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// Copyright Joyent, Inc. and other Node contributors.
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to permit
// persons to whom the Software is furnished to do so, subject to the
// following conditions:
//
// The above copyright notice and this permission notice shall be included
// in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
// USE OR OTHER DEALINGS IN THE SOFTWARE.
// 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,
} = primordials;
module.exports = Transform;
const {
ERR_METHOD_NOT_IMPLEMENTED,
} = require('internal/errors').codes;
const Duplex = require('internal/streams/duplex');
const { getHighWaterMark } = require('internal/streams/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,
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 (rState.ended) {
// If user has called this.push(null) we have to
// delay the callback to properly progate the new
// state.
process.nextTick(callback);
} else 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();
}
};
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