# all files / lib/ _stream_transform.js

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84.21% Branches 32/38
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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';

    module.exports = Transform;

    var Duplex = require('./_stream_duplex');

    /*<replacement>*/
    var util = require('core-util-is');
    util.inherits = require('inherits');
    /*</replacement>*/

    util.inherits(Transform, Duplex);

    function TransformState(stream) {
      this.afterTransform = function (er, data) {
        return afterTransform(stream, er, data);
      };

      this.needTransform = false;
      this.transforming = false;
      this.writecb = null;
      this.writechunk = null;
      this.writeencoding = null;
    }

    function afterTransform(stream, er, data) {
      var ts = stream._transformState;
      ts.transforming = false;

      var cb = ts.writecb;

      Iif (!cb) return stream.emit('error', new Error('no writecb in Transform class'));

      ts.writechunk = null;
      ts.writecb = null;

      if (data !== null && data !== undefined) stream.push(data);

      cb(er);

      var rs = stream._readableState;
      rs.reading = false;
      if (rs.needReadable || rs.length < rs.highWaterMark) {
        stream._read(rs.highWaterMark);
      }
    }

    function Transform(options) {
      Iif (!(this instanceof Transform)) return new Transform(options);

      Duplex.call(this, options);

      this._transformState = new TransformState(this);

      // when the writable side finishes, then flush out anything remaining.
      var stream = this;

      // start out asking for a readable event once data is transformed.
      this._readableState.needReadable = true;

      // 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;

      if (options) {
        if (typeof options.transform === 'function') this._transform = options.transform;

        if (typeof options.flush === 'function') this._flush = options.flush;
      }

      this.once('prefinish', function () {
        if (typeof this._flush === 'function') this._flush(function (er) {
          done(stream, er);
        });else done(stream);
      });
    }

    Transform.prototype.push = function (chunk, encoding) {
      this._transformState.needTransform = false;
      return Duplex.prototype.push.call(this, chunk, encoding);
    };

    // This is the part where you do stuff!
    // override this function in implementation classes.
    // 'chunk' is an input chunk.
    //
    // Call `push(newChunk)` to pass along transformed output
    // to the readable side.  You may call 'push' zero or more times.
    //
    // Call `cb(err)` when you are done with this chunk.  If you pass
    // an error, then that'll put the hurt on the whole operation.  If you
    // never call cb(), then you'll never get another chunk.
    Transform.prototype._transform = function (chunk, encoding, cb) {
      throw new Error('Not implemented');
    };

    Transform.prototype._write = function (chunk, encoding, cb) {
      var ts = this._transformState;
      ts.writecb = cb;
      ts.writechunk = chunk;
      ts.writeencoding = encoding;
      Eif (!ts.transforming) {
        var rs = this._readableState;
        if (ts.needTransform || rs.needReadable || rs.length < rs.highWaterMark) this._read(rs.highWaterMark);
      }
    };

    // Doesn't matter what the args are here.
    // _transform does all the work.
    // That we got here means that the readable side wants more data.
    Transform.prototype._read = function (n) {
      var ts = this._transformState;

      if (ts.writechunk !== null && ts.writecb && !ts.transforming) {
        ts.transforming = true;
        this._transform(ts.writechunk, ts.writeencoding, ts.afterTransform);
      } else {
        // mark that we need a transform, so that any data that comes in
        // will get processed, now that we've asked for it.
        ts.needTransform = true;
      }
    };

    function done(stream, er) {
      Iif (er) return stream.emit('error', er);

      // if there's nothing in the write buffer, then that means
      // that nothing more will ever be provided
      var ws = stream._writableState;
      var ts = stream._transformState;

      Iif (ws.length) throw new Error('Calling transform done when ws.length != 0');

      Iif (ts.transforming) throw new Error('Calling transform done when still transforming');

      return stream.push(null);
    }