import { appConfig } from '../../config/appConfig';

/**
 * Two independent limits, and the session dies at whichever comes first:
 * an idle window the server resets on every authenticated call, and an
 * absolute ceiling it never extends however active the user is.
 */
interface SessionWindow {
  idleTimeoutMs: number;
  /** Epoch ms of the server's absolute expiry, or null when unknown. */
  absoluteExpiryAt: number | null;
}

export type SessionExpiryReason = 'idle' | 'absolute';

/**
 * When we last exchanged an authenticated request with the server. This — not
 * anything the user did — is what the server's idle window is measured from,
 * so it is what expiry has to be judged on.
 */
let lastServerContactAt = Date.now();

/**
 * When the user last touched the screen. Tracked separately because most of
 * this app reads from fixtures: someone can tap around for five minutes without
 * a single authenticated call, and treating that as idle would sign them out
 * mid-use. Interaction cannot reset the server's timer by itself — only a call
 * can — so this drives a keep-alive rather than the expiry clock.
 */
let lastInteractionAt = Date.now();

let window: SessionWindow = {
  idleTimeoutMs: appConfig.security.sessionTimeoutMs,
  absoluteExpiryAt: null,
};

export const sessionManager = {
  /**
   * Adopt the limits the server reported at sign-in. `idleTimeoutSeconds` and
   * `expiresAt` come straight off the login response, so the app never has to
   * guess and never drifts from what the backend will enforce.
   */
  configure({
    idleTimeoutSeconds,
    expiresAt,
  }: {
    idleTimeoutSeconds?: number;
    expiresAt?: string;
  }) {
    const absolute = expiresAt ? Date.parse(expiresAt) : NaN;

    window = {
      idleTimeoutMs:
        typeof idleTimeoutSeconds === 'number' && idleTimeoutSeconds > 0
          ? idleTimeoutSeconds * 1000
          : appConfig.security.sessionTimeoutMs,
      absoluteExpiryAt: Number.isFinite(absolute) ? absolute : null,
    };

    lastServerContactAt = Date.now();
    lastInteractionAt = Date.now();
  },

  clear() {
    window = {
      idleTimeoutMs: appConfig.security.sessionTimeoutMs,
      absoluteExpiryAt: null,
    };
    lastServerContactAt = Date.now();
    lastInteractionAt = Date.now();
  },

  /** An authenticated call completed, so the server's idle window just reset. */
  touch() {
    lastServerContactAt = Date.now();
    lastInteractionAt = Date.now();
  },

  /**
   * The user touched the screen. Deliberately does not reset the expiry clock:
   * the server has not heard from us, so pretending otherwise would leave the
   * app believing it is signed in while the next real request 401s. It only
   * makes `shouldKeepAlive()` true, and the keep-alive call is what actually
   * moves the server's window.
   */
  noteInteraction() {
    lastInteractionAt = Date.now();
  },

  /**
   * Whether to spend a keep-alive call now.
   *
   * True only when the user has done something since we last spoke to the
   * server and the idle window is over halfway gone. The halfway threshold is
   * what keeps this to roughly one request per two and a half minutes instead
   * of one per tap.
   */
  shouldKeepAlive() {
    if (lastInteractionAt <= lastServerContactAt) {
      return false;
    }

    // Nothing to preserve if the session is already gone, and no point
    // extending the idle window when the absolute cap is what will bite.
    if (this.hasExpired() || this.getAbsoluteRemainingMs() <= window.idleTimeoutMs / 2) {
      return false;
    }

    return this.getIdleDurationMs() >= window.idleTimeoutMs / 2;
  },

  /**
   * Which limit will end the session first. The warning copy depends on it:
   * against the idle window carrying on using the app helps, and against the
   * absolute cap nothing does, so telling the user to "tap to stay signed in"
   * would be a lie.
   */
  nearestLimit(): SessionExpiryReason {
    return this.getAbsoluteRemainingMs() <= this.getIdleRemainingMs() ? 'absolute' : 'idle';
  },

  getIdleTimeoutMs() {
    return window.idleTimeoutMs;
  },

  getIdleDurationMs() {
    return Date.now() - lastServerContactAt;
  },

  getIdleRemainingMs() {
    return Math.max(0, window.idleTimeoutMs - this.getIdleDurationMs());
  },

  getAbsoluteRemainingMs() {
    if (window.absoluteExpiryAt === null) {
      return Number.POSITIVE_INFINITY;
    }

    return Math.max(0, window.absoluteExpiryAt - Date.now());
  },

  /** Milliseconds until the session dies, whichever limit bites first. */
  getRemainingMs() {
    return Math.min(this.getIdleRemainingMs(), this.getAbsoluteRemainingMs());
  },

  expiryReason(): SessionExpiryReason | null {
    if (this.getAbsoluteRemainingMs() <= 0) {
      return 'absolute';
    }

    if (this.getIdleRemainingMs() <= 0) {
      return 'idle';
    }

    return null;
  },

  hasExpired() {
    return this.expiryReason() !== null;
  },
};