# All files change-detector.ts

98.83% Statements 85/86
91.07% Branches 51/56
100% Functions 11/11
98.73% Lines 78/79

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     import {
      PropertySnapshot,
      ChangeDetectionResult,
      ChangeType,
      Severity,
      PropertyChange,
      DetectionConfig,
      Address,
    } from './types';
    import { matchRecords } from './matcher.service';

    const DEFAULT_DETECTION_CONFIG: DetectionConfig = {
      ownershipNameThreshold: 0.7,
      deedDateSensitivity: 0.9,
      taxAmountChangePercent: 15,
    };

    function classifyFieldChange(field: string, oldValue: unknown, newValue: unknown, config: DetectionConfig): PropertyChange {
      let changeType: ChangeType;

      switch (field) {
        case 'ownerName':
          changeType =
            typeof oldValue === 'string' && typeof newValue === 'string'
              ? isSignificantNameChange(oldValue, newValue, config)
                ? 'ownership_transfer'
                : 'metadata_change'
              : 'ownership_transfer';
          break;
        case 'deedDate':
          changeType = 'deed_change';
          break;
        case 'taxAmount':
          changeType = 'tax_change';
          break;
        case 'lienCount':
          changeType = (newValue as number) > (oldValue as number) ? 'lien_filing' : 'metadata_change';
          break;
        case 'taxId':
          changeType = 'deed_change';
          break;
        default:
          changeType = 'metadata_change';
      }

      return { field, oldValue, newValue, changeType };
    }

    function isSignificantNameChange(oldName: string, newName: string, config: DetectionConfig): boolean {
      const dummyAddress: Address = {
        streetNumber: '0',
        streetName: 'dummy',
        city: 'dummy',
        state: 'XX',
        zip: '00000',
      };

      const result = matchRecords(oldName, dummyAddress, newName, dummyAddress);
      return result.nameScore < config.ownershipNameThreshold;
    }

    function determineSeverity(changes: PropertyChange[], config: DetectionConfig): Severity {
      const severityOverrides = config.severityOverrides || {};

      const typeToSeverity: Record<ChangeType, Severity> = {
        ownership_transfer: severityOverrides['ownership_transfer'] || 'major',
        deed_change: severityOverrides['deed_change'] || 'moderate',
        lien_filing: severityOverrides['lien_filing'] || 'moderate',
        tax_change: severityOverrides['tax_change'] || 'minor',
        metadata_change: severityOverrides['metadata_change'] || 'minor',
      };

      const severityOrder: Severity[] = ['major', 'moderate', 'minor'];

      for (const change of changes) {
        const sev = typeToSeverity[change.changeType];
        const idx = severityOrder.indexOf(sev);
        if (idx === 0) return 'major';
      }

      for (const change of changes) {
        const sev = typeToSeverity[change.changeType];
        if (sev === 'moderate') return 'moderate';
      }

      return 'minor';
    }

    function computeChangeConfidence(changes: PropertyChange[], config: DetectionConfig): number {
      if (changes.length === 0) return 0;

      let totalConfidence = 0;
      for (const change of changes) {
        switch (change.changeType) {
          case 'ownership_transfer':
            totalConfidence += 0.95;
            break;
          case 'deed_change':
            totalConfidence += config.deedDateSensitivity;
            break;
          case 'tax_change': {
            const oldVal = change.oldValue as number;
            const newVal = change.newValue as number;
            const pctChange = oldVal ? Math.abs(newVal - oldVal) / oldVal * 100 : 100;
            totalConfidence += pctChange >= config.taxAmountChangePercent ? 0.85 : 0.5;
            break;
          }
          case 'lien_filing':
            totalConfidence += 0.9;
            break;
          default:
            totalConfidence += 0.4;
        }
      }

      return Math.round((totalConfidence / changes.length) * 1000) / 1000;
    }

    export function detectChanges(
      previous: PropertySnapshot,
      current: PropertySnapshot,
      config?: Partial<DetectionConfig>,
    ): ChangeDetectionResult {
      const effectiveConfig = { ...DEFAULT_DETECTION_CONFIG, ...config };
      const changes: PropertyChange[] = [];

      const fieldsToCompare: (keyof Omit<PropertySnapshot, 'id' | 'capturedAt' | 'propertyId'>)[] = [
        'ownerName',
        'deedDate',
        'taxId',
        'taxAmount',
        'lienCount',
        'propertyType',
      ];

      for (const field of fieldsToCompare) {
        const oldValue = previous[field];
        const newValue = current[field];

        if (oldValue !== newValue) {
          changes.push(classifyFieldChange(field, oldValue, newValue, effectiveConfig));
        }
      }

      const addressChanges = detectAddressChanges(previous.address, current.address);
      changes.push(...addressChanges);

      const severity = determineSeverity(changes, effectiveConfig);
      const confidence = computeChangeConfidence(changes, effectiveConfig);

      let changeType: ChangeType = 'metadata_change';
      if (changes.some(c => c.changeType === 'ownership_transfer')) {
        changeType = 'ownership_transfer';
      } else if (changes.some(c => c.changeType === 'deed_change')) {
        changeType = 'deed_change';
      } else if (changes.some(c => c.changeType === 'lien_filing')) {
        changeType = 'lien_filing';
      } else if (changes.some(c => c.changeType === 'tax_change')) {
        changeType = 'tax_change';
      }

      return {
        propertyId: previous.propertyId,
        changeType,
        severity,
        confidence,
        changes,
        previousSnapshot: previous,
        currentSnapshot: current,
        detectedAt: new Date().toISOString(),
      };
    }

    function detectAddressChanges(oldAddr: Address, newAddr: Address): PropertyChange[] {
      const changes: PropertyChange[] = [];

      const addressFields: (keyof Address)[] = ['streetNumber', 'streetName', 'streetType', 'unit', 'city', 'state', 'zip'];

      for (const field of addressFields) {
        const oldVal = oldAddr[field];
        const newVal = newAddr[field];
        if (oldVal !== newVal) {
          changes.push({
            field: `address.${field}`,
            oldValue: oldVal,
            newValue: newVal,
            changeType: 'metadata_change',
          });
        }
      }

      return changes;
    }

    export function shouldTriggerAlert(result: ChangeDetectionResult, minSeverity: Severity = 'moderate'): boolean {
      const severityOrder: Severity[] = ['minor', 'moderate', 'major'];
      const resultIdx = severityOrder.indexOf(result.severity);
      const minIdx = severityOrder.indexOf(minSeverity);
      return resultIdx >= minIdx && result.confidence >= 0.7;
    }

    export { classifyFieldChange, determineSeverity, computeChangeConfidence };