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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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@TimothyGu TimothyGu Oct 30, 2017

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I believe this is an implementation detail users don't have to know about.

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@thefourtheye thefourtheye Dec 1, 2017

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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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@BridgeAR BridgeAR Dec 6, 2017

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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()`.

42 changes: 33 additions & 9 deletions lib/string_decoder.js
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Expand Up @@ -41,36 +41,43 @@ 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);
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;
return;
}
this.lastNeed = 0;
this.lastTotal = 0;
this.lastChar = Buffer.allocUnsafe(nb);
this.lastChar = Buffer.allocUnsafe(this._nb);
}

StringDecoder.prototype.reset = StringDecoder;

StringDecoder.prototype.write = function(buf) {
if (buf.length === 0)
return '';

var r;
var i;
if (this.lastNeed) {
Expand All @@ -87,7 +94,7 @@ StringDecoder.prototype.write = function(buf) {
return r || '';
};

StringDecoder.prototype.end = utf8End;
StringDecoder.prototype.end = end;

// Returns only complete characters in a Buffer
StringDecoder.prototype.text = utf8Text;
Expand Down Expand Up @@ -282,3 +289,20 @@ function simpleWrite(buf) {
function simpleEnd(buf) {
return (buf && buf.length ? this.write(buf) : '');
}

function end(buf) {
let result = '';

if (this.encoding === 'utf16le')
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@mscdex mscdex Dec 1, 2017

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Might consider using a switch here.

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@thefourtheye thefourtheye Dec 5, 2017

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Ack.

result = utf16End.call(this, buf);
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@mscdex mscdex Dec 1, 2017

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If we're the only ones calling these methods directly, then we should be able to avoid the overhead of .call() and just pass an instance as another argument.

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@thefourtheye thefourtheye Dec 5, 2017

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Ack.

else if (this.encoding === 'utf8')
result = utf8End.call(this, buf);
else if (this.encoding === 'base64')
result = base64End.call(this, buf);
else
result = simpleEnd.call(this, buf);

this.reset();
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@mscdex mscdex Dec 1, 2017

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Would it be better to use this.reset(this.encoding) to avoid the switch in the constructor?

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@thefourtheye thefourtheye Dec 5, 2017

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We still cannot get rid of the switch in the constructor, right?

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@mscdex mscdex Dec 5, 2017 •
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Correct, but this.reset(this.encoding) would only hit that first if in the constructor, which might be better.


return result;
}
10 changes: 10 additions & 0 deletions test/parallel/test-string-decoder-end.js
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Expand Up @@ -66,3 +66,13 @@ 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, '€');
}