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string_decoder: make write after end to reset #16594

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string_decoder: make write after end to reset #16594

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string_decoder: make write after end to reset
thefourtheye Oct 29, 2017
dbf8fd2
sort the doc section
thefourtheye Oct 30, 2017
f34b251
make write hot path efficient
thefourtheye Nov 6, 2017
7c2286f
incorporate review comments
thefourtheye Dec 5, 2017
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11 changes: 11 additions & 0 deletions doc/api/string_decoder.md
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Expand Up @@ -66,6 +66,16 @@ substitution characters appropriate for the character encoding.
If the `buffer` argument is provided, one final call to `stringDecoder.write()`
is performed before returning the remaining input.

### stringDecoder.reset([encoding])
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I believe this is an implementation detail users don't have to know about.

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As it is, there is no way for the users to reset the string decoder, right? They have to create a new instance if needed. This will enable reusability.

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A new way to reset the state each time was not requested as far as I read the issue. So I also prefer not to expose the reset function. If I am correct the following should work.

const { StringDecoder } = require('string_decoder');
const decoder = new StringDecoder('utf8');

decoder.write(Buffer.from([0xE2, 0x82])); // => ''
decoder.end(); // => '�'
decoder.write(Buffer.of(0x61)); // => 'a'

<!-- YAML
added: REPLACEME
-->

* `encoding` {string} The character encoding the `StringDecoder` will use.
Defaults to the current `encoding` of the decoder.

Flushes all the internal data and the decoder will behave like a new object.

### stringDecoder.write(buffer)
<!-- YAML
added: v0.1.99
Expand All @@ -82,3 +92,4 @@ Returns a decoded string, ensuring that any incomplete multibyte characters at
the end of the `Buffer` are omitted from the returned string and stored in an
internal buffer for the next call to `stringDecoder.write()` or
`stringDecoder.end()`.

27 changes: 22 additions & 5 deletions lib/string_decoder.js
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Expand Up @@ -41,36 +41,52 @@ function normalizeEncoding(enc) {
// characters.
exports.StringDecoder = StringDecoder;
function StringDecoder(encoding) {
if (encoding === this.encoding && encoding !== undefined) {
this.lastNeed = 0;
this.lastTotal = 0;
this.lastChar = Buffer.allocUnsafe(this._nb || 0);
this._closed = false;
return;
}

this.encoding = normalizeEncoding(encoding);
var nb;
switch (this.encoding) {
case 'utf16le':
this.text = utf16Text;
this.end = utf16End;
nb = 4;
this._nb = 4;
break;
case 'utf8':
this.fillLast = utf8FillLast;
nb = 4;
this._nb = 4;
break;
case 'base64':
this.text = base64Text;
this.end = base64End;
nb = 3;
this._nb = 3;
break;
default:
this.write = simpleWrite;
this.end = simpleEnd;
this._nb = 0;
this._closed = false;
return;
}
this.lastNeed = 0;
this.lastTotal = 0;
this.lastChar = Buffer.allocUnsafe(nb);
this.lastChar = Buffer.allocUnsafe(this._nb);
this._closed = false;
}

StringDecoder.prototype.reset = StringDecoder;

StringDecoder.prototype.write = function(buf) {
if (this._closed === true)
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=== true is unnecessary

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If we explicitly compare strictly against the value, the performance would be better right?

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Not with TurboFan, no. I asked about this in another PR.

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For reference, I think this is the previous discussion in question: #16397 (comment)

this.reset();
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Can this logic be in StringDecoder.prototype.end instead? Make the per-encoding this.end switching in the constructor actually set to an internal symbol-named property, and use a shared StringDecoder.prototype.end that calls that internal encoding-specific function and clean up after itself. Would probably help with write performance too if StringDecoders of different encodings (e.g. UTF-8 which does not have an own property end and UTF-16LE that does) are used simultaneously.


if (buf.length === 0)
return '';

var r;
var i;
if (this.lastNeed) {
Expand Down Expand Up @@ -210,6 +226,7 @@ function utf8Text(buf, i) {
// character.
function utf8End(buf) {
const r = (buf && buf.length ? this.write(buf) : '');
this._closed = true;
if (this.lastNeed)
return r + '\ufffd';
return r;
Expand Down
30 changes: 30 additions & 0 deletions test/parallel/test-string-decoder-end.js
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Expand Up @@ -66,3 +66,33 @@ function testBuf(encoding, buf) {
assert.strictEqual(res1, res3, 'one byte at a time should match toString');
assert.strictEqual(res2, res3, 'all bytes at once should match toString');
}

{
// test to check if the write after end doesn't accumulate the data
const decoder = new SD('utf8');
const euroPart1 = Buffer.from([0xE2]);
const euroPart2 = Buffer.from([0x82, 0xAC]);
decoder.end(euroPart1);
const result = decoder.write(euroPart2);
assert.notStrictEqual(result, '€');
}

{
// test to check if write after end reopens the decoder
const decoder = new SD();
assert.strictEqual(decoder._closed, false);
decoder.end();
assert.strictEqual(decoder._closed, true);
decoder.write(Buffer.from([0xE2]));
assert.strictEqual(decoder._closed, false);
}

{
// test to check if reset after end reopens the decoder
const decoder = new SD();
assert.strictEqual(decoder._closed, false);
decoder.end();
assert.strictEqual(decoder._closed, true);
decoder.reset();
assert.strictEqual(decoder._closed, false);
}