pngjs/lib/chunkstream.js

"use strict";

let util = require("util");
let Stream = require("stream");

let ChunkStream = (module.exports = function () {
  Stream.call(this);

  this._buffers = [];
  this._buffered = 0;

  this._reads = [];
  this._paused = false;

  this._encoding = "utf8";
  this.writable = true;
});
util.inherits(ChunkStream, Stream);

ChunkStream.prototype.read = function (length, callback) {
  this._reads.push({
    length: Math.abs(length), // if length < 0 then at most this length
    allowLess: length < 0,
    func: callback,
  });

  process.nextTick(
    function () {
      this._process();

      // its paused and there is not enought data then ask for more
      if (this._paused && this._reads && this._reads.length > 0) {
        this._paused = false;

        this.emit("drain");
      }
    }.bind(this)
  );
};

ChunkStream.prototype.write = function (data, encoding) {
  if (!this.writable) {
    this.emit("error", new Error("Stream not writable"));
    return false;
  }

  let dataBuffer;
  if (Buffer.isBuffer(data)) {
    dataBuffer = data;
  } else {
    dataBuffer = Buffer.from(data, encoding || this._encoding);
  }

  this._buffers.push(dataBuffer);
  this._buffered += dataBuffer.length;

  this._process();

  // ok if there are no more read requests
  if (this._reads && this._reads.length === 0) {
    this._paused = true;
  }

  return this.writable && !this._paused;
};

ChunkStream.prototype.end = function (data, encoding) {
  if (data) {
    this.write(data, encoding);
  }

  this.writable = false;

  // already destroyed
  if (!this._buffers) {
    return;
  }

  // enqueue or handle end
  if (this._buffers.length === 0) {
    this._end();
  } else {
    this._buffers.push(null);
    this._process();
  }
};

ChunkStream.prototype.destroySoon = ChunkStream.prototype.end;

ChunkStream.prototype._end = function () {
  if (this._reads.length > 0) {
    this.emit("error", new Error("Unexpected end of input"));
  }

  this.destroy();
};

ChunkStream.prototype.destroy = function () {
  if (!this._buffers) {
    return;
  }

  this.writable = false;
  this._reads = null;
  this._buffers = null;

  this.emit("close");
};

ChunkStream.prototype._processReadAllowingLess = function (read) {
  // ok there is any data so that we can satisfy this request
  this._reads.shift(); // == read

  // first we need to peek into first buffer
  let smallerBuf = this._buffers[0];

  // ok there is more data than we need
  if (smallerBuf.length > read.length) {
    this._buffered -= read.length;
    this._buffers[0] = smallerBuf.slice(read.length);

    read.func.call(this, smallerBuf.slice(0, read.length));
  } else {
    // ok this is less than maximum length so use it all
    this._buffered -= smallerBuf.length;
    this._buffers.shift(); // == smallerBuf

    read.func.call(this, smallerBuf);
  }
};

ChunkStream.prototype._processRead = function (read) {
  this._reads.shift(); // == read

  let pos = 0;
  let count = 0;
  let data = Buffer.alloc(read.length);

  // create buffer for all data
  while (pos < read.length) {
    let buf = this._buffers[count++];
    let len = Math.min(buf.length, read.length - pos);

    buf.copy(data, pos, 0, len);
    pos += len;

    // last buffer wasn't used all so just slice it and leave
    if (len !== buf.length) {
      this._buffers[--count] = buf.slice(len);
    }
  }

  // remove all used buffers
  if (count > 0) {
    this._buffers.splice(0, count);
  }

  this._buffered -= read.length;

  read.func.call(this, data);
};

ChunkStream.prototype._process = function () {
  try {
    // as long as there is any data and read requests
    while (this._buffered > 0 && this._reads && this._reads.length > 0) {
      let read = this._reads[0];

      // read any data (but no more than length)
      if (read.allowLess) {
        this._processReadAllowingLess(read);
      } else if (this._buffered >= read.length) {
        // ok we can meet some expectations

        this._processRead(read);
      } else {
        // not enought data to satisfy first request in queue
        // so we need to wait for more
        break;
      }
    }

    if (this._buffers && !this.writable) {
      this._end();
    }
  } catch (ex) {
    this.emit("error", ex);
  }
};