var assert = require('assert');

// The Flow class
// ==============

// Flow is a [Duplex stream][1] subclass which implements HTTP/2 flow control. It is designed to be
// subclassed by [Connection](connection.html) and the `upstream` component of [Stream](stream.html).
// [1]: https://nodejs.org/api/stream.html#stream_class_stream_duplex

var Duplex  = require('stream').Duplex;

exports.Flow = Flow;

// Public API
// ----------

// * **Event: 'error' (type)**: signals an error
//
// * **setInitialWindow(size)**: the initial flow control window size can be changed *any time*
//   ([as described in the standard][1]) using this method
//
// [1]: https://tools.ietf.org/html/rfc7540#section-6.9.2

// API for child classes
// ---------------------

// * **new Flow([flowControlId])**: creating a new flow that will listen for WINDOW_UPDATES frames
//   with the given `flowControlId` (or every update frame if not given)
//
// * **_send()**: called when more frames should be pushed. The child class is expected to override
//   this (instead of the `_read` method of the Duplex class).
//
// * **_receive(frame, readyCallback)**: called when there's an incoming frame. The child class is
//   expected to override this (instead of the `_write` method of the Duplex class).
//
// * **push(frame): bool**: schedules `frame` for sending.
//
//   Returns `true` if it needs more frames in the output queue, `false` if the output queue is
//   full, and `null` if did not push the frame into the output queue (instead, it pushed it into
//   the flow control queue).
//
// * **read(limit): frame**: like the regular `read`, but the 'flow control size' (0 for non-DATA
//   frames, length of the payload for DATA frames) of the returned frame will be under `limit`.
//   Small exception: pass -1 as `limit` if the max. flow control size is 0. `read(0)` means the
//   same thing as [in the original API](https://nodejs.org/api/stream.html#stream_stream_read_0).
//
// * **getLastQueuedFrame(): frame**: returns the last frame in output buffers
//
// * **_log**: the Flow class uses the `_log` object of the parent

// Constructor
// -----------

// When a HTTP/2.0 connection is first established, new streams are created with an initial flow
// control window size of 65535 bytes.
var INITIAL_WINDOW_SIZE = 65535;

// `flowControlId` is needed if only specific WINDOW_UPDATEs should be watched.
function Flow(flowControlId) {
  Duplex.call(this, { objectMode: true });

  this._window = this._initialWindow = INITIAL_WINDOW_SIZE;
  this._flowControlId = flowControlId;
  this._queue = [];
  this._ended = false;
  this._received = 0;
}
Flow.prototype = Object.create(Duplex.prototype, { constructor: { value: Flow } });

// Incoming frames
// ---------------

// `_receive` is called when there's an incoming frame.
Flow.prototype._receive = function _receive(frame, callback) {
  throw new Error('The _receive(frame, callback) method has to be overridden by the child class!');
};

// `_receive` is called by `_write` which in turn is [called by Duplex][1] when someone `write()`s
// to the flow. It emits the 'receiving' event and notifies the window size tracking code if the
// incoming frame is a WINDOW_UPDATE.
// [1]: https://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback_1
Flow.prototype._write = function _write(frame, encoding, callback) {
  var sentToUs = (this._flowControlId === undefined) || (frame.stream === this._flowControlId);

  if (sentToUs && (frame.flags.END_STREAM || (frame.type === 'RST_STREAM'))) {
    this._ended = true;
  }

  if ((frame.type === 'DATA') && (frame.data.length > 0)) {
    this._receive(frame, function() {
      this._received += frame.data.length;
      if (!this._restoreWindowTimer) {
        this._restoreWindowTimer = setImmediate(this._restoreWindow.bind(this));
      }
      callback();
    }.bind(this));
  }

  else {
    this._receive(frame, callback);
  }

  if (sentToUs && (frame.type === 'WINDOW_UPDATE')) {
    this._updateWindow(frame);
  }
};

// `_restoreWindow` basically acknowledges the DATA frames received since it's last call. It sends
// a WINDOW_UPDATE that restores the flow control window of the remote end.
// TODO: push this directly into the output queue. No need to wait for DATA frames in the queue.
Flow.prototype._restoreWindow = function _restoreWindow() {
  delete this._restoreWindowTimer;
  if (!this._ended && (this._received > 0)) {
    this.push({
      type: 'WINDOW_UPDATE',
      flags: {},
      stream: this._flowControlId,
      window_size: this._received
    });
    this._received = 0;
  }
};

// Outgoing frames - sending procedure
// -----------------------------------

//                                         flow
//                +-------------------------------------------------+
//                |                                                 |
//                +--------+           +---------+                  |
//        read()  | output |  _read()  | flow    |  _send()         |
//     <----------|        |<----------| control |<-------------    |
//                | buffer |           | buffer  |                  |
//                +--------+           +---------+                  |
//                | input  |                                        |
//     ---------->|        |----------------------------------->    |
//       write()  | buffer |  _write()              _receive()      |
//                +--------+                                        |
//                |                                                 |
//                +-------------------------------------------------+

// `_send` is called when more frames should be pushed to the output buffer.
Flow.prototype._send = function _send() {
  throw new Error('The _send() method has to be overridden by the child class!');
};

// `_send` is called by `_read` which is in turn [called by Duplex][1] when it wants to have more
// items in the output queue.
// [1]: https://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback_1
Flow.prototype._read = function _read() {
  // * if the flow control queue is empty, then let the user push more frames
  if (this._queue.length === 0) {
    this._send();
  }

  // * if there are items in the flow control queue, then let's put them into the output queue (to
  //   the extent it is possible with respect to the window size and output queue feedback)
  else if (this._window > 0) {
    this._readableState.sync = true; // to avoid reentrant calls
    do {
      var moreNeeded = this._push(this._queue[0]);
      if (moreNeeded !== null) {
        this._queue.shift();
      }
    } while (moreNeeded && (this._queue.length > 0));
    this._readableState.sync = false;

    assert((!moreNeeded) ||                              // * output queue is full
           (this._queue.length === 0) ||                         // * flow control queue is empty
           (!this._window && (this._queue[0].type === 'DATA'))); // * waiting for window update
  }

  // * otherwise, come back when the flow control window is positive
  else {
    this.once('window_update', this._read);
  }
};

var MAX_PAYLOAD_SIZE = 4096; // Must not be greater than MAX_HTTP_PAYLOAD_SIZE which is 16383

// `read(limit)` is like the `read` of the Readable class, but it guarantess that the 'flow control
// size' (0 for non-DATA frames, length of the payload for DATA frames) of the returned frame will
// be under `limit`.
Flow.prototype.read = function read(limit) {
  if (limit === 0) {
    return Duplex.prototype.read.call(this, 0);
  } else if (limit === -1) {
    limit = 0;
  } else if ((limit === undefined) || (limit > MAX_PAYLOAD_SIZE)) {
    limit = MAX_PAYLOAD_SIZE;
  }

  // * Looking at the first frame in the queue without pulling it out if possible.
  var frame = this._readableState.buffer[0];
  if (!frame && !this._readableState.ended) {
    this._read();
    frame = this._readableState.buffer[0];
  }

  if (frame && (frame.type === 'DATA')) {
    // * If the frame is DATA, then there's two special cases:
    //   * if the limit is 0, we shouldn't return anything
    //   * if the size of the frame is larger than limit, then the frame should be split
    if (limit === 0) {
      return Duplex.prototype.read.call(this, 0);
    }

    else if (frame.data.length > limit) {
      this._log.trace({ frame: frame, size: frame.data.length, forwardable: limit },
        'Splitting out forwardable part of a DATA frame.');
      this.unshift({
        type: 'DATA',
        flags: {},
        stream: frame.stream,
        data: frame.data.slice(0, limit)
      });
      frame.data = frame.data.slice(limit);
    }
  }

  return Duplex.prototype.read.call(this);
};

// `_parentPush` pushes the given `frame` into the output queue
Flow.prototype._parentPush = function _parentPush(frame) {
  this._log.trace({ frame: frame }, 'Pushing frame into the output queue');

  if (frame && (frame.type === 'DATA') && (this._window !== Infinity)) {
    this._log.trace({ window: this._window, by: frame.data.length },
                    'Decreasing flow control window size.');
    this._window -= frame.data.length;
    assert(this._window >= 0);
  }

  return Duplex.prototype.push.call(this, frame);
};

// `_push(frame)` pushes `frame` into the output queue and decreases the flow control window size.
// It is capable of splitting DATA frames into smaller parts, if the window size is not enough to
// push the whole frame. The return value is similar to `push` except that it returns `null` if it
// did not push the whole frame to the output queue (but maybe it did push part of the frame).
Flow.prototype._push = function _push(frame) {
  var data = frame && (frame.type === 'DATA') && frame.data;
  var maxFrameLength = (this._window < 16384) ? this._window : 16384;

  if (!data || (data.length <= maxFrameLength)) {
    return this._parentPush(frame);
  }

  else if (this._window <= 0) {
    return null;
  }

  else {
    this._log.trace({ frame: frame, size: frame.data.length, forwardable: this._window },
                    'Splitting out forwardable part of a DATA frame.');
    frame.data = data.slice(maxFrameLength);
    this._parentPush({
      type: 'DATA',
      flags: {},
      stream: frame.stream,
      data: data.slice(0, maxFrameLength)
    });
    return null;
  }
};

// Push `frame` into the flow control queue, or if it's empty, then directly into the output queue
Flow.prototype.push = function push(frame) {
  if (frame === null) {
    this._log.debug('Enqueueing outgoing End Of Stream');
  } else {
    this._log.debug({ frame: frame }, 'Enqueueing outgoing frame');
  }

  var moreNeeded = null;
  if (this._queue.length === 0) {
    moreNeeded = this._push(frame);
  }

  if (moreNeeded === null) {
    this._queue.push(frame);
  }

  return moreNeeded;
};

// `getLastQueuedFrame` returns the last frame in output buffers. This is primarily used by the
// [Stream](stream.html) class to mark the last frame with END_STREAM flag.
Flow.prototype.getLastQueuedFrame = function getLastQueuedFrame() {
  var readableQueue = this._readableState.buffer;
  return this._queue[this._queue.length - 1] || readableQueue[readableQueue.length - 1];
};

// Outgoing frames - managing the window size
// ------------------------------------------

// Flow control window size is manipulated using the `_increaseWindow` method.
//
// * Invoking it with `Infinite` means turning off flow control. Flow control cannot be enabled
//   again once disabled. Any attempt to re-enable flow control MUST be rejected with a
//   FLOW_CONTROL_ERROR error code.
// * A sender MUST NOT allow a flow control window to exceed 2^31 - 1 bytes. The action taken
//   depends on it being a stream or the connection itself.

var WINDOW_SIZE_LIMIT = Math.pow(2, 31) - 1;

Flow.prototype._increaseWindow = function _increaseWindow(size) {
  if ((this._window === Infinity) && (size !== Infinity)) {
    this._log.error('Trying to increase flow control window after flow control was turned off.');
    this.emit('error', 'FLOW_CONTROL_ERROR');
  } else {
    this._log.trace({ window: this._window, by: size }, 'Increasing flow control window size.');
    this._window += size;
    if ((this._window !== Infinity) && (this._window > WINDOW_SIZE_LIMIT)) {
      this._log.error('Flow control window grew too large.');
      this.emit('error', 'FLOW_CONTROL_ERROR');
    } else {
      if (size != 0) {
        this.emit('window_update');
      }
    }
  }
};

// The `_updateWindow` method gets called every time there's an incoming WINDOW_UPDATE frame. It
// modifies the flow control window:
//
// * Flow control can be disabled for an individual stream by sending a WINDOW_UPDATE with the
//   END_FLOW_CONTROL flag set. The payload of a WINDOW_UPDATE frame that has the END_FLOW_CONTROL
//   flag set is ignored.
// * A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount
//   specified in the frame.
Flow.prototype._updateWindow = function _updateWindow(frame) {
  this._increaseWindow(frame.flags.END_FLOW_CONTROL ? Infinity : frame.window_size);
};

// A SETTINGS frame can alter the initial flow control window size for all current streams. When the
// value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream by
// calling the `setInitialWindow` method. The window size has to be modified by the difference
// between the new value and the old value.
Flow.prototype.setInitialWindow = function setInitialWindow(initialWindow) {
  this._increaseWindow(initialWindow - this._initialWindow);
  this._initialWindow = initialWindow;
};
