/**
 * Vinpin warm-session daemon (worker-process singleton).
 *
 * Owns the LIFECYCLE of the persistent warm Vinpin seat, on top of the browser
 * mechanics in VinpinDriverService:
 *   - a scheduler that warms the seat at 08:00 and tears it down at 21:00
 *     Europe/Istanbul (and warms on worker start if already inside the window),
 *   - an idle keepalive that nudges the RDS session every ~75s so it never drops,
 *   - `decode(vin)` — the entry the vinpin-decode processor calls. Inside business
 *     hours it makes sure the seat is warm (warming on-demand once), then delegates
 *     to the driver, which self-routes to the HOT warm path when warm and the
 *     proven COLD per-decode path otherwise. Outside hours it never holds the seat
 *     — the driver's cold path handles the decode and is torn down after.
 *
 * ADDITIVE + fail-safe: every warm operation degrades to the cold path, and
 * `decode()` never throws (the driver returns null on any failure). Gated by
 * VINPIN_ENABLED (feature flag) and VINPIN_WARM_DAEMON (kill-switch: set to
 * "false" to force the legacy per-decode cold path with the daemon inert).
 */

import { Logger } from "@nestjs/common";
import { getVinpinDriver } from "./vinpin-driver.service";
import type { VinpinDecodeResult, VinpinDriverService } from "./vinpin-driver.service";
import { VINPIN_WARM, isVinpinBusinessHours } from "./vinpin.constants";

/** Whether the warm daemon is allowed to run (feature flag + kill-switch). */
export function isVinpinWarmDaemonEnabled(): boolean {
  return process.env.VINPIN_ENABLED === "true" && process.env.VINPIN_WARM_DAEMON !== "false";
}

export interface VinpinDaemonDeps {
  /** Driver singleton (injectable for tests). Defaults to the process-wide one. */
  driver?: VinpinDriverService;
  /** Business-hours predicate (injectable so tests can drive the clock). */
  isBusinessHours?: () => boolean;
  /** Warm-daemon enabled predicate (injectable for tests). */
  isEnabled?: () => boolean;
  /** Monotonic-ish clock (injectable so tests can drive the warm-up backoff). */
  now?: () => number;
}

export class VinpinDaemonService {
  private readonly logger = new Logger(VinpinDaemonService.name);
  private readonly driver: VinpinDriverService;
  private readonly isBusinessHours: () => boolean;
  private readonly isEnabled: () => boolean;
  private readonly now: () => number;

  private schedulerTimer: ReturnType<typeof setInterval> | null = null;
  private keepaliveTimer: ReturnType<typeof setInterval> | null = null;
  private reconciling = false;
  private started = false;

  // ─── Warm-up backoff ─────────────────────────────────────
  /** Wall-clock time until which a re-warm is suppressed after a failed warmUp.
   *  Both reconcile() and decode()'s warm-on-demand honour this so a failing seat
   *  isn't hammered every ~60s (each failed attempt would leave a fresh dirty
   *  window). A successful warm resets it. */
  private warmCooldownUntil = 0;
  /** Current backoff span (ms): 0 when healthy, else base…max, doubling per
   *  consecutive failure. */
  private warmBackoffMs = 0;

  constructor(deps: VinpinDaemonDeps = {}) {
    this.driver = deps.driver ?? getVinpinDriver();
    this.isBusinessHours = deps.isBusinessHours ?? (() => isVinpinBusinessHours());
    this.isEnabled = deps.isEnabled ?? isVinpinWarmDaemonEnabled;
    this.now = deps.now ?? (() => Date.now());
  }

  /** True while a failed warmUp's cooldown is still in effect. */
  private inWarmCooldown(): boolean {
    return this.now() < this.warmCooldownUntil;
  }

  /**
   * Record a warm-up outcome and update the backoff. Success clears the cooldown;
   * failure sets/extends it (base, then exponential up to max). Returns `ok` so
   * callers can chain.
   */
  private noteWarmResult(ok: boolean): boolean {
    if (ok) {
      this.warmBackoffMs = 0;
      this.warmCooldownUntil = 0;
    } else {
      this.warmBackoffMs =
        this.warmBackoffMs === 0
          ? VINPIN_WARM.warmBackoffBaseMs
          : Math.min(this.warmBackoffMs * 2, VINPIN_WARM.warmBackoffMaxMs);
      this.warmCooldownUntil = this.now() + this.warmBackoffMs;
      this.logger.warn(
        `warmUp failed — backing off ${Math.round(this.warmBackoffMs / 1000)}s before the next attempt`,
      );
    }
    return ok;
  }

  /** Start the scheduler + keepalive loops (idempotent). */
  start(): void {
    if (this.started) return;
    this.started = true;
    // Reconcile once now (warm immediately if the worker booted inside hours).
    void this.reconcile();
    this.schedulerTimer = setInterval(() => {
      void this.reconcile();
    }, VINPIN_WARM.schedulerIntervalMs);
    this.keepaliveTimer = setInterval(() => {
      void this.driver.keepalivePing();
    }, VINPIN_WARM.keepaliveIntervalMs);
    // Don't keep the event loop alive just for these timers.
    this.schedulerTimer.unref?.();
    this.keepaliveTimer.unref?.();
    this.logger.log(
      `Vinpin warm daemon started (enabled=${this.isEnabled()}, hours ${VINPIN_WARM.businessStartHour}:00–${VINPIN_WARM.businessEndHour}:00 Europe/Istanbul)`,
    );
  }

  /** Stop the loops and tear the warm seat down (worker shutdown). */
  async stop(): Promise<void> {
    if (this.schedulerTimer) clearInterval(this.schedulerTimer);
    if (this.keepaliveTimer) clearInterval(this.keepaliveTimer);
    this.schedulerTimer = null;
    this.keepaliveTimer = null;
    this.started = false;
    await this.driver.teardownWarm().catch(() => undefined);
  }

  /**
   * Reconcile the warm seat against the schedule: warm up when enabled + inside
   * hours + not already warm; tear down when warm but disabled or outside hours.
   * Guards against overlapping runs (a slow warmUp must not stack). Never throws.
   */
  async reconcile(): Promise<void> {
    if (this.reconciling) return;
    this.reconciling = true;
    try {
      const enabled = this.isEnabled();
      const inHours = this.isBusinessHours();
      if (enabled && inHours && !this.driver.isWarm()) {
        if (this.inWarmCooldown()) {
          this.logger.debug("scheduler: in warm-up backoff — skipping this cycle");
        } else {
          this.logger.log("scheduler: inside business hours — warming the seat");
          this.noteWarmResult(await this.driver.warmUp());
        }
      } else if (this.driver.isWarm() && (!enabled || !inHours)) {
        this.logger.log("scheduler: outside business hours / disabled — tearing the seat down");
        await this.driver.teardownWarm();
        // Intentional teardown → clear any stale warm-up backoff so the next window
        // (e.g. tomorrow 08:00) isn't blocked by a leftover cooldown.
        this.warmBackoffMs = 0;
        this.warmCooldownUntil = 0;
      }
    } catch (err) {
      this.logger.warn(`reconcile failed: ${(err as Error).message}`);
    } finally {
      this.reconciling = false;
    }
  }

  /**
   * Decode entry the processor calls. Inside hours, ensure the seat is warm (warm
   * on-demand once), then delegate to the driver — which uses the hot warm path
   * when warm, else the cold per-decode path. Off-hours, delegate straight to the
   * driver's cold path (no seat held). Never throws — returns null on any failure.
   */
  async decode(vin: string): Promise<VinpinDecodeResult | null> {
    try {
      if (
        this.isEnabled() &&
        this.isBusinessHours() &&
        !this.driver.isWarm() &&
        !this.inWarmCooldown()
      ) {
        // Warm-on-demand: a decode arrived inside hours before the scheduler warmed
        // (e.g. right after 08:00, or after a drop). Best-effort — if it fails the
        // driver silently runs the cold path for this decode, and the backoff spaces
        // out the next warm attempt (respected by both this check and reconcile()).
        this.noteWarmResult(await this.driver.warmUp().catch(() => false));
      }
      return await this.driver.decode(vin);
    } catch (err) {
      // Defensive: driver.decode never throws, but guarantee the processor a null.
      this.logger.warn(`decode(${vin}) unexpected error: ${(err as Error).message}`);
      return null;
    }
  }
}

// ─── Worker singleton ────────────────────────────────────────
let daemonSingleton: VinpinDaemonService | null = null;
export function getVinpinDaemon(): VinpinDaemonService {
  if (!daemonSingleton) daemonSingleton = new VinpinDaemonService();
  return daemonSingleton;
}