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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+# 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