/**
 * Health Check Dashboard Module
 * Monitors service health, response times, and uptime
 * Provides SLA tracking and incident management
 */

const https = require('https');
const http = require('http');
const crypto = require('crypto');
const EventEmitter = require('events');
const fs = require('fs');
const path = require('path');
const paths = require('../../platform-paths');

// Persist health config + history alongside the other state files (services.json,
// config.json) rather than next to the source. In a container that data dir is the
// mounted /app/data volume, so uptime history survives container recreates/updates;
// previously these defaulted to __dirname (unmounted /app) and every recreate wiped
// the accumulated history, blanking the dashboard uptime bars. Explicit env vars
// still override.
const HEALTH_DATA_DIR = process.env.HEALTH_DATA_DIR || path.dirname(paths.configFile);
const HEALTH_CONFIG_FILE = process.env.HEALTH_CONFIG_FILE || path.join(HEALTH_DATA_DIR, 'health-config.json');
const HEALTH_HISTORY_FILE = process.env.HEALTH_HISTORY_FILE || path.join(HEALTH_DATA_DIR, 'health-history.json');

// Legacy locations (next to the source) used before the data-dir default. Read these
// once on first load if the new files are absent, so upgrading installs migrate their
// accumulated history/config instead of starting empty. The next save() rewrites to
// the new location.
const LEGACY_HEALTH_CONFIG_FILE = path.join(__dirname, 'health-config.json');
const LEGACY_HEALTH_HISTORY_FILE = path.join(__dirname, 'health-history.json');
const CHECK_INTERVAL = parseInt(process.env.HEALTH_CHECK_INTERVAL || '30000', 10); // 30 seconds
const MAX_CHECK_INTERVAL = parseInt(process.env.HEALTH_CHECK_MAX_INTERVAL || '300000', 10); // 5 minutes max backoff
const MAX_ENTRIES_PER_SERVICE = parseInt(process.env.HEALTH_MAX_ENTRIES || '500', 10); // Cap to prevent disk explosion
const HISTORY_RETENTION_DAYS = parseInt(process.env.HEALTH_HISTORY_RETENTION || '30', 10);

// DC-086: hysteresis thresholds for badge display.
// The raw probe result can flap on a single transient blip (Caddy reload,
// container CPU steal, network hiccup, mid-flight TLS handshake). Showing
// every probe result as-is to the dashboard creates the "perpetual flicker"
// UX. Asymmetric thresholds: going red is slow (don't false-alarm), going
// green is fast (don't keep showing red after recovery).
//   - DOWN_THRESHOLD = N consecutive "down" probes before the badge flips to red
//   - UP_THRESHOLD   = N consecutive "up" probes before the badge flips back to green
// Single probe flips to green on purpose — false-positive-green is much less
// painful than perpetual-red (operators notice red, ignore green).
function readPositiveIntEnv(name, fallback) {
  const raw = process.env[name];
  if (raw === undefined || raw === '') return fallback;
  const value = Number(raw);
  return Number.isSafeInteger(value) && value >= 1 ? value : fallback;
}

const DOWN_THRESHOLD = readPositiveIntEnv('HEALTH_DOWN_THRESHOLD', 2);
const UP_THRESHOLD = readPositiveIntEnv('HEALTH_UP_THRESHOLD', 1);

class HealthChecker extends EventEmitter {
  constructor() {
    super();
    this.config = this.loadConfig();
    this.history = this.loadHistory();
    this.currentStatus = new Map();
    // DC-086: the status the dashboard SHOULD display (post-hysteresis).
    // Distinct from currentStatus, which is the latest raw probe result.
    this.displayedStatus = new Map();
    // DC-086: counter of consecutive healthy/unhealthy probes since the
    // last displayed-status change. Reset to 0 whenever displayed status flips.
    this.consecutiveSinceChange = new Map();
    this.incidents = [];
    this.checking = false;
    this.checkInterval = null;
    this.consecutiveFailures = new Map(); // serviceId -> failure count
    this.serviceTimers = new Map(); // serviceId -> timer for per-service backoff
    // Invalidate probe completions that race with removal/reconfiguration.
    this.serviceGenerations = new Map(); // serviceId -> configuration generation
  }

  /**
   * Start health checking
   */
  start() {
    if (this.checking) return;

    this.checking = true;

    // Initial check
    this.checkAll();

    // Schedule periodic checks
    this.checkInterval = setInterval(() => this.checkAll(), CHECK_INTERVAL);
  }

  /**
   * Stop health checking
   */
  stop() {
    if (!this.checking) return;

    this.checking = false;

    if (this.checkInterval) {
      clearInterval(this.checkInterval);
      this.checkInterval = null;
    }

    // Clear per-service backoff timers
    for (const timer of this.serviceTimers.values()) {
      clearTimeout(timer);
    }
    this.serviceTimers.clear();
  }

  /**
   * Get the backoff interval for a service based on consecutive failures.
   * Doubles the interval for each failure, capped at MAX_CHECK_INTERVAL.
   */
  getBackoffInterval(serviceId) {
    const failures = this.consecutiveFailures.get(serviceId) || 0;
    if (failures === 0) return CHECK_INTERVAL;
    return Math.min(CHECK_INTERVAL * Math.pow(2, failures), MAX_CHECK_INTERVAL);
  }

  /**
   * Check all configured services
   */
  async checkAll() {
    const services = Object.entries(this.config.services || {});
    
    for (const [serviceId, config] of services) {
      if (config.enabled !== false) {
        try {
          await this.checkService(serviceId, config);
        } catch (error) {
          // Error logged via checkForIncidents
        }
      }
    }
    
    // Cleanup old history
    this.cleanupHistory();
  }

  /**
   * Check a single service
   */
  async checkService(serviceId, config) {
    const startTime = Date.now();
    const generation = this.serviceGenerations.get(serviceId) || 0;

    try {
      const result = await this.performHealthCheck(config);
      const responseTime = Date.now() - startTime;

      const status = {
        serviceId,
        timestamp: new Date().toISOString(),
        status: result.healthy ? 'up' : 'down',
        responseTime,
        statusCode: result.statusCode,
        message: result.message,
        details: result.details
      };

      if ((this.serviceGenerations.get(serviceId) || 0) !== generation) {
        return status;
      }

      // Track consecutive failures for exponential backoff
      if (result.healthy) {
        this.consecutiveFailures.delete(serviceId);
      } else {
        this.consecutiveFailures.set(serviceId, (this.consecutiveFailures.get(serviceId) || 0) + 1);
      }

      const previousStatus = this.currentStatus.get(serviceId);
      this.recordStatus(serviceId, status);
      this.checkForIncidents(serviceId, status, config, previousStatus);

      return status;
    } catch (error) {
      const responseTime = Date.now() - startTime;

      // Increment failure count for backoff
      this.consecutiveFailures.set(serviceId, (this.consecutiveFailures.get(serviceId) || 0) + 1);

      const status = {
        serviceId,
        timestamp: new Date().toISOString(),
        status: 'down',
        responseTime,
        error: error.message
      };

      if ((this.serviceGenerations.get(serviceId) || 0) !== generation) {
        return status;
      }

      const previousStatus = this.currentStatus.get(serviceId);
      this.recordStatus(serviceId, status);
      this.checkForIncidents(serviceId, status, config, previousStatus);

      return status;
    }
  }

  /**
   * Perform actual health check
   */
  async performHealthCheck(config) {
    const result = await this._doRequest(config, config.method || 'GET');
    // Fall back to GET if HEAD is not supported
    if ((result.statusCode === 501 || result.statusCode === 405) && (config.method || '').toUpperCase() === 'HEAD') {
      return this._doRequest({ ...config, method: 'GET' }, 'GET');
    }
    return result;
  }

  _doRequest(config, method) {
    return new Promise((resolve, reject) => {
      const url = new URL(config.url);
      const protocol = url.protocol === 'https:' ? https : http;

      // Merge user-supplied headers with the health-check marker. Caddy on
      // *.sami uses `forward_auth` for every non-API path and the auth gate
      // returns 401 for HEAD/GET without a session — without this marker the
      // probe never reaches the upstream service, and the authLimiter (20 req
      // / 15 min) on /auth/* would also rate-limit us after 20 probes. The
      // marker lets the Caddy snippet bypass forward_auth for probes that
      // originate from the local container network (see /etc/caddy/Caddyfile
      // `(dashcaddy_auth)` block).
      const headers = {
        ...(config.headers || {}),
        'X-DashCaddy-HealthCheck': '1'
      };

      const options = {
        hostname: url.hostname,
        port: url.port || (url.protocol === 'https:' ? 443 : 80),
        path: url.pathname + url.search,
        method,
        timeout: config.timeout || 20000,
        headers,
        rejectUnauthorized: false // Trust internal CA certs (.sami TLD)
      };

      const req = protocol.request(options, (res) => {
        let data = '';

        res.on('data', chunk => {
          data += chunk;
        });

        res.on('end', () => {
          const healthy = this.evaluateHealth(res.statusCode, data, config);

          resolve({
            healthy,
            statusCode: res.statusCode,
            message: healthy ? 'Service is healthy' : 'Service check failed',
            details: {
              headers: res.headers ? { server: res.headers.server } : undefined,  // Compact: disk explosion fix
              bodyLength: data.length
            }
          });
        });
      });

      req.on('error', (error) => {
        reject(error);
      });

      req.on('timeout', () => {
        req.destroy();
        reject(new Error('Health check timeout'));
      });

      if (config.body) {
        req.write(JSON.stringify(config.body));
      }

      req.end();
    });
  }

  /**
   * Evaluate if service is healthy based on response
   */
  evaluateHealth(statusCode, body, config) {
    // Check status code. Default expected codes include the usual 2xx/3xx
    // plus 401/403 (auth-walled UIs that still prove the service is up) and
    // 429 (rate-limited upstream — we hit the service, the service answered;
    // failing the check just because we're being throttled is wrong).
    const expectedCodes = config.expectedStatusCodes || [200, 201, 204, 301, 302, 303, 307, 308, 401, 403, 429];
    if (!expectedCodes.includes(statusCode)) {
      return false;
    }

    // Check response body if pattern specified
    if (config.expectedBodyPattern) {
      const regex = new RegExp(config.expectedBodyPattern);
      if (!regex.test(body)) {
        return false;
      }
    }

    // Check response body contains expected text
    if (config.expectedBodyContains) {
      if (!body.includes(config.expectedBodyContains)) {
        return false;
      }
    }

    return true;
  }

  /**
   * Compute the displayed status for a service given the latest raw probe
   * result. Applies asymmetric hysteresis:
   *   - Going DOWN: requires DOWN_THRESHOLD (default 2) consecutive "down"
   *     probes since the last display-state change. A single blip keeps the
   *     badge green.
   *   - Going UP: requires UP_THRESHOLD (default 1) consecutive "up" probes.
   *     Any single "up" after a down streak flips back to green so the badge
   *     doesn't linger red after the service has recovered.
   *
   * Returns the displayed status object (same shape as the raw status) so
   * recordStatus can use it both for the displayed map and as the broadcast
   * payload when the displayed status actually changes.
   */
  _computeDisplayedStatus(serviceId, rawStatus) {
    const currentDisplayed = this.displayedStatus.get(serviceId);
    const previousStatus = currentDisplayed ? currentDisplayed.status : null;

    // If no prior state, accept the raw probe as-is (first-check bootstrap).
    if (!previousStatus) {
      return rawStatus;
    }

    // Probe agrees with current displayed → no change, reset the counter so
    // a brief blip doesn't accumulate against the displayed state.
    if (rawStatus.status === previousStatus) {
      this.consecutiveSinceChange.set(serviceId, 0);
      return rawStatus;
    }

    // Probe disagrees with displayed. Bump the streak counter — this counts
    // CONSECUTIVE probes that disagree with what's shown, regardless of
    // whether the raw value itself changed between probes. That's what
    // makes "down, down" flip after threshold but "down, up, down" not flip.
    const prev = this.consecutiveSinceChange.get(serviceId) || 0;
    const next = prev + 1;

    if (rawStatus.status === 'down') {
      // Going DOWN: need DOWN_THRESHOLD consecutive probes that disagree
      // with the displayed "up" state.
      if (previousStatus === 'up' && next < DOWN_THRESHOLD) {
        this.consecutiveSinceChange.set(serviceId, next);
        // Keep the last internally-consistent displayed snapshot. Mixing the
        // raw failure metadata with status="up" would expose contradictory
        // API data (for example statusCode=500 on an "up" service).
        return currentDisplayed;
      }
      // Threshold met (or already down) — flip to red.
      this.consecutiveSinceChange.set(serviceId, 0);
      return rawStatus;
    }

    // rawStatus.status === 'up' (must be — the equal-to-displayed case above
    // already returned). Going UP after a down streak: need UP_THRESHOLD.
    if (previousStatus === 'down' && next < UP_THRESHOLD) {
      this.consecutiveSinceChange.set(serviceId, next);
      return currentDisplayed;
    }
    this.consecutiveSinceChange.set(serviceId, 0);
    return rawStatus;
  }

  /**
   * Record service status
   *
   * DC-086: history + consecutiveFailures are updated for EVERY probe
   * (operators want full probe history for postmortems). The dashboard's
   * `status-check` event is only emitted when the DISPLAYED status changes,
   * so the badge stops re-rendering on every probe.
   */
  recordStatus(serviceId, status) {
    // Update current (raw) status — used by checkForIncidents and history.
    this.currentStatus.set(serviceId, status);

    // Add raw probe to history (full fidelity — operators rely on this).
    if (!this.history[serviceId]) {
      this.history[serviceId] = [];
    }

    this.history[serviceId].push(status);

    // Cap entries to prevent unbounded growth (disk explosion fix)
    if (this.history[serviceId].length > MAX_ENTRIES_PER_SERVICE) {
      this.history[serviceId] = this.history[serviceId].slice(-MAX_ENTRIES_PER_SERVICE);
    }

    // Compute the post-hysteresis displayed status; only emit when it changes.
    // _computeDisplayedStatus compares the raw probe against the DISPLAYED
    // status (not the previous raw status), so the "consecutive since
    // change" counter doesn't depend on the order of writes here.
    const displayed = this._computeDisplayedStatus(serviceId, status);
    const previousDisplayed = this.displayedStatus.get(serviceId);
    const displayChanged =
      !previousDisplayed || previousDisplayed.status !== displayed.status;

    this.displayedStatus.set(serviceId, displayed);

    if (displayChanged) {
      // Emit with the displayed status so the dashboard renders the same
      // state the hysteresis just decided. The raw probe result is still
      // in `history` and `currentStatus` for anyone who wants it.
      this.emit('status-check', displayed);
    }

    // Save history periodically
    if (Math.random() < 0.05) { // 5% chance (every ~20 checks)
      this.saveHistory();
    }
  }

  /**
   * Check for incidents (downtime, slow response, etc.)
   */
  checkForIncidents(serviceId, status, config, previous = this.currentStatus.get(serviceId)) {
    
    // Check for status change (up -> down or down -> up)
    if (previous && previous.status !== status.status) {
      if (status.status === 'down') {
        this.createIncident(serviceId, 'outage', 'Service is down', status);
      } else if (status.status === 'up') {
        this.resolveIncident(serviceId, 'outage', status);
      }
    }

    // Check for slow response time
    const slowThreshold = config.slowResponseThreshold || 5000; // 5 seconds
    if (status.responseTime > slowThreshold) {
      this.createIncident(serviceId, 'slow-response', 
        `Response time ${status.responseTime}ms exceeds threshold ${slowThreshold}ms`, 
        status);
    }

    // Check SLA violations
    const sla = config.sla;
    if (sla) {
      const uptime = this.calculateUptime(serviceId, sla.period || 24);
      if (uptime < sla.target) {
        this.createIncident(serviceId, 'sla-violation',
          `Uptime ${uptime.toFixed(2)}% below SLA target ${sla.target}%`,
          status);
      }
    }
  }

  /**
   * Create a new incident
   */
  createIncident(serviceId, type, message, status) {
    // Check if similar incident already exists
    const existing = this.incidents.find(i => 
      i.serviceId === serviceId && 
      i.type === type && 
      i.status === 'open'
    );

    if (existing) {
      // Update existing incident
      existing.lastOccurrence = status.timestamp;
      existing.occurrences++;
      return;
    }

    // Create new incident
    const incident = {
      id: `incident-${crypto.randomUUID()}`,
      serviceId,
      type,
      message,
      status: 'open',
      severity: this.calculateSeverity(type),
      createdAt: status.timestamp,
      lastOccurrence: status.timestamp,
      occurrences: 1,
      details: status
    };

    this.incidents.push(incident);
    this.emit('incident-created', incident);
    
    this.emit('log', 'info', `Incident created: ${incident.id} - ${message}`);
  }

  /**
   * Resolve an incident
   */
  resolveIncident(serviceId, type, status) {
    const incident = this.incidents.find(i => 
      i.serviceId === serviceId && 
      i.type === type && 
      i.status === 'open'
    );

    if (incident) {
      incident.status = 'resolved';
      incident.resolvedAt = status.timestamp;
      incident.duration = new Date(incident.resolvedAt) - new Date(incident.createdAt);
      
      this.emit('incident-resolved', incident);
      this.emit('log', 'info', `Incident resolved: ${incident.id}`);
    }
  }

  /**
   * Calculate incident severity
   */
  calculateSeverity(type) {
    switch (type) {
      case 'outage':
        return 'critical';
      case 'sla-violation':
        return 'high';
      case 'slow-response':
        return 'medium';
      default:
        return 'low';
    }
  }

  /**
   * Calculate uptime percentage for a service
   */
  calculateUptime(serviceId, hours = 24) {
    const history = this.getServiceHistory(serviceId, hours);
    if (history.length === 0) return 100;

    const upChecks = history.filter(h => h.status === 'up').length;
    return (upChecks / history.length) * 100;
  }

  /**
   * Calculate average response time
   */
  calculateAverageResponseTime(serviceId, hours = 24) {
    const history = this.getServiceHistory(serviceId, hours);
    if (history.length === 0) return 0;

    const total = history.reduce((sum, h) => sum + (h.responseTime || 0), 0);
    return total / history.length;
  }

  /**
   * Get service history for specified time period
   */
  getServiceHistory(serviceId, hours = 24) {
    const cutoffTime = Date.now() - (hours * 60 * 60 * 1000);
    const history = this.history[serviceId] || [];
    
    return history.filter(h => 
      new Date(h.timestamp).getTime() > cutoffTime
    );
  }

  /**
   * Get current status for all services.
   *
   * DC-086: returns the DISPLAYED status (post-hysteresis), not the latest
   * raw probe. A page reload should show the same badge state the live
   * SSE stream is currently showing — otherwise an operator who reloads
   * the page after a single blip sees red even though the hysteresis kept
   * the badge green for them.
   */
  getCurrentStatus() {
    const result = {};

    for (const [serviceId, rawStatus] of this.currentStatus.entries()) {
      const config = this.config.services[serviceId];
      const uptime24h = this.calculateUptime(serviceId, 24);
      const uptime7d = this.calculateUptime(serviceId, 168);
      const avgResponseTime = this.calculateAverageResponseTime(serviceId, 24);

      // Prefer the displayed status if we've already computed one; fall back
      // to the raw probe on the very first call (before recordStatus has run).
      const displayed = this.displayedStatus.get(serviceId) || rawStatus;

      result[serviceId] = {
        ...displayed,
        name: config?.name || serviceId,
        uptime: {
          '24h': uptime24h,
          '7d': uptime7d
        },
        avgResponseTime,
        sla: config?.sla
      };
    }

    return result;
  }

  /**
   * Get service statistics
   */
  getServiceStats(serviceId, hours = 24) {
    const history = this.getServiceHistory(serviceId, hours);
    if (history.length === 0) return null;

    const upChecks = history.filter(h => h.status === 'up').length;
    const downChecks = history.length - upChecks;
    const responseTimes = history.map(h => h.responseTime || 0);
    
    return {
      serviceId,
      period: `${hours}h`,
      totalChecks: history.length,
      upChecks,
      downChecks,
      uptime: (upChecks / history.length) * 100,
      responseTime: {
        avg: responseTimes.reduce((a, b) => a + b, 0) / responseTimes.length,
        min: Math.min(...responseTimes),
        max: Math.max(...responseTimes),
        p95: this.calculatePercentile(responseTimes, 95),
        p99: this.calculatePercentile(responseTimes, 99)
      }
    };
  }

  /**
   * Calculate percentile
   */
  calculatePercentile(values, percentile) {
    const sorted = values.slice().sort((a, b) => a - b);
    const index = Math.ceil((percentile / 100) * sorted.length) - 1;
    return sorted[index] || 0;
  }

  /**
   * Get open incidents
   */
  getOpenIncidents() {
    return this.incidents.filter(i => i.status === 'open');
  }

  /**
   * Get incident history
   */
  getIncidentHistory(limit = 50) {
    return this.incidents.slice(-limit).reverse();
  }

  /**
   * Configure health check for a service
   */
  configureService(serviceId, config) {
    if (!this.config.services) {
      this.config.services = {};
    }

    this.serviceGenerations.set(serviceId, (this.serviceGenerations.get(serviceId) || 0) + 1);
    this.config.services[serviceId] = {
      enabled: config.enabled !== false,
      name: config.name || serviceId,
      url: config.url,
      method: config.method || 'GET',
      timeout: config.timeout || 20000,
      expectedStatusCodes: config.expectedStatusCodes || [200],
      expectedBodyPattern: config.expectedBodyPattern,
      expectedBodyContains: config.expectedBodyContains,
      slowResponseThreshold: config.slowResponseThreshold || 5000,
      sla: config.sla,
      headers: config.headers || {},
      body: config.body
    };

    this.saveConfig();
  }

  /**
   * Remove service configuration
   */
  removeService(serviceId) {
    this.serviceGenerations.set(serviceId, (this.serviceGenerations.get(serviceId) || 0) + 1);
    if (this.config.services) {
      delete this.config.services[serviceId];
      this.saveConfig();
    }

    this.currentStatus.delete(serviceId);
    this.displayedStatus.delete(serviceId);
    this.consecutiveSinceChange.delete(serviceId);
    this.consecutiveFailures.delete(serviceId);
    const timer = this.serviceTimers.get(serviceId);
    if (timer) clearTimeout(timer);
    this.serviceTimers.delete(serviceId);
    delete this.history[serviceId];
  }

  /**
   * Cleanup old history
   */
  cleanupHistory() {
    const cutoffTime = Date.now() - (HISTORY_RETENTION_DAYS * 24 * 60 * 60 * 1000);
    
    for (const serviceId in this.history) {
      this.history[serviceId] = this.history[serviceId].filter(h =>
        new Date(h.timestamp).getTime() > cutoffTime
      );
      // Also cap total entries per service
      if (this.history[serviceId].length > MAX_ENTRIES_PER_SERVICE) {
        this.history[serviceId] = this.history[serviceId].slice(-MAX_ENTRIES_PER_SERVICE);
      }
    }
  }

  /**
   * Load configuration
   */
  loadConfig() {
    try {
      const file = fs.existsSync(HEALTH_CONFIG_FILE) ? HEALTH_CONFIG_FILE
        : (HEALTH_CONFIG_FILE !== LEGACY_HEALTH_CONFIG_FILE && fs.existsSync(LEGACY_HEALTH_CONFIG_FILE) ? LEGACY_HEALTH_CONFIG_FILE : null);
      if (file) {
        return JSON.parse(fs.readFileSync(file, 'utf8'));
      }
    } catch (error) {
      this.emit('log', 'error', `Error loading config: ${error.message}`);
    }
    return { services: {} };
  }

  /**
   * Save configuration
   */
  saveConfig() {
    try {
      fs.writeFileSync(HEALTH_CONFIG_FILE, JSON.stringify(this.config, null, 2));
    } catch (error) {
      this.emit('log', 'error', `Error saving config: ${error.message}`);
    }
  }

  /**
   * Load history
   */
  loadHistory() {
    try {
      const file = fs.existsSync(HEALTH_HISTORY_FILE) ? HEALTH_HISTORY_FILE
        : (HEALTH_HISTORY_FILE !== LEGACY_HEALTH_HISTORY_FILE && fs.existsSync(LEGACY_HEALTH_HISTORY_FILE) ? LEGACY_HEALTH_HISTORY_FILE : null);
      if (file) {
        return JSON.parse(fs.readFileSync(file, 'utf8'));
      }
    } catch (error) {
      this.emit('log', 'error', `Error loading history: ${error.message}`);
    }
    return {};
  }

  /**
   * Save history
   */
  saveHistory() {
    try {
      fs.writeFileSync(HEALTH_HISTORY_FILE, JSON.stringify(this.history)); // Compact JSON (no pretty-print) to reduce file size
    } catch (error) {
      this.emit('log', 'error', `Error saving history: ${error.message}`);
    }
  }
}

// Export singleton instance
module.exports = new HealthChecker();