einvoice/STANDARDS_COMPLIANCE_PLAN.md

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

• Create rule registry with all EN16931, XRechnung, Peppol rule IDs (partial - EN16931 done)
• Build coverage tracking system (ValidationReport with coverage metrics)
• Set up validation result data model (ValidationResult interface with BT/BG references)
• Implement Schematron engine integration ✅ (Saxon-JS with official rules)

Phase 1: Core EN16931 Business Rules (Weeks 1-2) ✅ PARTIALLY COMPLETE

• Create EN16931BusinessRulesValidator class
• 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 ✅
• Implement VAT rules (BR-S-, BR-Z-, partial)
  • BR-S-01 to BR-S-03: Standard rated VAT ✅
  • BR-Z-01: Zero rated VAT ✅
• Add document level rules (BR-01 to BR-65) - ~25 rules implemented
  • BR-01 to BR-11: Mandatory fields ✅
  • BR-16: Invoice lines ✅
• 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
• Implement calculation verification:
  • Line totals (quantity × price) ✅
  • Tax base per category ✅
  • Header allowance/charge distribution ✅
  • Rounding and tolerance handling ✅ (ISO 4217 currency-aware)
• 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

• ISO 4217 currency codes (BR-CL-03, BR-CL-04) ✅
• ISO 3166 country codes (BR-CL-14, BR-CL-15, BR-CL-16) ✅
• UN/ECE 4461 payment means codes (BR-CL-16) ✅
• UNTDID 1001 document type codes (BR-CL-01) ✅
• VAT category codes (UNCL5305 - BR-CL-10) ✅
• 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:

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