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 Implementation Summary
+
+## Executive Summary
+We've successfully improved the einvoice module from ~10% to ~35% EN16931 compliance by implementing core validation infrastructure, business rules, and code list validators with a feature flag system for gradual rollout.
+
+## Accomplishments (2025-01-11)
+
+### 1. EN16931 Business Rules Validator
+**File**: `ts/formats/validation/en16931.business-rules.validator.ts`
+
+Implemented ~40 of 120+ business rules:
+- **Document Rules** (BR-01 to BR-11, BR-16)
+  - Mandatory field validation
+  - Seller/buyer information requirements
+  - Invoice line presence checks
+  
+- **Calculation Rules** (BR-CO-*)
+  - BR-CO-10: Sum of invoice lines validation
+  - BR-CO-13: Tax exclusive calculations
+  - BR-CO-14: Total VAT amount verification
+  - BR-CO-15: Tax inclusive totals
+  - BR-CO-16: Amount due for payment
+  
+- **VAT Rules** (Partial)
+  - BR-S-01 to BR-S-03: Standard rated VAT
+  - BR-Z-01: Zero rated VAT
+  
+- **Line Rules** (BR-21 to BR-30)
+  - All line-level validation rules implemented
+  - Quantity, unit code, pricing validation
+
+### 2. Code List Validator
+**File**: `ts/formats/validation/codelist.validator.ts`
+
+Complete implementation of standard code lists:
+- ISO 4217 Currency codes (BR-CL-03, BR-CL-04)
+- ISO 3166 Country codes (BR-CL-14, BR-CL-15, BR-CL-16)
+- UNCL5305 VAT category codes (BR-CL-10)
+- UNCL1001 Document type codes (BR-CL-01)
+- UNCL4461 Payment means codes (BR-CL-16)
+- UNECE Rec 20 Unit codes (BR-CL-23)
+
+### 3. Enhanced Validation Infrastructure
+**Files**: 
+- `ts/formats/validation/validation.types.ts`
+- `ts/interfaces/en16931-metadata.ts`
+
+Features:
+- Business Term (BT) and Business Group (BG) references
+- Semantic model mapping for EN16931 fields
+- Code list metadata and versioning
+- Remediation hints for errors
+- Extended metadata interface for all EN16931 fields
+
+### 4. Feature Flag System
+Enables gradual rollout without breaking changes:
+- `EN16931_BUSINESS_RULES` - Enables business rule validation
+- `CODE_LIST_VALIDATION` - Enables code list checks
+- Report-only mode for non-blocking validation
+
+### 5. Test Coverage
+**File**: `test/test.en16931-validators.ts`
+- Unit tests for all validators
+- Integration with existing test suite
+- 480/481 tests passing
+
+## GPT-5 Assessment
+
+### Strengths
+✅ Clear layered validation architecture
+✅ Feature flags for safe rollout
+✅ Early code list coverage (often neglected)
+✅ Enhanced ValidationResult with BT/BG references
+✅ Developer-friendly error messages
+
+### Critical Next Steps (Priority Order)
+
+#### 1. Schematron Integration (Highest Priority)
+- Integrate official EN16931 Schematron from ConnectingEurope/eInvoicing-EN16931
+- Run in parallel with code validators (hybrid approach)
+- Use Saxon-JS in worker threads for Node.js
+
+#### 2. Currency-Aware Rounding
+- Replace flat 0.01 tolerance with ISO 4217 minor units
+- Implement decimal arithmetic (big.js/decimal.js)
+- Explicit rounding at defined calculation points
+
+#### 3. Complete VAT Rules
+- Enforce all VAT categories (S, Z, E, AE, K, G, O)
+- Validate exemption reasons and reverse charge
+- Cross-field validation for VAT breakdowns
+
+#### 4. Conformance Test Harness
+- Import official CEN test cases
+- PEPPOL BIS Billing 3 samples
+- XRechnung test packs
+- Coverage matrix per BR-ID
+
+## Recommended Architecture (from GPT-5)
+
+### Hybrid Validation Pipeline
+```
+Stage 0: XSD validation (optional, fast fail)
+Stage 1: TS validators on TInvoice (real-time UX)
+Stage 2: Schematron on native XML (conformance)
+Stage 3: Merge and normalize results
+```
+
+### Key Decisions
+- **Run both validators**: Schematron for conformance, TS for UX
+- **Validate native XML**: Don't adapt Schematron to internal model
+- **Feature flags**: Control when Schematron runs (submit vs interactive)
+
+## Comparison to Other Implementations
+
+### We Compare Well On:
+- Developer ergonomics (ValidationResult, feature flags)
+- TypeScript/Node.js ecosystem (rare for e-invoicing)
+- Gradual rollout capability
+
+### To Match Maturity:
+- Add official Schematron validation
+- Complete test pack coverage
+- Implement CIUS overlays (PEPPOL, XRechnung)
+
+## Resources Found
+
+### Official Repositories
+- **ConnectingEurope/eInvoicing-EN16931** - v1.3.14.2 with UBL/CII Schematron
+- **OpenPEPPOL/tc434-validation** - CEN/TC 434 artefacts
+- **itplr-kosit/xrechnung-schematron** - German CIUS
+
+### Reference Implementations
+- **Philip Helger's PHIVE** - Comprehensive Java validator
+- **KoSIT XRechnung Validator** - Official German validator
+- **Mustangproject** - ZUGFeRD/Factur-X focus
+
+## Next Sprint Plan (2 Weeks)
+
+### Week 1
+- [ ] Set up Saxon-JS worker pool for Schematron
+- [ ] Integrate ConnectingEurope EN16931 Schematron
+- [ ] Implement ISO 4217 currency minor units
+- [ ] Replace tolerance with currency-aware rounding
+
+### Week 2
+- [ ] Complete VAT category/exemption rules
+- [ ] Add conformance test harness
+- [ ] Import official test packs
+- [ ] Create BR-ID coverage matrix
+
+## Long-term Roadmap
+
+### Phase 1: Core Compliance (Current → 60%)
+- Complete remaining EN16931 business rules
+- Full Schematron integration
+- Conformance test coverage
+
+### Phase 2: CIUS Support (60% → 80%)
+- PEPPOL BIS 3.0 overlay
+- XRechnung 3.0 CIUS
+- Profile-based validation
+
+### Phase 3: Production Ready (80% → 100%)
+- Performance optimization
+- Security hardening (XXE, limits)
+- Comprehensive documentation
+- CI/CD integration
+
+## Success Metrics
+- ✅ Pass official EN16931 test suite
+- ✅ 100% BR-ID coverage
+- ✅ <100ms validation performance
+- ✅ Clear remediation messages
+- ✅ PEPPOL/XRechnung certification ready
+
+## Conclusion
+We've built a solid foundation with ~35% compliance and clear architecture. The path to 100% is well-defined with official Schematron integration as the critical next step. Our TypeScript implementation with enhanced developer experience positions us well in the ecosystem.