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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;
if (!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) {
if (!(this instanceof Transform)) return new Transform(options);
Duplex.call(this, options);
this._transformState = new TransformState(this);
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;
}
// When the writable side finishes, then flush out anything remaining.
this.once('prefinish', function () {
if (typeof this._flush === 'function') this._flush(function (er, data) {
done(stream, er, data);
});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('_transform() is not implemented');
};
Transform.prototype._write = function (chunk, encoding, cb) {
var ts = this._transformState;
ts.writecb = cb;
ts.writechunk = chunk;
ts.writeencoding = encoding;
if (!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, data) {
if (er) return stream.emit('error', er);
if (data !== null && data !== undefined) stream.push(data);
// 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;
if (ws.length) throw new Error('Calling transform done when ws.length != 0');
if (ts.transforming) throw new Error('Calling transform done when still transforming');
return stream.push(null);
}
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