feat(validation): add validators

package.json

@@ -28,7 +28,8 @@
     "jsdom": "^24.1.3",
     "pako": "^2.1.0",
     "pdf-lib": "^1.17.1",
-    "xmldom": "^0.6.0"
+    "xmldom": "^0.6.0",
+    "xpath": "^0.0.34"
   },
   "repository": {
     "type": "git",

pnpm-lock.yaml

@@ -29,6 +29,9 @@ importers:
       xmldom:
         specifier: ^0.6.0
         version: 0.6.0
+      xpath:
+        specifier: ^0.0.34
+        version: 0.0.34
     devDependencies:
       '@git.zone/tsbuild':
         specifier: ^2.2.7
@@ -4340,6 +4343,10 @@ packages:
     resolution: {integrity: sha512-TEU+nJVUUnA4CYJFLvK5X9AOeH4KvDvhIfm0vV1GaQRtchnG0hgK5p8hw/xjv8cunWYCsiPCSDzObPyhEwq3KQ==}
     engines: {node: '>=0.4.0'}
 
+  xpath@0.0.34:
+    resolution: {integrity: sha512-FxF6+rkr1rNSQrhUNYrAFJpRXNzlDoMxeXN5qI84939ylEv3qqPFKa85Oxr6tDaJKqwW6KKyo2v26TSv3k6LeA==}
+    engines: {node: '>=0.6.0'}
+
   xtend@4.0.2:
     resolution: {integrity: sha512-LKYU1iAXJXUgAXn9URjiu+MWhyUXHsvfp7mcuYm9dSUKK0/CjtrUwFAxD82/mCWbtLsGjFIad0wIsod4zrTAEQ==}
     engines: {node: '>=0.4'}
@@ -10188,6 +10195,8 @@ snapshots:
 
   xmlhttprequest-ssl@2.1.2: {}
 
+  xpath@0.0.34: {}
+
   xtend@4.0.2: {}
 
   y18n@5.0.8: {}

test/test.validators.ts

@@ -0,0 +1,70 @@
+import { tap, expect } from '@push.rocks/tapbundle';
+import * as getInvoices from './assets/getasset.js';
+import { ValidatorFactory } from '../ts/formats/validator.factory.js';
+import { ValidationLevel } from '../ts/interfaces.js';
+import { validateXml } from '../ts/index.js';
+
+// Test ValidatorFactory format detection
+tap.test('ValidatorFactory should detect UBL format', async () => {
+  const path = getInvoices.invoices.XMLRechnung.UBL['EN16931_Einfach.ubl.xml'];
+  const invoice = await getInvoices.getInvoice(path);
+  const xml = invoice.toString('utf8');
+  
+  const validator = ValidatorFactory.createValidator(xml);
+  expect(validator.constructor.name).toBeTypeOf('string');
+  expect(validator.constructor.name).toInclude('UBL');
+});
+
+tap.test('ValidatorFactory should detect CII/Factur-X format', async () => {
+  const path = getInvoices.invoices.XMLRechnung.CII['EN16931_Einfach.cii.xml'];
+  const invoice = await getInvoices.getInvoice(path);
+  const xml = invoice.toString('utf8');
+  
+  const validator = ValidatorFactory.createValidator(xml);
+  expect(validator.constructor.name).toBeTypeOf('string');
+  expect(validator.constructor.name).toInclude('FacturX');
+});
+
+// Test UBL validation
+tap.test('UBL validator should validate valid XML at syntax level', async () => {
+  const path = getInvoices.invoices.XMLRechnung.UBL['EN16931_Einfach.ubl.xml'];
+  const invoice = await getInvoices.getInvoice(path);
+  const xml = invoice.toString('utf8');
+  
+  const result = validateXml(xml, ValidationLevel.SYNTAX);
+  expect(result.valid).toBeTrue();
+  expect(result.errors.length).toEqual(0);
+});
+
+// Test CII validation
+tap.test('CII validator should validate valid XML at syntax level', async () => {
+  const path = getInvoices.invoices.XMLRechnung.CII['EN16931_Einfach.cii.xml'];
+  const invoice = await getInvoices.getInvoice(path);
+  const xml = invoice.toString('utf8');
+  
+  const result = validateXml(xml, ValidationLevel.SYNTAX);
+  expect(result.valid).toBeTrue();
+  expect(result.errors.length).toEqual(0);
+});
+
+// Test XInvoice integration
+tap.test('XInvoice class should validate invoices on load when requested', async () => {
+  // Import XInvoice dynamically to prevent circular dependencies
+  const { XInvoice } = await import('../ts/index.js');
+  const invoice = new XInvoice();
+  
+  // Load a UBL invoice with validation
+  const path = getInvoices.invoices.XMLRechnung.UBL['EN16931_Einfach.ubl.xml'];
+  const invoiceBuffer = await getInvoices.getInvoice(path);
+  const xml = invoiceBuffer.toString('utf8');
+  
+  // Add XML with validation enabled
+  await invoice.addXmlString(xml, true);
+  
+  // Check validation results
+  expect(invoice.isValid()).toBeTrue();
+  expect(invoice.getValidationErrors().length).toEqual(0);
+});
+
+// Mark the test file as complete
+tap.start();

ts/classes.xinvoice.ts

@@ -11,6 +11,8 @@ import {
 import { FacturXEncoder } from './formats/facturx.encoder.js';
 import { DecoderFactory } from './formats/decoder.factory.js';
 import { BaseDecoder } from './formats/base.decoder.js';
+import { ValidatorFactory } from './formats/validator.factory.js';
+import { BaseValidator } from './formats/base.validator.js';
 
 export class XInvoice {
   private xmlString: string;
@@ -19,6 +21,10 @@ export class XInvoice {
 
   private encoderInstance = new FacturXEncoder(); 
   private decoderInstance: BaseDecoder;
+  private validatorInstance: BaseValidator;
+  
+  // Validation errors from last validation
+  private validationErrors: interfaces.ValidationError[] = [];
   
   constructor() {
     // Decoder will be initialized when we have XML data
@@ -28,7 +34,7 @@ export class XInvoice {
     this.pdfUint8Array = Uint8Array.from(pdfBuffer);
   }
 
-  public async addXmlString(xmlString: string): Promise<void> {
+  public async addXmlString(xmlString: string, validate: boolean = false): Promise<void> {
     // Basic XML validation - just check if it starts with <?xml
     if (!xmlString || !xmlString.trim().startsWith('<?xml')) {
       throw new Error('Invalid XML: Missing XML declaration');
@@ -39,6 +45,58 @@ export class XInvoice {
     
     // Initialize the decoder with the XML string using the factory
     this.decoderInstance = DecoderFactory.createDecoder(xmlString);
+    
+    // Initialize the validator with the XML string using the factory
+    this.validatorInstance = ValidatorFactory.createValidator(xmlString);
+    
+    // Validate the XML if requested
+    if (validate) {
+      await this.validate();
+    }
+  }
+  
+  /**
+   * Validates the XML against the appropriate validation rules
+   * @param level Validation level (syntax, semantic, business)
+   * @returns Validation result
+   */
+  public async validate(level: interfaces.ValidationLevel = interfaces.ValidationLevel.SYNTAX): Promise<interfaces.ValidationResult> {
+    if (!this.xmlString) {
+      throw new Error('No XML to validate. Use addXmlString() first.');
+    }
+    
+    if (!this.validatorInstance) {
+      // Initialize the validator with the XML string if not already done
+      this.validatorInstance = ValidatorFactory.createValidator(this.xmlString);
+    }
+    
+    // Run validation
+    const result = this.validatorInstance.validate(level);
+    
+    // Store validation errors
+    this.validationErrors = result.errors;
+    
+    return result;
+  }
+  
+  /**
+   * Checks if the document is valid based on the last validation
+   * @returns True if the document is valid
+   */
+  public isValid(): boolean {
+    if (!this.validatorInstance) {
+      return false;
+    }
+    
+    return this.validatorInstance.isValid();
+  }
+  
+  /**
+   * Gets validation errors from the last validation
+   * @returns Array of validation errors
+   */
+  public getValidationErrors(): interfaces.ValidationError[] {
+    return this.validationErrors;
   }
 
   public async addLetterData(letterData: plugins.tsclass.business.ILetter): Promise<void> {
@@ -183,13 +241,28 @@ export class XInvoice {
     if (xmlContent.includes('CrossIndustryInvoice') || 
         xmlContent.includes('rsm:') || 
         xmlContent.includes('ram:')) {
-      return 'ZUGFeRD/CII';
+      
+      // Check for specific profiles
+      if (xmlContent.includes('factur-x') || xmlContent.includes('Factur-X')) {
+        return 'Factur-X';
+      }
+      if (xmlContent.includes('zugferd') || xmlContent.includes('ZUGFeRD')) {
+        return 'ZUGFeRD';
+      }
+      
+      return 'CII';
     }
     
     // Check for UBL
     if (xmlContent.includes('<Invoice') || 
         xmlContent.includes('ubl:Invoice') || 
         xmlContent.includes('oasis:names:specification:ubl')) {
+      
+      // Check for XRechnung
+      if (xmlContent.includes('xrechnung') || xmlContent.includes('XRechnung')) {
+        return 'XRechnung';
+      }
+      
       return 'UBL';
     }
     
@@ -202,6 +275,44 @@ export class XInvoice {
     // For unknown formats, return generic
     return 'Unknown';
   }
+  
+  /**
+   * Gets the invoice format as an enum value
+   * @returns InvoiceFormat enum value
+   */
+  public getFormat(): interfaces.InvoiceFormat {
+    if (!this.xmlString) {
+      return interfaces.InvoiceFormat.UNKNOWN;
+    }
+    
+    const formatString = this.identifyXmlFormat(this.xmlString);
+    
+    switch (formatString) {
+      case 'UBL':
+        return interfaces.InvoiceFormat.UBL;
+      case 'XRechnung':
+        return interfaces.InvoiceFormat.XRECHNUNG;
+      case 'CII':
+        return interfaces.InvoiceFormat.CII;
+      case 'ZUGFeRD':
+        return interfaces.InvoiceFormat.ZUGFERD;
+      case 'Factur-X':
+        return interfaces.InvoiceFormat.FACTURX;
+      case 'FatturaPA':
+        return interfaces.InvoiceFormat.FATTURAPA;
+      default:
+        return interfaces.InvoiceFormat.UNKNOWN;
+    }
+  }
+  
+  /**
+   * Checks if the invoice is in a specific format
+   * @param format Format to check
+   * @returns True if the invoice is in the specified format
+   */
+  public isFormat(format: interfaces.InvoiceFormat): boolean {
+    return this.getFormat() === format;
+  }
 
   public async getParsedXmlData(): Promise<interfaces.IXInvoice> {
     if (!this.xmlString && !this.pdfUint8Array) {

ts/formats/base.validator.ts

@@ -0,0 +1,64 @@
+import { ValidationLevel } from '../interfaces.js';
+import type { ValidationResult, ValidationError } from '../interfaces.js';
+
+/**
+ * Base validator class that defines common validation functionality
+ * for all invoice format validators
+ */
+export abstract class BaseValidator {
+  protected xml: string;
+  protected errors: ValidationError[] = [];
+
+  constructor(xml: string) {
+    this.xml = xml;
+  }
+
+  /**
+   * Validates XML against the specified level of validation
+   * @param level Validation level (syntax, semantic, business)
+   * @returns Result of validation
+   */
+  abstract validate(level?: ValidationLevel): ValidationResult;
+
+  /**
+   * Gets all validation errors found during validation
+   * @returns Array of validation errors
+   */
+  public getValidationErrors(): ValidationError[] {
+    return this.errors;
+  }
+
+  /**
+   * Checks if the document is valid
+   * @returns True if no validation errors were found
+   */
+  public isValid(): boolean {
+    return this.errors.length === 0;
+  }
+
+  /**
+   * Validates XML against schema
+   * @returns True if schema validation passed
+   */
+  protected abstract validateSchema(): boolean;
+
+  /**
+   * Validates business rules
+   * @returns True if business rule validation passed
+   */
+  protected abstract validateBusinessRules(): boolean;
+
+  /**
+   * Adds an error to the validation errors list
+   * @param code Error code
+   * @param message Error message
+   * @param location Location in the XML where the error occurred
+   */
+  protected addError(code: string, message: string, location: string = ''): void {
+    this.errors.push({
+      code,
+      message,
+      location
+    });
+  }
+}

ts/formats/facturx.validator.ts

@@ -0,0 +1,322 @@
+import { BaseValidator } from './base.validator.js';
+import { ValidationLevel } from '../interfaces.js';
+import type { ValidationResult, ValidationError } from '../interfaces.js';
+import * as xpath from 'xpath';
+import { DOMParser } from 'xmldom';
+
+/**
+ * Validator for Factur-X/ZUGFeRD invoice format
+ * Implements validation rules according to EN16931 and Factur-X specification
+ */
+export class FacturXValidator extends BaseValidator {
+  // XML namespaces for Factur-X/ZUGFeRD
+  private static NS_RSMT = 'urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100';
+  private static NS_RAM = 'urn:un:unece:uncefact:data:standard:ReusableAggregateBusinessInformationEntity:100';
+  private static NS_UDT = 'urn:un:unece:uncefact:data:standard:UnqualifiedDataType:100';
+  
+  // XML document for processing
+  private xmlDoc: Document | null = null;
+  
+  // Factur-X profile (BASIC, EN16931, EXTENDED, etc.)
+  private profile: string = '';
+  
+  constructor(xml: string) {
+    super(xml);
+    
+    try {
+      // Parse XML document
+      this.xmlDoc = new DOMParser().parseFromString(xml, 'application/xml');
+      
+      // Determine Factur-X profile
+      this.detectProfile();
+    } catch (error) {
+      this.addError('FX-PARSE', `Failed to parse XML: ${error}`, '/');
+    }
+  }
+  
+  /**
+   * Validates the Factur-X invoice against the specified level
+   * @param level Validation level
+   * @returns Validation result
+   */
+  public validate(level: ValidationLevel = ValidationLevel.SYNTAX): ValidationResult {
+    // Reset errors
+    this.errors = [];
+    
+    // Check if document was parsed successfully
+    if (!this.xmlDoc) {
+      return {
+        valid: false,
+        errors: this.errors,
+        level: level
+      };
+    }
+    
+    // Perform validation based on level
+    let valid = true;
+    
+    if (level === ValidationLevel.SYNTAX) {
+      valid = this.validateSchema();
+    } else if (level === ValidationLevel.SEMANTIC) {
+      valid = this.validateSchema() && this.validateStructure();
+    } else if (level === ValidationLevel.BUSINESS) {
+      valid = this.validateSchema() && 
+              this.validateStructure() && 
+              this.validateBusinessRules();
+    }
+    
+    return {
+      valid,
+      errors: this.errors,
+      level
+    };
+  }
+  
+  /**
+   * Validates XML against schema
+   * @returns True if schema validation passed
+   */
+  protected validateSchema(): boolean {
+    // Basic schema validation (simplified for now)
+    if (!this.xmlDoc) return false;
+    
+    // Check for root element
+    const root = this.xmlDoc.documentElement;
+    if (!root || root.nodeName !== 'rsm:CrossIndustryInvoice') {
+      this.addError('FX-SCHEMA-1', 'Root element must be rsm:CrossIndustryInvoice', '/');
+      return false;
+    }
+    
+    // Check for required namespaces
+    if (!root.lookupNamespaceURI('rsm') || !root.lookupNamespaceURI('ram')) {
+      this.addError('FX-SCHEMA-2', 'Required namespaces rsm and ram must be declared', '/');
+      return false;
+    }
+    
+    return true;
+  }
+  
+  /**
+   * Validates structure of the XML document
+   * @returns True if structure validation passed
+   */
+  private validateStructure(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    let valid = true;
+    
+    // Check for required main sections
+    const sections = [
+      'rsm:ExchangedDocumentContext',
+      'rsm:ExchangedDocument',
+      'rsm:SupplyChainTradeTransaction'
+    ];
+    
+    for (const section of sections) {
+      if (!this.exists(section)) {
+        this.addError('FX-STRUCT-1', `Required section ${section} is missing`, '/rsm:CrossIndustryInvoice');
+        valid = false;
+      }
+    }
+    
+    // Check for SupplyChainTradeTransaction sections
+    if (this.exists('rsm:SupplyChainTradeTransaction')) {
+      const tradeSubsections = [
+        'ram:ApplicableHeaderTradeAgreement',
+        'ram:ApplicableHeaderTradeDelivery',
+        'ram:ApplicableHeaderTradeSettlement'
+      ];
+      
+      for (const subsection of tradeSubsections) {
+        if (!this.exists(`rsm:SupplyChainTradeTransaction/ram:ApplicableHeaderTradeTransaction/${subsection}`)) {
+          this.addError('FX-STRUCT-2', `Required subsection ${subsection} is missing`, 
+            '/rsm:CrossIndustryInvoice/rsm:SupplyChainTradeTransaction/ram:ApplicableHeaderTradeTransaction');
+          valid = false;
+        }
+      }
+    }
+    
+    return valid;
+  }
+  
+  /**
+   * Validates business rules
+   * @returns True if business rule validation passed
+   */
+  protected validateBusinessRules(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    let valid = true;
+    
+    // BR-16: Amount due for payment (BT-115) = Invoice total amount with VAT (BT-112) - Paid amount (BT-113)
+    valid = this.validateAmounts() && valid;
+    
+    // BR-CO-3: Value added tax point date (BT-7) and Value added tax point date code (BT-8) are mutually exclusive
+    valid = this.validateMutuallyExclusiveFields() && valid;
+    
+    // BR-S-1: An Invoice that contains a line (BG-25) where the Invoiced item VAT category code (BT-151) is "Standard rated" 
+    // shall contain the Seller VAT Identifier (BT-31), the Seller tax registration identifier (BT-32) 
+    // and/or the Seller tax representative VAT identifier (BT-63).
+    valid = this.validateSellerVatIdentifier() && valid;
+    
+    return valid;
+  }
+  
+  /**
+   * Detects Factur-X profile from the XML
+   */
+  private detectProfile(): void {
+    if (!this.xmlDoc) return;
+    
+    // Look for profile identifier
+    const profileNode = xpath.select1(
+      'string(//rsm:ExchangedDocumentContext/ram:GuidelineSpecifiedDocumentContextParameter/ram:ID)', 
+      this.xmlDoc
+    );
+    
+    if (profileNode) {
+      const profileText = profileNode.toString();
+      
+      if (profileText.includes('BASIC')) {
+        this.profile = 'BASIC';
+      } else if (profileText.includes('EN16931')) {
+        this.profile = 'EN16931';
+      } else if (profileText.includes('EXTENDED')) {
+        this.profile = 'EXTENDED';
+      } else if (profileText.includes('MINIMUM')) {
+        this.profile = 'MINIMUM';
+      }
+    }
+  }
+  
+  /**
+   * Validates amount calculations in the invoice
+   * @returns True if amount validation passed
+   */
+  private validateAmounts(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    try {
+      // Extract amounts
+      const totalAmount = this.getNumberValue(
+        '//ram:ApplicableHeaderTradeSettlement/ram:SpecifiedTradeSettlementHeaderMonetarySummation/ram:GrandTotalAmount'
+      );
+      
+      const paidAmount = this.getNumberValue(
+        '//ram:ApplicableHeaderTradeSettlement/ram:SpecifiedTradeSettlementHeaderMonetarySummation/ram:TotalPrepaidAmount'
+      ) || 0;
+      
+      const dueAmount = this.getNumberValue(
+        '//ram:ApplicableHeaderTradeSettlement/ram:SpecifiedTradeSettlementHeaderMonetarySummation/ram:DuePayableAmount'
+      );
+      
+      // Calculate expected due amount
+      const expectedDueAmount = totalAmount - paidAmount;
+      
+      // Compare with a small tolerance for rounding errors
+      if (Math.abs(dueAmount - expectedDueAmount) > 0.01) {
+        this.addError(
+          'BR-16', 
+          `Amount due for payment (${dueAmount}) must equal Invoice total amount with VAT (${totalAmount}) - Paid amount (${paidAmount})`,
+          '//ram:ApplicableHeaderTradeSettlement/ram:SpecifiedTradeSettlementHeaderMonetarySummation'
+        );
+        return false;
+      }
+      
+      return true;
+    } catch (error) {
+      this.addError('FX-AMOUNT', `Error validating amounts: ${error}`, '/');
+      return false;
+    }
+  }
+  
+  /**
+   * Validates mutually exclusive fields
+   * @returns True if validation passed
+   */
+  private validateMutuallyExclusiveFields(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    try {
+      // Check for VAT point date and code (BR-CO-3)
+      const vatPointDate = this.exists('//ram:ApplicableHeaderTradeSettlement/ram:ApplicableTradeTax/ram:TaxPointDate');
+      const vatPointDateCode = this.exists('//ram:ApplicableHeaderTradeSettlement/ram:ApplicableTradeTax/ram:DueDateTypeCode');
+      
+      if (vatPointDate && vatPointDateCode) {
+        this.addError(
+          'BR-CO-3', 
+          'Value added tax point date and Value added tax point date code are mutually exclusive',
+          '//ram:ApplicableHeaderTradeSettlement/ram:ApplicableTradeTax'
+        );
+        return false;
+      }
+      
+      return true;
+    } catch (error) {
+      this.addError('FX-MUTUAL', `Error validating mutually exclusive fields: ${error}`, '/');
+      return false;
+    }
+  }
+  
+  /**
+   * Validates seller VAT identifier requirements
+   * @returns True if validation passed
+   */
+  private validateSellerVatIdentifier(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    try {
+      // Check if there are any standard rated line items
+      const standardRatedItems = this.exists(
+        '//ram:SpecifiedLineTradeSettlement/ram:ApplicableTradeTax/ram:CategoryCode[text()="S"]'
+      );
+      
+      if (standardRatedItems) {
+        // Check for seller VAT identifier
+        const sellerVatId = this.exists('//ram:ApplicableHeaderTradeAgreement/ram:SellerTradeParty/ram:SpecifiedTaxRegistration/ram:ID[@schemeID="VA"]');
+        const sellerTaxId = this.exists('//ram:ApplicableHeaderTradeAgreement/ram:SellerTradeParty/ram:SpecifiedTaxRegistration/ram:ID[@schemeID="FC"]');
+        const sellerTaxRepId = this.exists('//ram:ApplicableHeaderTradeAgreement/ram:SellerTaxRepresentativeTradeParty/ram:SpecifiedTaxRegistration/ram:ID[@schemeID="VA"]');
+        
+        if (!sellerVatId && !sellerTaxId && !sellerTaxRepId) {
+          this.addError(
+            'BR-S-1', 
+            'An Invoice with standard rated items must contain the Seller VAT Identifier, Tax registration identifier or Tax representative VAT identifier',
+            '//ram:ApplicableHeaderTradeAgreement/ram:SellerTradeParty'
+          );
+          return false;
+        }
+      }
+      
+      return true;
+    } catch (error) {
+      this.addError('FX-VAT', `Error validating seller VAT identifier: ${error}`, '/');
+      return false;
+    }
+  }
+  
+  /**
+   * Helper method to check if a node exists
+   * @param xpathExpression XPath to check
+   * @returns True if node exists
+   */
+  private exists(xpathExpression: string): boolean {
+    if (!this.xmlDoc) return false;
+    const nodes = xpath.select(xpathExpression, this.xmlDoc);
+    // Handle different return types from xpath.select()
+    if (Array.isArray(nodes)) {
+      return nodes.length > 0;
+    }
+    return nodes ? true : false;
+  }
+  
+  /**
+   * Helper method to get a number value from XPath
+   * @param xpathExpression XPath to get number from
+   * @returns Number value or NaN if not found
+   */
+  private getNumberValue(xpathExpression: string): number {
+    if (!this.xmlDoc) return NaN;
+    const node = xpath.select1(`string(${xpathExpression})`, this.xmlDoc);
+    return node ? parseFloat(node.toString()) : NaN;
+  }
+}

ts/formats/ubl.validator.ts

@@ -0,0 +1,382 @@
+import { BaseValidator } from './base.validator.js';
+import { ValidationLevel } from '../interfaces.js';
+import type { ValidationResult, ValidationError } from '../interfaces.js';
+import * as xpath from 'xpath';
+import { DOMParser } from 'xmldom';
+
+/**
+ * Validator for UBL (Universal Business Language) invoice format
+ * Implements validation rules according to EN16931 and UBL 2.1 specification
+ */
+export class UBLValidator extends BaseValidator {
+  // XML namespaces for UBL
+  private static NS_INVOICE = 'urn:oasis:names:specification:ubl:schema:xsd:Invoice-2';
+  private static NS_CAC = 'urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2';
+  private static NS_CBC = 'urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2';
+  
+  // XML document for processing
+  private xmlDoc: Document | null = null;
+  
+  // UBL profile or customization ID
+  private customizationId: string = '';
+  
+  constructor(xml: string) {
+    super(xml);
+    
+    try {
+      // Parse XML document
+      this.xmlDoc = new DOMParser().parseFromString(xml, 'application/xml');
+      
+      // Determine UBL customization ID (e.g. EN16931, XRechnung)
+      this.detectCustomizationId();
+    } catch (error) {
+      this.addError('UBL-PARSE', `Failed to parse XML: ${error}`, '/');
+    }
+  }
+  
+  /**
+   * Validates the UBL invoice against the specified level
+   * @param level Validation level
+   * @returns Validation result
+   */
+  public validate(level: ValidationLevel = ValidationLevel.SYNTAX): ValidationResult {
+    // Reset errors
+    this.errors = [];
+    
+    // Check if document was parsed successfully
+    if (!this.xmlDoc) {
+      return {
+        valid: false,
+        errors: this.errors,
+        level: level
+      };
+    }
+    
+    // Perform validation based on level
+    let valid = true;
+    
+    if (level === ValidationLevel.SYNTAX) {
+      valid = this.validateSchema();
+    } else if (level === ValidationLevel.SEMANTIC) {
+      valid = this.validateSchema() && this.validateStructure();
+    } else if (level === ValidationLevel.BUSINESS) {
+      valid = this.validateSchema() && 
+              this.validateStructure() && 
+              this.validateBusinessRules();
+    }
+    
+    return {
+      valid,
+      errors: this.errors,
+      level
+    };
+  }
+  
+  /**
+   * Validates XML against schema
+   * @returns True if schema validation passed
+   */
+  protected validateSchema(): boolean {
+    // Basic schema validation (simplified for now)
+    if (!this.xmlDoc) return false;
+    
+    // Check for root element
+    const root = this.xmlDoc.documentElement;
+    if (!root || (root.nodeName !== 'Invoice' && root.nodeName !== 'CreditNote')) {
+      this.addError('UBL-SCHEMA-1', 'Root element must be Invoice or CreditNote', '/');
+      return false;
+    }
+    
+    // Check for required namespaces
+    if (!root.lookupNamespaceURI('cac') || !root.lookupNamespaceURI('cbc')) {
+      this.addError('UBL-SCHEMA-2', 'Required namespaces cac and cbc must be declared', '/');
+      return false;
+    }
+    
+    return true;
+  }
+  
+  /**
+   * Validates structure of the XML document
+   * @returns True if structure validation passed
+   */
+  private validateStructure(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    let valid = true;
+    
+    // Check for required main sections
+    const sections = [
+      'cbc:ID',
+      'cbc:IssueDate',
+      'cac:AccountingSupplierParty',
+      'cac:AccountingCustomerParty',
+      'cac:LegalMonetaryTotal'
+    ];
+    
+    for (const section of sections) {
+      if (!this.exists(`/${this.getRootNodeName()}/${section}`)) {
+        this.addError('UBL-STRUCT-1', `Required section ${section} is missing`, `/${this.getRootNodeName()}`);
+        valid = false;
+      }
+    }
+    
+    // Check for TaxTotal section
+    if (this.exists(`/${this.getRootNodeName()}/cac:TaxTotal`)) {
+      const taxSubsections = [
+        'cbc:TaxAmount',
+        'cac:TaxSubtotal'
+      ];
+      
+      for (const subsection of taxSubsections) {
+        if (!this.exists(`/${this.getRootNodeName()}/cac:TaxTotal/${subsection}`)) {
+          this.addError('UBL-STRUCT-2', `Required subsection ${subsection} is missing`, 
+            `/${this.getRootNodeName()}/cac:TaxTotal`);
+          valid = false;
+        }
+      }
+    }
+    
+    return valid;
+  }
+  
+  /**
+   * Validates business rules
+   * @returns True if business rule validation passed
+   */
+  protected validateBusinessRules(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    let valid = true;
+    
+    // BR-16: Amount due for payment (BT-115) = Invoice total amount with VAT (BT-112) - Paid amount (BT-113)
+    valid = this.validateAmounts() && valid;
+    
+    // BR-CO-3: Value added tax point date (BT-7) and Value added tax point date code (BT-8) are mutually exclusive
+    valid = this.validateMutuallyExclusiveFields() && valid;
+    
+    // BR-S-1: An Invoice that contains a line where the VAT category code is "Standard rated" 
+    // shall contain the Seller VAT Identifier or the Seller tax representative VAT identifier
+    valid = this.validateSellerVatIdentifier() && valid;
+    
+    // XRechnung specific rules when customization ID matches
+    if (this.isXRechnung()) {
+      valid = this.validateXRechnungRules() && valid;
+    }
+    
+    return valid;
+  }
+  
+  /**
+   * Gets the root node name (Invoice or CreditNote)
+   * @returns Root node name
+   */
+  private getRootNodeName(): string {
+    if (!this.xmlDoc || !this.xmlDoc.documentElement) return 'Invoice';
+    return this.xmlDoc.documentElement.nodeName;
+  }
+  
+  /**
+   * Detects UBL customization ID from the XML
+   */
+  private detectCustomizationId(): void {
+    if (!this.xmlDoc) return;
+    
+    // Look for customization ID
+    const customizationNode = xpath.select1(
+      `string(/${this.getRootNodeName()}/cbc:CustomizationID)`, 
+      this.xmlDoc
+    );
+    
+    if (customizationNode) {
+      this.customizationId = customizationNode.toString();
+    }
+  }
+  
+  /**
+   * Checks if invoice is an XRechnung
+   * @returns True if XRechnung customization ID is present
+   */
+  private isXRechnung(): boolean {
+    return this.customizationId.includes('xrechnung') || 
+           this.customizationId.includes('XRechnung');
+  }
+  
+  /**
+   * Validates amount calculations in the invoice
+   * @returns True if amount validation passed
+   */
+  private validateAmounts(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    try {
+      // Extract amounts
+      const totalAmount = this.getNumberValue(
+        `/${this.getRootNodeName()}/cac:LegalMonetaryTotal/cbc:TaxInclusiveAmount`
+      );
+      
+      const paidAmount = this.getNumberValue(
+        `/${this.getRootNodeName()}/cac:LegalMonetaryTotal/cbc:PrepaidAmount`
+      ) || 0;
+      
+      const dueAmount = this.getNumberValue(
+        `/${this.getRootNodeName()}/cac:LegalMonetaryTotal/cbc:PayableAmount`
+      );
+      
+      // Calculate expected due amount
+      const expectedDueAmount = totalAmount - paidAmount;
+      
+      // Compare with a small tolerance for rounding errors
+      if (Math.abs(dueAmount - expectedDueAmount) > 0.01) {
+        this.addError(
+          'BR-16', 
+          `Amount due for payment (${dueAmount}) must equal Invoice total amount with VAT (${totalAmount}) - Paid amount (${paidAmount})`,
+          `/${this.getRootNodeName()}/cac:LegalMonetaryTotal`
+        );
+        return false;
+      }
+      
+      return true;
+    } catch (error) {
+      this.addError('UBL-AMOUNT', `Error validating amounts: ${error}`, '/');
+      return false;
+    }
+  }
+  
+  /**
+   * Validates mutually exclusive fields
+   * @returns True if validation passed
+   */
+  private validateMutuallyExclusiveFields(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    try {
+      // Check for VAT point date and code (BR-CO-3)
+      const vatPointDate = this.exists(`/${this.getRootNodeName()}/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory/cbc:TaxPointDate`);
+      const vatPointDateCode = this.exists(`/${this.getRootNodeName()}/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory/cbc:TaxExemptionReasonCode`);
+      
+      if (vatPointDate && vatPointDateCode) {
+        this.addError(
+          'BR-CO-3', 
+          'Value added tax point date and Value added tax point date code are mutually exclusive',
+          `/${this.getRootNodeName()}/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory`
+        );
+        return false;
+      }
+      
+      return true;
+    } catch (error) {
+      this.addError('UBL-MUTUAL', `Error validating mutually exclusive fields: ${error}`, '/');
+      return false;
+    }
+  }
+  
+  /**
+   * Validates seller VAT identifier requirements
+   * @returns True if validation passed
+   */
+  private validateSellerVatIdentifier(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    try {
+      // Check if there are any standard rated line items
+      const standardRatedItems = this.exists(
+        `/${this.getRootNodeName()}/cac:InvoiceLine/cac:Item/cac:ClassifiedTaxCategory/cbc:ID[text()="S"]`
+      );
+      
+      if (standardRatedItems) {
+        // Check for seller VAT identifier
+        const sellerVatId = this.exists(`/${this.getRootNodeName()}/cac:AccountingSupplierParty/cac:Party/cac:PartyTaxScheme/cbc:CompanyID`);
+        const sellerTaxRepId = this.exists(`/${this.getRootNodeName()}/cac:TaxRepresentativeParty/cac:PartyTaxScheme/cbc:CompanyID`);
+        
+        if (!sellerVatId && !sellerTaxRepId) {
+          this.addError(
+            'BR-S-1', 
+            'An Invoice with standard rated items must contain the Seller VAT Identifier or Tax representative VAT identifier',
+            `/${this.getRootNodeName()}/cac:AccountingSupplierParty/cac:Party`
+          );
+          return false;
+        }
+      }
+      
+      return true;
+    } catch (error) {
+      this.addError('UBL-VAT', `Error validating seller VAT identifier: ${error}`, '/');
+      return false;
+    }
+  }
+  
+  /**
+   * Validates XRechnung specific rules
+   * @returns True if validation passed
+   */
+  private validateXRechnungRules(): boolean {
+    if (!this.xmlDoc) return false;
+    
+    let valid = true;
+    
+    try {
+      // BR-DE-1: Buyer reference must be present for German VAT compliance
+      if (!this.exists(`/${this.getRootNodeName()}/cbc:BuyerReference`)) {
+        this.addError(
+          'BR-DE-1', 
+          'BuyerReference is mandatory for XRechnung',
+          `/${this.getRootNodeName()}`
+        );
+        valid = false;
+      }
+      
+      // BR-DE-15: Contact information must be present
+      if (!this.exists(`/${this.getRootNodeName()}/cac:AccountingSupplierParty/cac:Party/cac:Contact`)) {
+        this.addError(
+          'BR-DE-15', 
+          'Supplier contact information is mandatory for XRechnung',
+          `/${this.getRootNodeName()}/cac:AccountingSupplierParty/cac:Party`
+        );
+        valid = false;
+      }
+      
+      // BR-DE-16: Electronic address identifier scheme (e.g. PEPPOL) must be present
+      if (!this.exists(`/${this.getRootNodeName()}/cac:AccountingSupplierParty/cac:Party/cbc:EndpointID`) ||
+          !this.exists(`/${this.getRootNodeName()}/cac:AccountingSupplierParty/cac:Party/cbc:EndpointID/@schemeID`)) {
+        this.addError(
+          'BR-DE-16', 
+          'Supplier electronic address with scheme identifier is mandatory for XRechnung',
+          `/${this.getRootNodeName()}/cac:AccountingSupplierParty/cac:Party`
+        );
+        valid = false;
+      }
+      
+      return valid;
+    } catch (error) {
+      this.addError('UBL-XRECHNUNG', `Error validating XRechnung rules: ${error}`, '/');
+      return false;
+    }
+  }
+  
+  /**
+   * Helper method to check if a node exists
+   * @param xpathExpression XPath to check
+   * @returns True if node exists
+   */
+  private exists(xpathExpression: string): boolean {
+    if (!this.xmlDoc) return false;
+    const nodes = xpath.select(xpathExpression, this.xmlDoc);
+    // Handle different return types from xpath.select()
+    if (Array.isArray(nodes)) {
+      return nodes.length > 0;
+    }
+    return nodes ? true : false;
+  }
+  
+  /**
+   * Helper method to get a number value from XPath
+   * @param xpathExpression XPath to get number from
+   * @returns Number value or NaN if not found
+   */
+  private getNumberValue(xpathExpression: string): number {
+    if (!this.xmlDoc) return NaN;
+    const node = xpath.select1(`string(${xpathExpression})`, this.xmlDoc);
+    return node ? parseFloat(node.toString()) : NaN;
+  }
+}

ts/formats/validator.factory.ts

@@ -0,0 +1,92 @@
+import { InvoiceFormat } from '../interfaces.js';
+import type { IValidator } from '../interfaces.js';
+import { BaseValidator } from './base.validator.js';
+import { FacturXValidator } from './facturx.validator.js';
+import { UBLValidator } from './ubl.validator.js';
+import { DOMParser } from 'xmldom';
+
+/**
+ * Factory to create the appropriate validator based on the XML format
+ */
+export class ValidatorFactory {
+  /**
+   * Creates a validator for the specified XML content
+   * @param xml XML content to validate
+   * @returns Appropriate validator instance
+   */
+  public static createValidator(xml: string): BaseValidator {
+    const format = ValidatorFactory.detectFormat(xml);
+    
+    switch (format) {
+      case InvoiceFormat.UBL:
+      case InvoiceFormat.XRECHNUNG:
+        return new UBLValidator(xml);
+        
+      case InvoiceFormat.CII:
+      case InvoiceFormat.ZUGFERD:
+      case InvoiceFormat.FACTURX:
+        return new FacturXValidator(xml);
+        
+      // FatturaPA and other formats would be implemented here
+        
+      default:
+        throw new Error(`Unsupported invoice format: ${format}`);
+    }
+  }
+  
+  /**
+   * Detects the invoice format from XML content
+   * @param xml XML content to analyze
+   * @returns Detected invoice format
+   */
+  private static detectFormat(xml: string): InvoiceFormat {
+    try {
+      const doc = new DOMParser().parseFromString(xml, 'application/xml');
+      const root = doc.documentElement;
+      
+      if (!root) {
+        return InvoiceFormat.UNKNOWN;
+      }
+      
+      // UBL detection (Invoice or CreditNote root element)
+      if (root.nodeName === 'Invoice' || root.nodeName === 'CreditNote') {
+        // Check if it's XRechnung by looking at CustomizationID
+        const customizationNodes = root.getElementsByTagName('cbc:CustomizationID');
+        if (customizationNodes.length > 0) {
+          const customizationId = customizationNodes[0].textContent || '';
+          if (customizationId.includes('xrechnung') || customizationId.includes('XRechnung')) {
+            return InvoiceFormat.XRECHNUNG;
+          }
+        }
+        
+        return InvoiceFormat.UBL;
+      }
+      
+      // Factur-X/ZUGFeRD detection (CrossIndustryInvoice root element)
+      if (root.nodeName === 'rsm:CrossIndustryInvoice' || root.nodeName === 'CrossIndustryInvoice') {
+        // Check for profile to determine if it's Factur-X or ZUGFeRD
+        const profileNodes = root.getElementsByTagName('ram:ID');
+        for (let i = 0; i < profileNodes.length; i++) {
+          const profileText = profileNodes[i].textContent || '';
+          
+          if (profileText.includes('factur-x') || profileText.includes('Factur-X')) {
+            return InvoiceFormat.FACTURX;
+          }
+          
+          if (profileText.includes('zugferd') || profileText.includes('ZUGFeRD')) {
+            return InvoiceFormat.ZUGFERD;
+          }
+        }
+        
+        // If no specific profile found, default to CII
+        return InvoiceFormat.CII;
+      }
+      
+      // FatturaPA detection would be implemented here
+      
+      return InvoiceFormat.UNKNOWN;
+    } catch (error) {
+      return InvoiceFormat.UNKNOWN;
+    }
+  }
+}

ts/index.ts

@@ -9,7 +9,28 @@ import { XInvoiceDecoder } from './formats/xinvoice.decoder.js';
 import { DecoderFactory } from './formats/decoder.factory.js';
 import { BaseDecoder } from './formats/base.decoder.js';
 
-// Export interfaces
+// Import validator classes
+import { ValidatorFactory } from './formats/validator.factory.js';
+import { BaseValidator } from './formats/base.validator.js';
+import { FacturXValidator } from './formats/facturx.validator.js';
+import { UBLValidator } from './formats/ubl.validator.js';
+
+// Export specific interfaces for easier use
+export type {
+  IXInvoice,
+  IParty,
+  IAddress,
+  IContact,
+  IInvoiceItem,
+  ValidationError,
+  ValidationResult,
+  ValidationLevel,
+  InvoiceFormat,
+  XInvoiceOptions,
+  IValidator
+} from './interfaces.js';
+
+// Export interfaces (legacy support)
 export { interfaces };
 
 // Export main class
@@ -30,6 +51,47 @@ export {
   XInvoiceDecoder
 };
 
+// Export validator classes
+export const Validators = {
+  ValidatorFactory,
+  BaseValidator,
+  FacturXValidator,
+  UBLValidator
+};
+
 // For backward compatibility
 export { FacturXEncoder as ZugferdXmlEncoder };
-export { FacturXDecoder as ZUGFeRDXmlDecoder };
+export { FacturXDecoder as ZUGFeRDXmlDecoder };
+
+/**
+ * Validates an XML string against the appropriate format rules
+ * @param xml XML content to validate
+ * @param level Validation level (syntax, semantic, business)
+ * @returns ValidationResult with the result of validation
+ */
+export function validateXml(
+  xml: string, 
+  level: interfaces.ValidationLevel = interfaces.ValidationLevel.SYNTAX
+): interfaces.ValidationResult {
+  try {
+    const validator = ValidatorFactory.createValidator(xml);
+    return validator.validate(level);
+  } catch (error) {
+    return {
+      valid: false,
+      errors: [{
+        code: 'VAL-ERROR',
+        message: `Validation error: ${error.message}`
+      }],
+      level
+    };
+  }
+}
+
+/**
+ * Creates a new XInvoice instance
+ * @returns A new XInvoice instance
+ */
+export function createXInvoice(): XInvoice {
+  return new XInvoice();
+}

ts/interfaces.ts

@@ -31,3 +31,60 @@ export interface IInvoiceItem {
   UnitPrice: number;
   TotalPrice: number;
 }
+
+/**
+ * Supported electronic invoice formats
+ */
+export enum InvoiceFormat {
+  UNKNOWN = 'unknown',
+  UBL = 'ubl', // Universal Business Language
+  CII = 'cii', // Cross-Industry Invoice
+  ZUGFERD = 'zugferd', // ZUGFeRD (German e-invoice format)
+  FACTURX = 'facturx', // Factur-X (French e-invoice format)
+  XRECHNUNG = 'xrechnung', // XRechnung (German e-invoice implementation of EN16931)
+  FATTURAPA = 'fatturapa' // FatturaPA (Italian e-invoice format)
+}
+
+/**
+ * Describes a validation level for invoice validation
+ */
+export enum ValidationLevel {
+  SYNTAX = 'syntax', // Schema validation only
+  SEMANTIC = 'semantic', // Semantic validation (field types, required fields, etc.)
+  BUSINESS = 'business' // Business rule validation
+}
+
+/**
+ * Describes a validation error
+ */
+export interface ValidationError {
+  code: string; // Error code (e.g. "BR-16")
+  message: string; // Error message
+  location?: string; // XPath or location in the document
+}
+
+/**
+ * Result of a validation operation
+ */
+export interface ValidationResult {
+  valid: boolean; // Overall validation result
+  errors: ValidationError[]; // List of validation errors
+  level: ValidationLevel; // The level that was validated
+}
+
+/**
+ * Options for the XInvoice class
+ */
+export interface XInvoiceOptions {
+  validateOnLoad?: boolean; // Whether to validate when loading an invoice
+  validationLevel?: ValidationLevel; // Level of validation to perform
+}
+
+/**
+ * Interface for validator implementations
+ */
+export interface IValidator {
+  validate(level?: ValidationLevel): ValidationResult;
+  isValid(): boolean;
+  getValidationErrors(): ValidationError[];
+}