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string_decoder: reimplement in C++
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Implement string decoder in C++. The perks are a decent speed boost
(for decoding, whereas creation show some performance degradation),
that this can now be used more easily to add native decoding support
to C++ streams and (arguably) more readable variable names.

PR-URL: #18537
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
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@addaleax @BridgeAR
addaleax authored and BridgeAR committed Feb 10, 2018
1 parent de848ac commit 180af17
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Showing 7 changed files with 478 additions and 238 deletions.
285 changes: 47 additions & 238 deletions lib/string_decoder.js
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Original file line number Diff line number Diff line change
Expand Up @@ -22,10 +22,23 @@
'use strict';

const { Buffer } = require('buffer');
const {
kIncompleteCharactersStart,
kIncompleteCharactersEnd,
kMissingBytes,
kBufferedBytes,
kEncodingField,
kSize,
decode,
flush,
encodings
} = internalBinding('string_decoder');
const internalUtil = require('internal/util');
const errors = require('internal/errors');
const isEncoding = Buffer[internalUtil.kIsEncodingSymbol];

const kNativeDecoder = Symbol('kNativeDecoder');

// Do not cache `Buffer.isEncoding` when checking encoding names as some
// modules monkey-patch it to support additional encodings
function normalizeEncoding(enc) {
Expand All @@ -36,258 +49,54 @@ function normalizeEncoding(enc) {
return nenc || enc;
}

const encodingsMap = {};
for (var i = 0; i < encodings.length; ++i)
encodingsMap[encodings[i]] = i;

// StringDecoder provides an interface for efficiently splitting a series of
// buffers into a series of JS strings without breaking apart multi-byte
// characters.
exports.StringDecoder = StringDecoder;
function StringDecoder(encoding) {
this.encoding = normalizeEncoding(encoding);
var nb;
switch (this.encoding) {
case 'utf16le':
this.text = utf16Text;
this.end = utf16End;
nb = 4;
break;
case 'utf8':
this.fillLast = utf8FillLast;
nb = 4;
break;
case 'base64':
this.text = base64Text;
this.end = base64End;
nb = 3;
break;
default:
this.write = simpleWrite;
this.end = simpleEnd;
return;
}
this.lastNeed = 0;
this.lastTotal = 0;
this.lastChar = Buffer.allocUnsafe(nb);
}

StringDecoder.prototype.write = function(buf) {
if (buf.length === 0)
return '';
var r;
var i;
if (this.lastNeed) {
r = this.fillLast(buf);
if (r === undefined)
return '';
i = this.lastNeed;
this.lastNeed = 0;
} else {
i = 0;
}
if (i < buf.length)
return (r ? r + this.text(buf, i) : this.text(buf, i));
return r || '';
};

StringDecoder.prototype.end = utf8End;

// Returns only complete characters in a Buffer
StringDecoder.prototype.text = utf8Text;

// Attempts to complete a partial non-UTF-8 character using bytes from a Buffer
StringDecoder.prototype.fillLast = function(buf) {
if (this.lastNeed <= buf.length) {
buf.copy(this.lastChar, this.lastTotal - this.lastNeed, 0, this.lastNeed);
return this.lastChar.toString(this.encoding, 0, this.lastTotal);
}
buf.copy(this.lastChar, this.lastTotal - this.lastNeed, 0, buf.length);
this.lastNeed -= buf.length;
};

// Checks the type of a UTF-8 byte, whether it's ASCII, a leading byte, or a
// continuation byte. If an invalid byte is detected, -2 is returned.
function utf8CheckByte(byte) {
if (byte <= 0x7F)
return 0;
else if (byte >> 5 === 0x06)
return 2;
else if (byte >> 4 === 0x0E)
return 3;
else if (byte >> 3 === 0x1E)
return 4;
return (byte >> 6 === 0x02 ? -1 : -2);
}

// Checks at most 3 bytes at the end of a Buffer in order to detect an
// incomplete multi-byte UTF-8 character. The total number of bytes (2, 3, or 4)
// needed to complete the UTF-8 character (if applicable) are returned.
function utf8CheckIncomplete(self, buf, i) {
var j = buf.length - 1;
if (j < i)
return 0;
var nb = utf8CheckByte(buf[j]);
if (nb >= 0) {
if (nb > 0)
self.lastNeed = nb - 1;
return nb;
}
if (--j < i || nb === -2)
return 0;
nb = utf8CheckByte(buf[j]);
if (nb >= 0) {
if (nb > 0)
self.lastNeed = nb - 2;
return nb;
}
if (--j < i || nb === -2)
return 0;
nb = utf8CheckByte(buf[j]);
if (nb >= 0) {
if (nb > 0) {
if (nb === 2)
nb = 0;
else
self.lastNeed = nb - 3;
}
return nb;
}
return 0;
}

// Validates as many continuation bytes for a multi-byte UTF-8 character as
// needed or are available. If we see a non-continuation byte where we expect
// one, we "replace" the validated continuation bytes we've seen so far with
// a single UTF-8 replacement character ('\ufffd'), to match v8's UTF-8 decoding
// behavior. The continuation byte check is included three times in the case
// where all of the continuation bytes for a character exist in the same buffer.
// It is also done this way as a slight performance increase instead of using a
// loop.
function utf8CheckExtraBytes(self, buf, p) {
if ((buf[0] & 0xC0) !== 0x80) {
self.lastNeed = 0;
return '\ufffd';
}
if (self.lastNeed > 1 && buf.length > 1) {
if ((buf[1] & 0xC0) !== 0x80) {
self.lastNeed = 1;
return '\ufffd';
}
if (self.lastNeed > 2 && buf.length > 2) {
if ((buf[2] & 0xC0) !== 0x80) {
self.lastNeed = 2;
return '\ufffd';
}
}
class StringDecoder {
constructor(encoding) {
this.encoding = normalizeEncoding(encoding);
this[kNativeDecoder] = Buffer.alloc(kSize);
this[kNativeDecoder][kEncodingField] = encodingsMap[this.encoding];
}
}

// Attempts to complete a multi-byte UTF-8 character using bytes from a Buffer.
function utf8FillLast(buf) {
const p = this.lastTotal - this.lastNeed;
var r = utf8CheckExtraBytes(this, buf, p);
if (r !== undefined)
return r;
if (this.lastNeed <= buf.length) {
buf.copy(this.lastChar, p, 0, this.lastNeed);
return this.lastChar.toString(this.encoding, 0, this.lastTotal);
write(buf) {
if (typeof buf === 'string')
return buf;
if (!ArrayBuffer.isView(buf))
throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'buf',
['Buffer', 'Uint8Array', 'ArrayBufferView']);
return decode(this[kNativeDecoder], buf);
}
buf.copy(this.lastChar, p, 0, buf.length);
this.lastNeed -= buf.length;
}

// Returns all complete UTF-8 characters in a Buffer. If the Buffer ended on a
// partial character, the character's bytes are buffered until the required
// number of bytes are available.
function utf8Text(buf, i) {
const total = utf8CheckIncomplete(this, buf, i);
if (!this.lastNeed)
return buf.toString('utf8', i);
this.lastTotal = total;
const end = buf.length - (total - this.lastNeed);
buf.copy(this.lastChar, 0, end);
return buf.toString('utf8', i, end);
}

// 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) {
this.lastNeed = 0;
this.lastTotal = 0;
return r + '\ufffd';
end(buf) {
let ret = '';
if (buf !== undefined)
ret = this.write(buf);
if (this[kNativeDecoder][kBufferedBytes] > 0)
ret += flush(this[kNativeDecoder]);
return ret;
}
return r;
}

// UTF-16LE typically needs two bytes per character, but even if we have an even
// number of bytes available, we need to check if we end on a leading/high
// surrogate. In that case, we need to wait for the next two bytes in order to
// decode the last character properly.
function utf16Text(buf, i) {
if ((buf.length - i) % 2 === 0) {
const r = buf.toString('utf16le', i);
if (r) {
const c = r.charCodeAt(r.length - 1);
if (c >= 0xD800 && c <= 0xDBFF) {
this.lastNeed = 2;
this.lastTotal = 4;
this.lastChar[0] = buf[buf.length - 2];
this.lastChar[1] = buf[buf.length - 1];
return r.slice(0, -1);
}
}
return r;
}
this.lastNeed = 1;
this.lastTotal = 2;
this.lastChar[0] = buf[buf.length - 1];
return buf.toString('utf16le', i, buf.length - 1);
}
/* Everything below this line is undocumented legacy stuff. */

// 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;
this.lastNeed = 0;
this.lastTotal = 0;
return r + this.lastChar.toString('utf16le', 0, end);
text(buf, offset) {
this[kNativeDecoder][kMissingBytes] = 0;
this[kNativeDecoder][kBufferedBytes] = 0;
return this.write(buf.slice(offset));
}
return r;
}

function base64Text(buf, i) {
const n = (buf.length - i) % 3;
if (n === 0)
return buf.toString('base64', i);
this.lastNeed = 3 - n;
this.lastTotal = 3;
if (n === 1) {
this.lastChar[0] = buf[buf.length - 1];
} else {
this.lastChar[0] = buf[buf.length - 2];
this.lastChar[1] = buf[buf.length - 1];
get lastTotal() {
return this[kNativeDecoder][kBufferedBytes] + this.lastNeed;
}
return buf.toString('base64', i, buf.length - n);
}


function base64End(buf) {
const r = (buf && buf.length ? this.write(buf) : '');
if (this.lastNeed) {
const end = 3 - this.lastNeed;
this.lastNeed = 0;
this.lastTotal = 0;
return r + this.lastChar.toString('base64', 0, end);
get lastChar() {
return this[kNativeDecoder].subarray(kIncompleteCharactersStart,
kIncompleteCharactersEnd);
}
return r;
}

// Pass bytes on through for single-byte encodings (e.g. ascii, latin1, hex)
function simpleWrite(buf) {
return buf.toString(this.encoding);
}

function simpleEnd(buf) {
return (buf && buf.length ? this.write(buf) : '');
}
exports.StringDecoder = StringDecoder;
4 changes: 4 additions & 0 deletions node.gyp
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Expand Up @@ -326,6 +326,7 @@
'src/signal_wrap.cc',
'src/spawn_sync.cc',
'src/string_bytes.cc',
'src/string_decoder.cc',
'src/string_search.cc',
'src/stream_base.cc',
'src/stream_wrap.cc',
Expand Down Expand Up @@ -379,6 +380,8 @@
'src/req_wrap.h',
'src/req_wrap-inl.h',
'src/string_bytes.h',
'src/string_decoder.h',
'src/string_decoder-inl.h',
'src/stream_base.h',
'src/stream_base-inl.h',
'src/stream_wrap.h',
Expand Down Expand Up @@ -989,6 +992,7 @@
'<(obj_path)<(obj_separator)node_url.<(obj_suffix)',
'<(obj_path)<(obj_separator)util.<(obj_suffix)',
'<(obj_path)<(obj_separator)string_bytes.<(obj_suffix)',
'<(obj_path)<(obj_separator)string_decoder.<(obj_suffix)',
'<(obj_path)<(obj_separator)string_search.<(obj_suffix)',
'<(obj_path)<(obj_separator)stream_base.<(obj_suffix)',
'<(obj_path)<(obj_separator)node_constants.<(obj_suffix)',
Expand Down
1 change: 1 addition & 0 deletions src/node_internals.h
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Original file line number Diff line number Diff line change
Expand Up @@ -120,6 +120,7 @@ struct sockaddr;
V(signal_wrap) \
V(spawn_sync) \
V(stream_wrap) \
V(string_decoder) \
V(tcp_wrap) \
V(timer_wrap) \
V(trace_events) \
Expand Down
38 changes: 38 additions & 0 deletions src/string_decoder-inl.h
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@@ -0,0 +1,38 @@
#ifndef SRC_STRING_DECODER_INL_H_
#define SRC_STRING_DECODER_INL_H_

#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS

#include "string_decoder.h"
#include "util.h"

namespace node {

void StringDecoder::SetEncoding(enum encoding encoding) {
state_[kBufferedBytes] = 0;
state_[kMissingBytes] = 0;
state_[kEncodingField] = encoding;
}

enum encoding StringDecoder::Encoding() const {
return static_cast<enum encoding>(state_[kEncodingField]);
}

unsigned StringDecoder::BufferedBytes() const {
return state_[kBufferedBytes];
}

unsigned StringDecoder::MissingBytes() const {
return state_[kMissingBytes];
}

char* StringDecoder::IncompleteCharacterBuffer() {
return reinterpret_cast<char*>(state_ + kIncompleteCharactersStart);
}


} // namespace node

#endif // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS

#endif // SRC_STRING_DECODER_INL_H_
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