IOTConnect-Web/node_modules/readable-stream/lib/internal/streams/transform.js

// Copyright Joyent, Inc. and other Node contributors.
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// a transform stream is a readable/writable stream where you do
// something with the data.  Sometimes it's called a "filter",
// but that's not a great name for it, since that implies a thing where
// some bits pass through, and others are simply ignored.  (That would
// be a valid example of a transform, of course.)
//
// While the output is causally related to the input, it's not a
// necessarily symmetric or synchronous transformation.  For example,
// a zlib stream might take multiple plain-text writes(), and then
// emit a single compressed chunk some time in the future.
//
// Here's how this works:
//
// The Transform stream has all the aspects of the readable and writable
// stream classes.  When you write(chunk), that calls _write(chunk,cb)
// internally, and returns false if there's a lot of pending writes
// buffered up.  When you call read(), that calls _read(n) until
// there's enough pending readable data buffered up.
//
// In a transform stream, the written data is placed in a buffer.  When
// _read(n) is called, it transforms the queued up data, calling the
// buffered _write cb's as it consumes chunks.  If consuming a single
// written chunk would result in multiple output chunks, then the first
// outputted bit calls the readcb, and subsequent chunks just go into
// the read buffer, and will cause it to emit 'readable' if necessary.
//
// This way, back-pressure is actually determined by the reading side,
// since _read has to be called to start processing a new chunk.  However,
// a pathological inflate type of transform can cause excessive buffering
// here.  For example, imagine a stream where every byte of input is
// interpreted as an integer from 0-255, and then results in that many
// bytes of output.  Writing the 4 bytes {ff,ff,ff,ff} would result in
// 1kb of data being output.  In this case, you could write a very small
// amount of input, and end up with a very large amount of output.  In
// such a pathological inflating mechanism, there'd be no way to tell
// the system to stop doing the transform.  A single 4MB write could
// cause the system to run out of memory.
//
// However, even in such a pathological case, only a single written chunk
// would be consumed, and then the rest would wait (un-transformed) until
// the results of the previous transformed chunk were consumed.

'use strict'

const { ObjectSetPrototypeOf, Symbol } = require('../../ours/primordials')
module.exports = Transform
const { ERR_METHOD_NOT_IMPLEMENTED } = require('../../ours/errors').codes
const Duplex = require('./duplex')
const { getHighWaterMark } = require('./state')
ObjectSetPrototypeOf(Transform.prototype, Duplex.prototype)
ObjectSetPrototypeOf(Transform, Duplex)
const kCallback = Symbol('kCallback')
function Transform(options) {
  if (!(this instanceof Transform)) return new Transform(options)

  // TODO (ronag): This should preferably always be
  // applied but would be semver-major. Or even better;
  // make Transform a Readable with the Writable interface.
  const readableHighWaterMark = options ? getHighWaterMark(this, options, 'readableHighWaterMark', true) : null
  if (readableHighWaterMark === 0) {
    // A Duplex will buffer both on the writable and readable side while
    // a Transform just wants to buffer hwm number of elements. To avoid
    // buffering twice we disable buffering on the writable side.
    options = {
      ...options,
      highWaterMark: null,
      readableHighWaterMark,
      // TODO (ronag): 0 is not optimal since we have
      // a "bug" where we check needDrain before calling _write and not after.
      // Refs: https://github.com/nodejs/node/pull/32887
      // Refs: https://github.com/nodejs/node/pull/35941
      writableHighWaterMark: options.writableHighWaterMark || 0
    }
  }
  Duplex.call(this, options)

  // We have implemented the _read method, and done the other things
  // that Readable wants before the first _read call, so unset the
  // sync guard flag.
  this._readableState.sync = false
  this[kCallback] = null
  if (options) {
    if (typeof options.transform === 'function') this._transform = options.transform
    if (typeof options.flush === 'function') this._flush = options.flush
  }

  // When the writable side finishes, then flush out anything remaining.
  // Backwards compat. Some Transform streams incorrectly implement _final
  // instead of or in addition to _flush. By using 'prefinish' instead of
  // implementing _final we continue supporting this unfortunate use case.
  this.on('prefinish', prefinish)
}
function final(cb) {
  if (typeof this._flush === 'function' && !this.destroyed) {
    this._flush((er, data) => {
      if (er) {
        if (cb) {
          cb(er)
        } else {
          this.destroy(er)
        }
        return
      }
      if (data != null) {
        this.push(data)
      }
      this.push(null)
      if (cb) {
        cb()
      }
    })
  } else {
    this.push(null)
    if (cb) {
      cb()
    }
  }
}
function prefinish() {
  if (this._final !== final) {
    final.call(this)
  }
}
Transform.prototype._final = final
Transform.prototype._transform = function (chunk, encoding, callback) {
  throw new ERR_METHOD_NOT_IMPLEMENTED('_transform()')
}
Transform.prototype._write = function (chunk, encoding, callback) {
  const rState = this._readableState
  const wState = this._writableState
  const length = rState.length
  this._transform(chunk, encoding, (err, val) => {
    if (err) {
      callback(err)
      return
    }
    if (val != null) {
      this.push(val)
    }
    if (
      wState.ended ||
      // Backwards compat.
      length === rState.length ||
      // Backwards compat.
      rState.length < rState.highWaterMark
    ) {
      callback()
    } else {
      this[kCallback] = callback
    }
  })
}
Transform.prototype._read = function () {
  if (this[kCallback]) {
    const callback = this[kCallback]
    this[kCallback] = null
    callback()
  }
}