import { Hono } from "hono";

/**
 * The longest a readiness probe waits on the database before answering `503` (ADR 0044). Short
 * enough for a Compose health check or an external monitor to treat a stall as "not ready" rather
 * than as a hung probe, and well inside the shared pool's own 10s connection and operation
 * timeouts, which bound the underlying query independently.
 */
export const READINESS_DATABASE_TIMEOUT_MS = 2_000;

export const HEALTH_LIVE_PATH = "/api/v1/health/live";
export const HEALTH_READY_PATH = "/api/v1/health/ready";

export interface ApiHealthOptions {
  /**
   * One trivial database round trip. Resolves when the database answered; rejects on any failure.
   * The rejection is never read — not its message, code, or cause — so nothing a driver says can
   * reach a response. The route bounds the wait and never overlaps two calls; this function only
   * has to perform the query.
   */
  readonly checkDatabase: () => Promise<void>;
  /** Whether module startup has completed. `false` again from the moment shutdown begins, so a
   * draining instance stops reporting ready while it finishes in-flight requests. */
  readonly isModuleStartupComplete: () => boolean;
}

const LIVE_BODY = Object.freeze({ status: "live" });
const READY_BODY = Object.freeze({ status: "ready" });
const NOT_READY_BODY = Object.freeze({ code: "NOT_READY" });

/**
 * Wraps `checkDatabase` so at most one call is ever unsettled, and no probe waits longer than
 * `timeoutMs`.
 *
 * Every probe arriving while a check is unsettled shares that check's outcome, including after its
 * deadline has passed: the deadline resolves the shared outcome to `false`, so those later probes
 * answer immediately. A new check starts only once the previous call has actually settled. A timed-
 * out query still occupies a pool connection until the database or the pool's own timeouts end it,
 * so starting a fresh query on every probe would let a stalled database drain the pool one health
 * check at a time.
 */
function createBoundedDatabaseCheck(
  checkDatabase: () => Promise<void>,
  timeoutMs: number,
): () => Promise<boolean> {
  let inFlight: Promise<boolean> | undefined;

  return () => {
    if (inFlight !== undefined) {
      return inFlight;
    }
    let timer: ReturnType<typeof setTimeout> | undefined;
    // The executor runs `checkDatabase` synchronously, so a synchronous throw is a rejection like
    // any other rather than an exception escaping the route.
    const settled = new Promise<void>((resolve) => resolve(checkDatabase())).then(
      () => true,
      () => false,
    );
    const outcome = Promise.race([
      settled,
      new Promise<false>((resolve) => {
        timer = setTimeout(() => resolve(false), timeoutMs);
      }),
    ]);
    inFlight = outcome;
    void settled.then(() => {
      clearTimeout(timer);
      inFlight = undefined;
    });
    return outcome;
  };
}

/** The same fixed refusal `./module-realtime-routes.js` uses: Hono lets a GET handler answer HEAD,
 * and these routes accept GET alone. */
function methodNotAllowed(): Response {
  return new Response(null, {
    headers: { Allow: "GET", "Cache-Control": "no-store" },
    status: 405,
  });
}

/**
 * Unauthenticated liveness and readiness (ADR 0044), for Compose health checks and external
 * monitoring. Neither route resolves a principal or reads a cookie, and both return fixed bodies
 * that name no module, version, error, or connection detail; `pnpm doctor` remains the detailed
 * operator report. They live under `/api/v1/health/`, a sibling of the `/api/v1/modules/` prefix that
 * installed modules own entirely, so no module route can shadow them.
 */
export function createHealthRoutes(options: ApiHealthOptions): Hono {
  const checkDatabase = createBoundedDatabaseCheck(
    options.checkDatabase,
    READINESS_DATABASE_TIMEOUT_MS,
  );
  const routes = new Hono();

  routes.all(HEALTH_LIVE_PATH, (c) => {
    if (c.req.method !== "GET") {
      return methodNotAllowed();
    }
    c.header("Cache-Control", "no-store");
    return c.json(LIVE_BODY, 200);
  });

  routes.all(HEALTH_READY_PATH, async (c) => {
    if (c.req.method !== "GET") {
      return methodNotAllowed();
    }
    c.header("Cache-Control", "no-store");
    if (!options.isModuleStartupComplete()) {
      return c.json(NOT_READY_BODY, 503);
    }
    const databaseReady = await checkDatabase();
    // Checked again afterwards: shutdown may have begun while the query was in flight.
    return databaseReady && options.isModuleStartupComplete()
      ? c.json(READY_BODY, 200)
      : c.json(NOT_READY_BODY, 503);
  });

  return routes;
}