/**
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								 * refer:
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								 *   * @atimb "Real keep-alive HTTP agent": https://gist.github.com/2963672
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								 *   * https://github.com/joyent/node/blob/master/lib/http.js
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								 *   * https://github.com/joyent/node/blob/master/lib/https.js
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								 *   * https://github.com/joyent/node/blob/master/lib/_http_agent.js
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								 */
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								'use strict';
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								const OriginalAgent = require('./_http_agent').Agent;
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								const ms = require('humanize-ms');
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								class Agent extends OriginalAgent {
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								  constructor(options) {
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								    options = options || {};
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								    options.keepAlive = options.keepAlive !== false;
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								    // default is keep-alive and 15s free socket timeout
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								    if (options.freeSocketKeepAliveTimeout === undefined) {
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								      options.freeSocketKeepAliveTimeout = 15000;
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								    }
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								    // Legacy API: keepAliveTimeout should be rename to `freeSocketKeepAliveTimeout`
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								    if (options.keepAliveTimeout) {
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								      options.freeSocketKeepAliveTimeout = options.keepAliveTimeout;
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								    }
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								    options.freeSocketKeepAliveTimeout = ms(options.freeSocketKeepAliveTimeout);
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								    // Sets the socket to timeout after timeout milliseconds of inactivity on the socket.
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								    // By default is double free socket keepalive timeout.
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								    if (options.timeout === undefined) {
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								      options.timeout = options.freeSocketKeepAliveTimeout * 2;
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								      // make sure socket default inactivity timeout >= 30s
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								      if (options.timeout < 30000) {
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								        options.timeout = 30000;
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								      }
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								    }
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								    options.timeout = ms(options.timeout);
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								    super(options);
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								    this.createSocketCount = 0;
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								    this.createSocketCountLastCheck = 0;
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								    this.createSocketErrorCount = 0;
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								    this.createSocketErrorCountLastCheck = 0;
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								    this.closeSocketCount = 0;
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								    this.closeSocketCountLastCheck = 0;
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								    // socket error event count
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								    this.errorSocketCount = 0;
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								    this.errorSocketCountLastCheck = 0;
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								    this.requestCount = 0;
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								    this.requestCountLastCheck = 0;
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								    this.timeoutSocketCount = 0;
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								    this.timeoutSocketCountLastCheck = 0;
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								    this.on('free', s => {
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								      this.requestCount++;
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								      // last enter free queue timestamp
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								      s.lastFreeTime = Date.now();
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								    });
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								    this.on('timeout', () => {
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								      this.timeoutSocketCount++;
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								    });
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								    this.on('close', () => {
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								      this.closeSocketCount++;
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								    });
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								    this.on('error', () => {
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								      this.errorSocketCount++;
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								    });
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								  }
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								  createSocket(req, options, cb) {
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								    super.createSocket(req, options, (err, socket) => {
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								      if (err) {
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								        this.createSocketErrorCount++;
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								        return cb(err);
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								      }
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								      if (this.keepAlive) {
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								        // Disable Nagle's algorithm: http://blog.caustik.com/2012/04/08/scaling-node-js-to-100k-concurrent-connections/
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								        // https://fengmk2.com/benchmark/nagle-algorithm-delayed-ack-mock.html
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								        socket.setNoDelay(true);
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								      }
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								      this.createSocketCount++;
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								      cb(null, socket);
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								    });
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								  }
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								  get statusChanged() {
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								    const changed = this.createSocketCount !== this.createSocketCountLastCheck ||
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								      this.createSocketErrorCount !== this.createSocketErrorCountLastCheck ||
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								      this.closeSocketCount !== this.closeSocketCountLastCheck ||
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								      this.errorSocketCount !== this.errorSocketCountLastCheck ||
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								      this.timeoutSocketCount !== this.timeoutSocketCountLastCheck ||
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								      this.requestCount !== this.requestCountLastCheck;
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								    if (changed) {
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								      this.createSocketCountLastCheck = this.createSocketCount;
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								      this.createSocketErrorCountLastCheck = this.createSocketErrorCount;
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								      this.closeSocketCountLastCheck = this.closeSocketCount;
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								      this.errorSocketCountLastCheck = this.errorSocketCount;
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								      this.timeoutSocketCountLastCheck = this.timeoutSocketCount;
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								      this.requestCountLastCheck = this.requestCount;
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								    }
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								    return changed;
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								  }
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								  getCurrentStatus() {
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								    return {
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								      createSocketCount: this.createSocketCount,
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								      createSocketErrorCount: this.createSocketErrorCount,
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								      closeSocketCount: this.closeSocketCount,
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								      errorSocketCount: this.errorSocketCount,
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								      timeoutSocketCount: this.timeoutSocketCount,
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								      requestCount: this.requestCount,
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								      freeSockets: inspect(this.freeSockets),
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								      sockets: inspect(this.sockets),
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								      requests: inspect(this.requests),
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								    };
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								  }
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								}
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								module.exports = Agent;
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								function inspect(obj) {
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								  const res = {};
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								  for (const key in obj) {
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								    res[key] = obj[key].length;
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								  }
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								  return res;
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								}
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