/*
 * Copyright 2019 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

import * as http2 from 'http2';
import { ChannelCredentials } from './channel-credentials';
import { Metadata } from './metadata';
import { Http2CallStream } from './call-stream';
import { ChannelOptions } from './channel-options';
import { PeerCertificate, checkServerIdentity } from 'tls';
import { ConnectivityState } from './channel';
import { BackoffTimeout } from './backoff-timeout';
import { getDefaultAuthority } from './resolver';
import * as logging from './logging';
import { LogVerbosity } from './constants';

const { version: clientVersion } = require('../../package.json');

const TRACER_NAME = 'subchannel';

function trace(text: string): void {
  logging.trace(LogVerbosity.DEBUG, TRACER_NAME, text);
}

const MIN_CONNECT_TIMEOUT_MS = 20000;
const INITIAL_BACKOFF_MS = 1000;
const BACKOFF_MULTIPLIER = 1.6;
const MAX_BACKOFF_MS = 120000;
const BACKOFF_JITTER = 0.2;

/* setInterval and setTimeout only accept signed 32 bit integers. JS doesn't
 * have a constant for the max signed 32 bit integer, so this is a simple way
 * to calculate it */
const KEEPALIVE_TIME_MS = ~(1 << 31);
const KEEPALIVE_TIMEOUT_MS = 20000;

export type ConnectivityStateListener = (
  subchannel: Subchannel,
  previousState: ConnectivityState,
  newState: ConnectivityState
) => void;

const {
  HTTP2_HEADER_AUTHORITY,
  HTTP2_HEADER_CONTENT_TYPE,
  HTTP2_HEADER_METHOD,
  HTTP2_HEADER_PATH,
  HTTP2_HEADER_TE,
  HTTP2_HEADER_USER_AGENT,
} = http2.constants;

/**
 * Get a number uniformly at random in the range [min, max)
 * @param min
 * @param max
 */
function uniformRandom(min: number, max: number) {
  return Math.random() * (max - min) + min;
}

export class Subchannel {
  /**
   * The subchannel's current connectivity state. Invariant: `session` === `null`
   * if and only if `connectivityState` is IDLE or TRANSIENT_FAILURE.
   */
  private connectivityState: ConnectivityState = ConnectivityState.IDLE;
  /**
   * The underlying http2 session used to make requests.
   */
  private session: http2.ClientHttp2Session | null = null;
  /**
   * Indicates that the subchannel should transition from TRANSIENT_FAILURE to
   * CONNECTING instead of IDLE when the backoff timeout ends.
   */
  private continueConnecting = false;
  /**
   * A list of listener functions that will be called whenever the connectivity
   * state changes. Will be modified by `addConnectivityStateListener` and
   * `removeConnectivityStateListener`
   */
  private stateListeners: ConnectivityStateListener[] = [];

  /**
   * A list of listener functions that will be called when the underlying
   * socket disconnects. Used for ending active calls with an UNAVAILABLE
   * status.
   */
  private disconnectListeners: (() => void)[] = [];

  private backoffTimeout: BackoffTimeout;

  /**
   * The complete user agent string constructed using channel args.
   */
  private userAgent: string;

  /**
   * The amount of time in between sending pings
   */
  private keepaliveTimeMs: number = KEEPALIVE_TIME_MS;
  /**
   * The amount of time to wait for an acknowledgement after sending a ping
   */
  private keepaliveTimeoutMs: number = KEEPALIVE_TIMEOUT_MS;
  /**
   * Timer reference for timeout that indicates when to send the next ping
   */
  private keepaliveIntervalId: NodeJS.Timer;
  /**
   * Timer reference tracking when the most recent ping will be considered lost
   */
  private keepaliveTimeoutId: NodeJS.Timer;

  /**
   * Tracks calls with references to this subchannel
   */
  private callRefcount = 0;
  /**
   * Tracks channels and subchannel pools with references to this subchannel
   */
  private refcount = 0;

  /**
   * A class representing a connection to a single backend.
   * @param channelTarget The target string for the channel as a whole
   * @param subchannelAddress The address for the backend that this subchannel
   *     will connect to
   * @param options The channel options, plus any specific subchannel options
   *     for this subchannel
   * @param credentials The channel credentials used to establish this
   *     connection
   */
  constructor(
    private channelTarget: string,
    private subchannelAddress: string,
    private options: ChannelOptions,
    private credentials: ChannelCredentials
  ) {
    // Build user-agent string.
    this.userAgent = [
      options['grpc.primary_user_agent'],
      `grpc-node-js/${clientVersion}`,
      options['grpc.secondary_user_agent'],
    ]
      .filter(e => e)
      .join(' '); // remove falsey values first

    if ('grpc.keepalive_time_ms' in options) {
      this.keepaliveTimeMs = options['grpc.keepalive_time_ms']!;
    }
    if ('grpc.keepalive_timeout_ms' in options) {
      this.keepaliveTimeoutMs = options['grpc.keepalive_timeout_ms']!;
    }
    this.keepaliveIntervalId = setTimeout(() => {}, 0);
    clearTimeout(this.keepaliveIntervalId);
    this.keepaliveTimeoutId = setTimeout(() => {}, 0);
    clearTimeout(this.keepaliveTimeoutId);
    this.backoffTimeout = new BackoffTimeout(() => {
      if (this.continueConnecting) {
        this.transitionToState(
          [ConnectivityState.TRANSIENT_FAILURE, ConnectivityState.CONNECTING],
          ConnectivityState.CONNECTING
        );
      } else {
        this.transitionToState(
          [ConnectivityState.TRANSIENT_FAILURE, ConnectivityState.CONNECTING],
          ConnectivityState.IDLE
        );
      }
    });
  }

  /**
   * Start a backoff timer with the current nextBackoff timeout
   */
  private startBackoff() {
    this.backoffTimeout.runOnce();
  }

  private stopBackoff() {
    this.backoffTimeout.stop();
    this.backoffTimeout.reset();
  }

  private sendPing() {
    this.keepaliveTimeoutId = setTimeout(() => {
      this.transitionToState([ConnectivityState.READY], ConnectivityState.IDLE);
    }, this.keepaliveTimeoutMs);
    this.session!.ping(
      (err: Error | null, duration: number, payload: Buffer) => {
        clearTimeout(this.keepaliveTimeoutId);
      }
    );
  }

  private startKeepalivePings() {
    this.keepaliveIntervalId = setInterval(() => {
      this.sendPing();
    }, this.keepaliveTimeMs);
    this.sendPing();
  }

  private stopKeepalivePings() {
    clearInterval(this.keepaliveIntervalId);
    clearTimeout(this.keepaliveTimeoutId);
  }

  private startConnectingInternal() {
    const connectionOptions: http2.SecureClientSessionOptions =
      this.credentials._getConnectionOptions() || {};
    let addressScheme = 'http://';
    if ('secureContext' in connectionOptions) {
      addressScheme = 'https://';
      // If provided, the value of grpc.ssl_target_name_override should be used
      // to override the target hostname when checking server identity.
      // This option is used for testing only.
      if (this.options['grpc.ssl_target_name_override']) {
        const sslTargetNameOverride = this.options[
          'grpc.ssl_target_name_override'
        ]!;
        connectionOptions.checkServerIdentity = (
          host: string,
          cert: PeerCertificate
        ): Error | undefined => {
          return checkServerIdentity(sslTargetNameOverride, cert);
        };
        connectionOptions.servername = sslTargetNameOverride;
      } else {
        connectionOptions.servername = getDefaultAuthority(this.channelTarget);
      }
    }
    this.session = http2.connect(
      addressScheme + this.subchannelAddress,
      connectionOptions
    );
    this.session.unref();
    this.session.once('connect', () => {
      this.transitionToState(
        [ConnectivityState.CONNECTING],
        ConnectivityState.READY
      );
    });
    this.session.once('close', () => {
      this.transitionToState(
        [ConnectivityState.CONNECTING, ConnectivityState.READY],
        ConnectivityState.TRANSIENT_FAILURE
      );
    });
    this.session.once('goaway', () => {
      this.transitionToState(
        [ConnectivityState.CONNECTING, ConnectivityState.READY],
        ConnectivityState.IDLE
      );
    });
    this.session.once('error', error => {
      /* Do nothing here. Any error should also trigger a close event, which is
       * where we want to handle that. */
    });
  }

  /**
   * Initiate a state transition from any element of oldStates to the new
   * state. If the current connectivityState is not in oldStates, do nothing.
   * @param oldStates The set of states to transition from
   * @param newState The state to transition to
   * @returns True if the state changed, false otherwise
   */
  private transitionToState(
    oldStates: ConnectivityState[],
    newState: ConnectivityState
  ): boolean {
    if (oldStates.indexOf(this.connectivityState) === -1) {
      return false;
    }
    trace(this.subchannelAddress + ' ' + ConnectivityState[this.connectivityState] + ' -> ' + ConnectivityState[newState]);
    const previousState = this.connectivityState;
    this.connectivityState = newState;
    switch (newState) {
      case ConnectivityState.READY:
        this.stopBackoff();
        this.session!.socket.once('close', () => {
          for (const listener of this.disconnectListeners) {
            listener();
          }
        });
        break;
      case ConnectivityState.CONNECTING:
        this.startBackoff();
        this.startConnectingInternal();
        this.continueConnecting = false;
        break;
      case ConnectivityState.TRANSIENT_FAILURE:
        this.session = null;
        this.stopKeepalivePings();
        break;
      case ConnectivityState.IDLE:
        /* Stopping the backoff timer here is probably redundant because we
         * should only transition to the IDLE state as a result of the timer
         * ending, but we still want to reset the backoff timeout. */
        this.stopBackoff();
        this.session = null;
        this.stopKeepalivePings();
        break;
      default:
        throw new Error(`Invalid state: unknown ConnectivityState ${newState}`);
    }
    /* We use a shallow copy of the stateListeners array in case a listener
     * is removed during this iteration */
    for (const listener of [...this.stateListeners]) {
      listener(this, previousState, newState);
    }
    return true;
  }

  /**
   * Check if the subchannel associated with zero calls and with zero channels.
   * If so, shut it down.
   */
  private checkBothRefcounts() {
    /* If no calls, channels, or subchannel pools have any more references to
     * this subchannel, we can be sure it will never be used again. */
    if (this.callRefcount === 0 && this.refcount === 0) {
      this.transitionToState(
        [
          ConnectivityState.CONNECTING,
          ConnectivityState.IDLE,
          ConnectivityState.READY,
        ],
        ConnectivityState.TRANSIENT_FAILURE
      );
    }
  }

  callRef() {
    if (this.callRefcount === 0) {
      if (this.session) {
        this.session.ref();
      }
      this.startKeepalivePings();
    }
    this.callRefcount += 1;
  }

  callUnref() {
    this.callRefcount -= 1;
    if (this.callRefcount === 0) {
      if (this.session) {
        this.session.unref();
      }
      this.stopKeepalivePings();
      this.checkBothRefcounts();
    }
  }

  ref() {
    this.refcount += 1;
  }

  unref() {
    this.refcount -= 1;
    this.checkBothRefcounts();
  }

  unrefIfOneRef(): boolean {
    if (this.refcount === 1) {
      this.unref();
      return true;
    }
    return false;
  }

  /**
   * Start a stream on the current session with the given `metadata` as headers
   * and then attach it to the `callStream`. Must only be called if the
   * subchannel's current connectivity state is READY.
   * @param metadata
   * @param callStream
   */
  startCallStream(metadata: Metadata, callStream: Http2CallStream) {
    const headers = metadata.toHttp2Headers();
    headers[HTTP2_HEADER_AUTHORITY] = callStream.getHost();
    headers[HTTP2_HEADER_USER_AGENT] = this.userAgent;
    headers[HTTP2_HEADER_CONTENT_TYPE] = 'application/grpc';
    headers[HTTP2_HEADER_METHOD] = 'POST';
    headers[HTTP2_HEADER_PATH] = callStream.getMethod();
    headers[HTTP2_HEADER_TE] = 'trailers';
    const http2Stream = this.session!.request(headers);
    callStream.attachHttp2Stream(http2Stream, this);
  }

  /**
   * If the subchannel is currently IDLE, start connecting and switch to the
   * CONNECTING state. If the subchannel is current in TRANSIENT_FAILURE,
   * the next time it would transition to IDLE, start connecting again instead.
   * Otherwise, do nothing.
   */
  startConnecting() {
    /* First, try to transition from IDLE to connecting. If that doesn't happen
     * because the state is not currently IDLE, check if it is
     * TRANSIENT_FAILURE, and if so indicate that it should go back to
     * connecting after the backoff timer ends. Otherwise do nothing */
    if (
      !this.transitionToState(
        [ConnectivityState.IDLE],
        ConnectivityState.CONNECTING
      )
    ) {
      if (this.connectivityState === ConnectivityState.TRANSIENT_FAILURE) {
        this.continueConnecting = true;
      }
    }
  }

  /**
   * Get the subchannel's current connectivity state.
   */
  getConnectivityState() {
    return this.connectivityState;
  }

  /**
   * Add a listener function to be called whenever the subchannel's
   * connectivity state changes.
   * @param listener
   */
  addConnectivityStateListener(listener: ConnectivityStateListener) {
    this.stateListeners.push(listener);
  }

  /**
   * Remove a listener previously added with `addConnectivityStateListener`
   * @param listener A reference to a function previously passed to
   *     `addConnectivityStateListener`
   */
  removeConnectivityStateListener(listener: ConnectivityStateListener) {
    const listenerIndex = this.stateListeners.indexOf(listener);
    if (listenerIndex > -1) {
      this.stateListeners.splice(listenerIndex, 1);
    }
  }

  addDisconnectListener(listener: () => void) {
    this.disconnectListeners.push(listener);
  }

  removeDisconnectListener(listener: () => void) {
    const listenerIndex = this.disconnectListeners.indexOf(listener);
    if (listenerIndex > -1) {
      this.disconnectListeners.splice(listenerIndex, 1);
    }
  }

  /**
   * Reset the backoff timeout, and immediately start connecting if in backoff.
   */
  resetBackoff() {
    this.backoffTimeout.reset();
    this.transitionToState(
      [ConnectivityState.TRANSIENT_FAILURE],
      ConnectivityState.CONNECTING
    );
  }

  getAddress(): string {
    return this.subchannelAddress;
  }
}