class DataViewStream {
  _grow = false
  _byteOffset = 0
  _view = null
  _textDecoder = new TextDecoder()
  _textEncoder = new TextEncoder()
 
  get byteOffset() { return this._byteOffset }
  get byteLength() { return this._view.byteLength }
  get bytesLeft() { return this._view.byteLength - this._offset }
  get buffer() { return this._view.buffer }
 
  constructor(buffer=null, grow=false) {
    this._grow = grow
    this._view = new DataView(buffer || new ArrayBuffer(0))
 
    for (const name of Object.getOwnPropertyNames(DataView.prototype)) {
      const match = name.match(/^([gs]et)([^\d]+)(\d+)$/)
      if (!match) continue
 
      const variant = match[1]
      const type = match[2]
      const bits = match[3]
      const bytes = bits / 8
 
      this[name] = function(...args) {
        return this._view[name](this._byteOffset, ...args)
      }
 
      if (variant === 'get') {
        this[`read${type}${bits}`] = function(...args) {
          const res = this._view[name](this._byteOffset, ...args)
          this._byteOffset += bytes
          return res
        }
      } else {
        this[`write${type}${bits}`] = function(...args) {
          this.expectBytes(bits/8)
          
          const res = this._view[name](this._byteOffset, ...args)
          this._byteOffset += bytes
          return res
        }
      }
    }
  }
  
  setCanGrow(grow) {
  	this._grow = grow
  }
 
  seek(byteOffset) {
    this._byteOffset = byteOffset
  }
  
  appendBytes(num) {
    if (!this._grow) throw new Error('Cannot grow')
    const oldBuffer = new Uint8Array(this._view.buffer)
    const newBuffer = new Uint8Array(new ArrayBuffer(oldBuffer.length + num))
    newBuffer.set(oldBuffer)
    this._view = new DataView(newBuffer.buffer)
  }
  
  expectBytes(byteCount) {
    if (!this._grow) return
    
    const missingBytes = (this.bytesLeft - byteCount) * -1
    if (missingBytes > 0) {
      this.appendBytes(missingBytes)
    }
  }
 
  // Buffer
  getBuffer(len=-1) {
    if (len === -1) len = this._view.byteLength - this._byteOffset
    const byteOffset = this._view.byteOffset + this._byteOffset
    return this._view.buffer.slice(byteOffset, byteOffset + len)
  }
 
  readBuffer(len=-1) {
    const buffer = this.getBuffer(len)
    this._byteOffset += buffer.byteLength
    return buffer
  }
 
  setBuffer(buffer) {
    this.expectBytes(buffer.byteLength)
    const bytes = new Uint8Array(buffer)
    for (let i=0; i<bytes.length; i++) {
      this._view.setUint8(this._byteOffset + i, bytes[i])
    }
  }
 
  writeBuffer(buffer) {
    this.setBuffer(buffer)
    this._byteOffset += buffer.byteLength
  }
 
  // Prefixed String
  getPString(byteLen=4) {
    const byteOffset = this._byteOffset
    let len = -1
    if (byteLen === 1) len = this.readInt8()
    else if (byteLen === 2) len = this.readInt16()
    else if (byteLen === 4) len = this.readInt32()
    const string = len > 0 ? this.readString(len) : ''
    this.seek(byteOffset)
    return string
  }
 
  readPString(byteLen=4) {
    const string = this.getPString(byteLen)
    this._byteOffset += byteLen + string.length
    return string
  }
 
  setPString(string, byteLen=4) {
    const byteOffset = this._byteOffset
    this.writePString(string, byteLen)
    this.seek(byteOffset)
  }
 
  writePString(string, byteLen=4) {
    if (byteLen === 1) this.writeInt8(string.length || -1)
    else if (byteLen === 2) this.writeInt16(string.length || -1)
    else if (byteLen === 4) this.writeInt32(string.length || -1)
    if (string.length > 0) this.writeString(string)
  }
 
  // String
  getString(len) {
    return this._textDecoder.decode(this.getBuffer(len))
  }
 
  readString(len) {
    const string = this.getString(len)
    this._byteOffset += len
    return string
  }
 
  setString(string) {
    this.setBuffer(this._textEncoder.encode(string).buffer)
  }
 
  writeString(string) {
    this.writeBuffer(this._textEncoder.encode(string).buffer)
  }
}