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string_decoder: make write after end to reset #16594

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b555911
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])

@TimothyGuTimothyGuOct 30, 2017

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

@thefourtheyethefourtheyeDec 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.

@BridgeARBridgeARDec 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');constdecoder=newStringDecoder('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()`.

65 changes: 42 additions & 23 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 DownExpand Up@@ -208,9 +215,9 @@ function utf8Text(buf, i) {

// For UTF-8, a replacement character is added when ending on a partial
// character.
function utf8End(buf) {
const r = (buf && buf.length ? this.write(buf) : '');
if (this.lastNeed)
function utf8End(ctx, buf) {
const r = (buf && buf.length ? ctx.write(buf) : '');
if (ctx.lastNeed)
return r + '\ufffd';
return r;
}
Expand DownExpand Up@@ -242,11 +249,11 @@ function utf16Text(buf, i) {

// For UTF-16LE we do not explicitly append special replacement characters if we
// end on a partial character, we simply let v8 handle that.
function utf16End(buf) {
const r = (buf && buf.length ? this.write(buf) : '');
if (this.lastNeed) {
const end = this.lastTotal - this.lastNeed;
return r + this.lastChar.toString('utf16le', 0, end);
function utf16End(ctx, buf) {
const r = (buf && buf.length ? ctx.write(buf) : '');
if (ctx.lastNeed) {
const end = ctx.lastTotal - ctx.lastNeed;
return r + ctx.lastChar.toString('utf16le', 0, end);
}
return r;
}
Expand All@@ -267,10 +274,10 @@ function base64Text(buf, i) {
}


function base64End(buf) {
const r = (buf && buf.length ? this.write(buf) : '');
if (this.lastNeed)
return r + this.lastChar.toString('base64', 0, 3 - this.lastNeed);
function base64End(ctx, buf) {
const r = (buf && buf.length ? ctx.write(buf) : '');
if (ctx.lastNeed)
return r + ctx.lastChar.toString('base64', 0, 3 - ctx.lastNeed);
return r;
}

Expand All@@ -279,6 +286,18 @@ function simpleWrite(buf) {
return buf.toString(this.encoding);
}

function simpleEnd(buf) {
return (buf && buf.length ? this.write(buf) : '');
function simpleEnd(ctx, buf) {
return (buf && buf.length ? ctx.write(buf) : '');
}

const endMappings = new Map([
['utf16le', utf16End],
['utf8', utf8End],
['base64', base64End]
]);

function end(buf) {
const result = (endMappings.get(this.encoding) || simpleEnd)(this, buf);

@BridgeARBridgeARDec 6, 2017

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This looks "elegant" but using a switch case for the encoding and calling the function directly is probably faster.

this.reset(this.encoding);
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, '€');
}