ktg-plugin-marketplace/plugins/llm-security-copilot/scanners/lib/diff-engine.mjs

// diff-engine.mjs — Baseline storage, finding fingerprinting, and diff categorization.
// Compares scan results against a stored baseline to classify findings as:
//   new       — present in current scan, absent from baseline
//   resolved  — present in baseline, absent from current scan
//   unchanged — matched between baseline and current (line drift ≤3)
//   moved     — same finding, different location (line drift >3 or file renamed)
// Zero external dependencies.

import { createHash } from 'node:crypto';
import { existsSync, mkdirSync, readFileSync, writeFileSync } from 'node:fs';
import { join, resolve } from 'node:path';

// ---------------------------------------------------------------------------
// Configuration
// ---------------------------------------------------------------------------
const LINE_FUZZY_THRESHOLD = 3; // ±3 lines = unchanged, >3 = moved

// ---------------------------------------------------------------------------
// Target hashing — deterministic key for baseline storage
// ---------------------------------------------------------------------------

/**
 * Create a stable hash for a target path to use as baseline filename.
 * Uses the resolved absolute path so the same directory always maps
 * to the same baseline regardless of how it was referenced.
 * @param {string} targetPath
 * @returns {string} 12-char hex hash
 */
export function targetHash(targetPath) {
  const resolved = resolve(targetPath);
  return createHash('sha256').update(resolved).digest('hex').slice(0, 12);
}

// ---------------------------------------------------------------------------
// Finding fingerprinting — identity that survives line drift
// ---------------------------------------------------------------------------

/**
 * Generate a stable fingerprint for a finding.
 * Combines scanner prefix + file + title + evidence to create an identity
 * that is independent of line number (line drift is handled separately).
 * @param {object} finding - A finding object from output.mjs
 * @returns {string} hex fingerprint
 */
export function fingerprintFinding(finding) {
  const parts = [
    finding.scanner || '',
    finding.file || '',
    finding.title || '',
    // Evidence provides content-level identity — two different findings
    // in the same file with different evidence are distinct findings.
    finding.evidence || '',
  ];
  return createHash('sha256').update(parts.join('\x00')).digest('hex').slice(0, 16);
}

// ---------------------------------------------------------------------------
// Baseline I/O
// ---------------------------------------------------------------------------

/**
 * Resolve the baseline file path for a given target.
 * @param {string} baselinesDir - Path to reports/baselines/
 * @param {string} targetPath
 * @returns {string} Full path to baseline JSON file
 */
export function baselinePath(baselinesDir, targetPath) {
  return join(baselinesDir, `${targetHash(targetPath)}.json`);
}

/**
 * Save scan results as a baseline.
 * @param {string} baselinesDir - Path to reports/baselines/
 * @param {string} targetPath - The scanned target
 * @param {object} scanEnvelope - Full scan output envelope from scan-orchestrator
 * @returns {string} Path to saved baseline file
 */
export function saveBaseline(baselinesDir, targetPath, scanEnvelope) {
  if (!existsSync(baselinesDir)) {
    mkdirSync(baselinesDir, { recursive: true });
  }
  const filePath = baselinePath(baselinesDir, targetPath);

  // Store a compact baseline: metadata + fingerprinted findings
  const baseline = {
    meta: {
      target: scanEnvelope.meta.target,
      timestamp: scanEnvelope.meta.timestamp,
      version: '1', // baseline format version
    },
    aggregate: scanEnvelope.aggregate,
    findings: extractFindings(scanEnvelope),
  };

  writeFileSync(filePath, JSON.stringify(baseline, null, 2) + '\n');
  return filePath;
}

/**
 * Load a baseline from disk.
 * @param {string} baselinesDir
 * @param {string} targetPath
 * @returns {object|null} Baseline object or null if not found
 */
export function loadBaseline(baselinesDir, targetPath) {
  const filePath = baselinePath(baselinesDir, targetPath);
  if (!existsSync(filePath)) return null;
  try {
    return JSON.parse(readFileSync(filePath, 'utf8'));
  } catch {
    return null;
  }
}

// ---------------------------------------------------------------------------
// Finding extraction — flatten all scanner results into fingerprinted list
// ---------------------------------------------------------------------------

/**
 * Extract all findings from a scan envelope, adding fingerprints.
 * @param {object} scanEnvelope
 * @returns {object[]} Array of { fingerprint, scanner, severity, title, file, line, evidence, owasp, recommendation }
 */
export function extractFindings(scanEnvelope) {
  const findings = [];
  for (const [scannerName, result] of Object.entries(scanEnvelope.scanners || {})) {
    for (const f of result.findings || []) {
      findings.push({
        fingerprint: fingerprintFinding(f),
        scanner: f.scanner || scannerName.toUpperCase().slice(0, 3),
        severity: f.severity,
        title: f.title,
        file: f.file || null,
        line: f.line || null,
        evidence: f.evidence || null,
        owasp: f.owasp || null,
        recommendation: f.recommendation || null,
      });
    }
  }
  return findings;
}

// ---------------------------------------------------------------------------
// Diff algorithm
// ---------------------------------------------------------------------------

/**
 * Compare current scan findings against a baseline.
 *
 * Matching strategy (priority order):
 * 1. Exact: fingerprint + file + line within ±LINE_FUZZY_THRESHOLD → unchanged
 * 2. Moved: fingerprint matches but file or line drifted beyond threshold → moved
 * 3. Unmatched current findings → new
 * 4. Unmatched baseline findings → resolved
 *
 * @param {object[]} baselineFindings - From loadBaseline().findings
 * @param {object[]} currentFindings - From extractFindings()
 * @returns {object} { new, resolved, unchanged, moved, summary }
 */
export function diffFindings(baselineFindings, currentFindings) {
  // Index baseline findings by fingerprint for O(n) lookup
  // Multiple findings can share a fingerprint (same pattern, different locations)
  const baselineByFp = new Map();
  for (const f of baselineFindings) {
    const existing = baselineByFp.get(f.fingerprint) || [];
    existing.push({ ...f, matched: false });
    baselineByFp.set(f.fingerprint, existing);
  }

  const results = {
    new: [],
    resolved: [],
    unchanged: [],
    moved: [],
  };

  // Pass 1: Match current findings against baseline
  for (const current of currentFindings) {
    const candidates = baselineByFp.get(current.fingerprint);
    if (!candidates) {
      results.new.push(current);
      continue;
    }

    // Try exact match first (same file, line within threshold)
    let matched = false;
    for (const baseline of candidates) {
      if (baseline.matched) continue;
      if (baseline.file === current.file && isLineClose(baseline.line, current.line)) {
        baseline.matched = true;
        results.unchanged.push({
          ...current,
          baseline_line: baseline.line,
        });
        matched = true;
        break;
      }
    }
    if (matched) continue;

    // Try moved match (fingerprint matches, location differs)
    for (const baseline of candidates) {
      if (baseline.matched) continue;
      baseline.matched = true;
      results.moved.push({
        ...current,
        previous_file: baseline.file,
        previous_line: baseline.line,
      });
      matched = true;
      break;
    }
    if (matched) continue;

    // All candidates consumed — this is new
    results.new.push(current);
  }

  // Pass 2: Unmatched baseline findings are resolved
  for (const candidates of baselineByFp.values()) {
    for (const baseline of candidates) {
      if (!baseline.matched) {
        const { matched: _, ...finding } = baseline;
        results.resolved.push(finding);
      }
    }
  }

  // Summary
  results.summary = {
    new: results.new.length,
    resolved: results.resolved.length,
    unchanged: results.unchanged.length,
    moved: results.moved.length,
    total_current: currentFindings.length,
    total_baseline: baselineFindings.length,
    baseline_timestamp: null, // caller fills in
  };

  return results;
}

/**
 * Check if two line numbers are within the fuzzy threshold.
 * Null lines always match (some findings are file-level, not line-level).
 * @param {number|null} a
 * @param {number|null} b
 * @returns {boolean}
 */
function isLineClose(a, b) {
  if (a == null || b == null) return true;
  return Math.abs(a - b) <= LINE_FUZZY_THRESHOLD;
}

// ---------------------------------------------------------------------------
// High-level API — used by scan-orchestrator
// ---------------------------------------------------------------------------

/**
 * Run a full diff cycle: load baseline, compare, return diff results.
 * @param {string} baselinesDir
 * @param {string} targetPath
 * @param {object} scanEnvelope - Current scan results
 * @returns {object|null} Diff results with summary, or null if no baseline exists
 */
export function diffAgainstBaseline(baselinesDir, targetPath, scanEnvelope) {
  const baseline = loadBaseline(baselinesDir, targetPath);
  if (!baseline) return null;

  const currentFindings = extractFindings(scanEnvelope);
  const diff = diffFindings(baseline.findings, currentFindings);
  diff.summary.baseline_timestamp = baseline.meta.timestamp;

  return diff;
}