ ] Add Peppol Schematron layer
- [ ] Implement endpoint ID validation (0088:xxxxxxxxx)
- [ ] Add document type ID validation
- [ ] Party identification scheme validation
- [ ] Process ID validation

### Phase 5: Factur-X Profiles (Week 6)
- [ ] Implement profile detection
- [ ] Add profile-specific validators:
  - MINIMUM: Only BT-1, BT-2, BT-3
  - BASIC: Core fields
  - EN16931: Full compliance
  - EXTENDED: Additional structured data
- [ ] Profile-based field cardinality enforcement

### Phase 6: Code List Validators ✅ COMPLETE
- [x] ISO 4217 currency codes (BR-CL-03, BR-CL-04) ✅
- [x] ISO 3166 country codes (BR-CL-14, BR-CL-15, BR-CL-16) ✅
- [x] UN/ECE 4461 payment means codes (BR-CL-16) ✅
- [x] UNTDID 1001 document type codes (BR-CL-01) ✅
- [x] VAT category codes (UNCL5305 - BR-CL-10) ✅
- [x] UNECE Rec 20 unit codes (BR-CL-23) ✅

## Technical Architecture

### Layered Validation Approach:
1. **Schema validation**: XSD for UBL/CII
2. **Schematron packs**: EN16931, CIUS, code lists
3. **Programmatic engine**: Calculations and relationships

### API Design:
```typescript
interface ValidationOptions {
  format?: 'ubl' | 'cii';
  profile?: 'EN16931' | 'XRechnung_3.0' | 'Peppol_BIS_3.0' | 'FacturX_Basic';
  tolerance?: number; // Default 0.01
  strictMode?: boolean;
}

interface ValidationResult {
  ruleId: string;
  severity: 'error' | 'warning' | 'info';
  message: string;
  location?: string; // XPath
  context?: any;
}
```

### Security Considerations:
- Disable DTD/XXE in XML parsing
- Enforce document size limits
- Sandbox XSLT execution
- Validate only trusted rule packs

## Success Criteria
- Pass official test suites for each standard
- 100% coverage of mandatory rules
- Performance: <100ms for full validation
- Clear error messages with rule IDs and locations

## Resources Needed
- EN16931 Schematron from official sources
- XRechnung artifacts for current version
- Peppol BIS 3.0 Schematron
- Factur-X profile documentation
- Official test invoices for each standard

## Risk Mitigation
- Version pinning for rule packs
- Snapshot testing for regression detection
- Configurable tolerances for calculations
- Layer precedence for conflicting rules

## Immediate Next Steps
1. ~~Set up Saxon-JS for Schematron integration~~ ✅ Complete
2. ~~Download official EN16931 Schematron files~~ ✅ Complete
3. ~~Create hybrid validation pipeline~~ ✅ Complete
4. ~~Implement ISO 4217 currency-aware rounding~~ ✅ Complete
5. ~~Complete remaining VAT category rules~~ ✅ Complete
6. ~~Add conformance test harness~~ ✅ Complete
7. Implement decimal arithmetic library (next priority)
8. Add XRechnung CIUS support
9. Implement PEPPOL BIS 3.0 overlay

## Accomplishments (2025-01-11)

### Implemented Components:
1. **EN16931BusinessRulesValidator** (`ts/formats/validation/en16931.business-rules.validator.ts`)
   - ~40 business rules implemented
   - Document, calculation, VAT, and line-level validation
   - Currency-aware calculation verification with ISO 4217 support

2. **CodeListValidator** (`ts/formats/validation/codelist.validator.ts`)
   - All major code lists validated
   - Currency, country, tax category, payment means, unit codes
   - Context-aware validation with exemption reasons

3. **Enhanced ValidationResult Interface** (`ts/formats/validation/validation.types.ts`)
   - Business Term (BT) and Business Group (BG) references
   - Semantic model mapping
   - Code list metadata
   - Remediation hints

4. **Extended Metadata Support** (`ts/interfaces/en16931-metadata.ts`)
   - Comprehensive EN16931 metadata fields
   - Delivery addresses, payment accounts
   - Allowances and charges structure

5. **Feature Flag Integration**
   - Gradual rollout capability
   - Backward compatibility maintained
   - Separate flags for EN16931 and code lists

6. **ISO 4217 Currency-Aware Rounding** (`ts/formats/utils/currency.utils.ts`)
   - Complete ISO 4217 currency minor units database
   - 7 rounding modes (HALF_UP, HALF_DOWN, HALF_EVEN, UP, DOWN, CEILING, FLOOR)
   - CurrencyCalculator class for EN16931 calculations
   - Replaces flat 0.01 tolerance with currency-specific tolerances

7. **Saxon-JS Schematron Integration** (`ts/formats/validation/schematron.*.ts`)
   - Saxon-JS for XSLT 3.0 processing
   - Official EN16931 Schematron files downloaded (v1.3.14)
   - Worker thread pool for non-blocking validation
   - Hybrid validator combining TypeScript and Schematron
   - Automatic format detection (UBL/CII)
   - SVRL parsing and result integration

### Test Coverage:
- Unit tests for validators (`test/test.en16931-validators.ts`)
- Currency utilities tests (`test/test.currency-utils.ts`) - 7/7 passing
- Schematron infrastructure tests (`test/test.schematron-validator.ts`) - 9/9 passing
- Integration with existing test suite
- 496/497 tests passing overall (added 16 new tests)

8. **VAT Categories Validator** (`ts/formats/validation/vat-categories.validator.ts`)
   - Complete implementation of all VAT category business rules
   - BR-S-* (Standard rate), BR-Z-* (Zero rated), BR-E-* (Exempt)
   - BR-AE-* (Reverse charge), BR-K-* (Intra-community), BR-G-* (Export)
   - BR-O-* (Out of scope services)
   - Cross-category validation rules

9. **Conformance Test Harness** (`ts/formats/validation/conformance.harness.ts`)
   - Automated testing against official samples
   - BR coverage matrix generation
   - HTML coverage reports
   - Support for PEPPOL and CEN test suites
   - Performance metrics collection

10. **Test Sample Downloader** (`scripts/download-test-samples.ts`)
   - Automated download from official repositories
   - PEPPOL BIS 3.0 examples
   - CEN TC434 test files
   - Metadata tracking

11. **XML to EInvoice Converter** (`ts/formats/converters/xml-to-einvoice.converter.ts`)
   - Basic UBL and CII parsing
   - Integration with conformance testing

### Next Priority Items:
1. ~~Set up Saxon-JS for Schematron integration~~ ✅ COMPLETE
2. ~~Integrate official EN16931 Schematron from ConnectingEurope~~ ✅ COMPLETE
3. ~~Complete remaining VAT category rules~~ ✅ COMPLETE
4. ~~Add conformance test harness with official test packs~~ ✅ COMPLETE
5. Implement decimal arithmetic for even more precision
6. Add XRechnung CIUS layer
7. Implement PEPPOL BIS 3.0 support