text: /* @flow */
  
  /**
   * Original RenderStream implmentation by Sasha Aickin (@aickin)
   * Licensed under the Apache License, Version 2.0
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Modified by Evan You (@yyx990803)
   */
  
  import stream from 'stream'
  import { createWriteFunction } from './write'
  
  export default class RenderStream extends stream.Readable {
    buffer: string;
    render: Function;
    expectedSize: number;
    stackDepth: number;
    write: Function;
    next: Function;
    end: Function;
    done: boolean;
  
    constructor (render: Function) {
      super()
      this.buffer = ''
      this.render = render
      this.expectedSize = 0
      this.stackDepth = 0
  
      this.write = createWriteFunction((text, next) => {
        const n = this.expectedSize
        this.buffer += text
        if (this.buffer.length >= n) {
          this.next = next
          this.pushBySize(n)
          return true // we will decide when to call next
        }
      }, err => {
        this.emit('error', err)
      })
  
      this.end = () => {
        // the rendering is finished; we should push out the last of the buffer.
        this.done = true
        this.push(this.buffer)
      }
    }
  
    pushBySize (n: number) {
      const bufferToPush = this.buffer.substring(0, n)
      this.buffer = this.buffer.substring(n)
      this.push(bufferToPush)
    }
  
    tryRender () {
      try {
        this.render(this.write, this.end)
      } catch (e) {
        this.emit('error', e)
      }
    }
  
    tryNext () {
      try {
        this.next()
      } catch (e) {
        this.emit('error', e)
      }
    }
  
    _read (n: number) {
      this.expectedSize = n
      // it's possible that the last chunk added bumped the buffer up to > 2 * n,
      // which means we will need to go through multiple read calls to drain it
      // down to < n.
      if (this.done) {
        this.push(null)
        return
      }
      if (this.buffer.length >= n) {
        this.pushBySize(n)
        return
      }
      if (!this.next) {
        // start the rendering chain.
        this.tryRender()
      } else {
        // continue with the rendering.
        this.tryNext()
      }
    }
  }
  