text: const events = require('events');
  
  const utils = require('./utils');
  
  // =============================================================================
  // FlowControl - Used to slow down streaming to manageable speed
  // Implements a subset of Stream.Duplex: pipe() and write()
  class FlowControl extends events.EventEmitter {
    constructor(options) {
      super();
  
      this.options = options = options || {};
  
      // Buffer queue
      this.queue = [];
  
      // Consumer streams
      this.pipes = [];
  
      // Down-stream flow-control instances
      this.children = [];
  
      // Up-stream flow-control instances
      this.parent = options.parent;
  
      // Ensure we don't flush more than once at a time
      this.flushing = false;
  
      // determine timeout for flow control delays
      if (options.gc) {
        const {gc} = options;
        if (gc.getTimeout) {
          this.getTimeout = gc.getTimeout;
        } else {
          // heap size below which we don't bother delaying
          const threshold = gc.threshold !== undefined ? gc.threshold : 150000000;
          // convert from heapsize to ms timeout
          const divisor = gc.divisor !== undefined ? gc.divisor : 500000;
          this.getTimeout = function() {
            const memory = process.memoryUsage();
            const heapSize = memory.heapTotal;
            return heapSize < threshold ? 0 : Math.floor(heapSize / divisor);
          };
        }
      } else {
        this.getTimeout = null;
      }
    }
  
    get name() {
      return ['FlowControl', this.parent ? 'Child' : 'Root', this.corked ? 'corked' : 'open'].join(' ');
    }
  
    get corked() {
      // We remain corked while we have children and at least one has data to consume
      return this.children.length > 0 && this.children.some(child => child.queue && child.queue.length);
    }
  
    get stem() {
      // the decision to stem the incoming data depends on whether the children are corked
      // and how many buffers we have backed up
      return this.corked || !this.queue || this.queue.length > 2;
    }
  
    _write(dst, data, encoding) {
      // Write to a single destination and return a promise
  
      return new Promise((resolve, reject) => {
        dst.write(data, encoding, error => {
          if (error) {
            reject(error);
          } else {
            resolve();
          }
        });
      });
    }
  
    async _pipe(chunk) {
      // Write chunk to all pipes. A chunk with no data is the end
      const promises = [];
      this.pipes.forEach(pipe => {
        if (chunk.data) {
          if (pipe.sync) {
            pipe.stream.write(chunk.data, chunk.encoding);
          } else {
            promises.push(this._write(pipe.stream, chunk.data, chunk.encoding));
          }
        } else {
          pipe.stream.end();
        }
      });
      if (!promises.length) {
        promises.push(Promise.resolve());
      }
      await Promise.all(promises);
      try {
        chunk.callback();
      } catch (e) {
        // quietly ignore
      }
    }
  
    _animate() {
      let count = 0;
      const seq = ['|', '/', '-', '\\'];
      const cr = '\u001b[0G'; // was '\033[0G'
      return setInterval(() => {
        process.stdout.write(seq[count++ % 4] + cr);
      }, 100);
    }
  
    _delay() {
      // in certain situations it may be useful to delay processing (e.g. for GC)
      const timeout = this.getTimeout && this.getTimeout();
      if (timeout) {
        return new Promise(resolve => {
          const anime = this._animate();
          setTimeout(() => {
            clearInterval(anime);
            resolve();
          }, timeout);
        });
      }
      return Promise.resolve();
    }
  
    async _flush() {
      // If/while not corked and we have buffers to send, send them
      if (this.queue && !this.flushing && !this.corked) {
        if (this.queue.length) {
          this.flushing = true;
          await this._delay();
          await this._pipe(this.queue.shift());
          setImmediate(() => {
            this.flushing = false;
            this._flush();
          });
        }
  
        if (!this.stem) {
          // Signal up-stream that we're ready for more data
          this.emit('drain');
        }
      }
    }
  
    write(data, encoding, callback) {
      // Called by up-stream pipe
      if (encoding instanceof Function) {
        callback = encoding;
        encoding = 'utf8';
      }
      callback = callback || utils.nop;
  
      if (!this.queue) {
        throw new Error('Cannot write to stream after end');
      }
  
      // Always queue chunks and then flush
      this.queue.push({
        data,
        encoding,
        callback,
      });
      this._flush();
  
      // restrict further incoming data if we have backed up buffers or
      // the children are still busy
      const stemFlow = this.corked || this.queue.length > 3;
      return !stemFlow;
    }
  
    end() {
      // Signal from up-stream
      this.queue.push({
        callback: () => {
          this.queue = null;
          this.emit('finish');
        },
      });
      this._flush();
    }
  
    pipe(stream, options) {
      options = options || {};
  
      // some streams don't call callbacks
      const sync = options.sync || false;
  
      this.pipes.push({
        stream,
        sync,
      });
    }
  
    unpipe(stream) {
      this.pipes = this.pipes.filter(pipe => pipe.stream !== stream);
    }
  
    createChild() {
      // Create a down-stream flow-control
      const options = Object.assign({parent: this}, this.options);
      const child = new FlowControl(options);
      this.children.push(child);
  
      child.on('drain', () => {
        // a child is ready for more
        this._flush();
      });
  
      child.on('finish', () => {
        // One child has finished its stream. Remove it and continue
        this.children = this.children.filter(item => item !== child);
        this._flush();
      });
  
      return child;
    }
  }
  
  module.exports = FlowControl;
  