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string_decoder.js
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224 lines (188 loc) · 6.85 KB
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'use strict';
const Buffer = require('buffer').Buffer;
function assertEncoding(encoding) {
// Do not cache `Buffer.isEncoding`, some modules monkey-patch it to support
// additional encodings
if (encoding && !Buffer.isEncoding(encoding)) {
throw new Error('Unknown encoding: ' + encoding);
}
}
// StringDecoder provides an interface for efficiently splitting a series of
// buffers into a series of JS strings without breaking apart multi-byte
// characters. CESU-8 is handled as part of the UTF-8 encoding.
//
// @TODO Handling all encodings inside a single object makes it very difficult
// to reason about this code, so it should be split up in the future.
// @TODO There should be a utf8-strict encoding that rejects invalid UTF-8 code
// points as used by CESU-8.
const StringDecoder = exports.StringDecoder = function(encoding) {
this.encoding = (encoding || 'utf8').toLowerCase().replace(/[-_]/, '');
assertEncoding(encoding);
switch (this.encoding) {
case 'utf8':
// CESU-8 represents each of Surrogate Pair by 3-bytes
this.surrogateSize = 3;
break;
case 'ucs2':
case 'utf16le':
// UTF-16 represents each of Surrogate Pair by 2-bytes
this.surrogateSize = 2;
this.detectIncompleteChar = utf16DetectIncompleteChar;
break;
case 'base64':
// Base-64 stores 3 bytes in 4 chars, and pads the remainder.
this.surrogateSize = 3;
this.detectIncompleteChar = base64DetectIncompleteChar;
break;
default:
this.write = passThroughWrite;
return;
}
// Enough space to store all bytes of a single character. UTF-8 needs 4
// bytes, but CESU-8 may require up to 6 (3 bytes per surrogate).
this.charBuffer = new Buffer(6);
// Number of bytes received for the current incomplete multi-byte character.
this.charReceived = 0;
// Number of bytes expected for the current incomplete multi-byte character.
this.charLength = 0;
};
// write decodes the given buffer and returns it as JS string that is
// guaranteed to not contain any partial multi-byte characters. Any partial
// character found at the end of the buffer is buffered up, and will be
// returned when calling write again with the remaining bytes.
//
// Note: Converting a Buffer containing an orphan surrogate to a String
// currently works, but converting a String to a Buffer (via `new Buffer`, or
// Buffer#write) will replace incomplete surrogates with the unicode
// replacement character. See https://codereview.chromium.org/121173009/ .
StringDecoder.prototype.write = function(buffer) {
var charStr = '';
var buflen = buffer.length;
var charBuffer = this.charBuffer;
var charLength = this.charLength;
var charReceived = this.charReceived;
var surrogateSize = this.surrogateSize;
var encoding = this.encoding;
// if our last write ended with an incomplete multibyte character
while (charLength) {
// determine how many remaining bytes this buffer has to offer for this char
var diff = charLength - charReceived;
var available = (buflen >= diff) ? diff : buflen;
// add the new bytes to the char buffer
buffer.copy(charBuffer, charReceived, 0, available);
charReceived += available;
if (charReceived < charLength) {
// still not enough chars in this buffer? wait for more ...
this.charLength = charLength;
this.charReceived = charReceived;
return '';
}
// remove bytes belonging to the current character from the buffer
buffer = buffer.slice(available, buflen);
buflen = buffer.length;
// get the character that was split
charStr = charBuffer.toString(encoding, 0, charLength);
// CESU-8: lead surrogate (D800-DBFF) is also the incomplete character
var charCode = charStr.charCodeAt(charStr.length - 1);
if (charCode >= 0xD800 && charCode <= 0xDBFF) {
charLength += surrogateSize;
charStr = '';
continue;
}
charReceived = charLength = 0;
// if there are no more bytes in this buffer, just emit our char
if (buflen === 0) {
this.charLength = charLength;
this.charReceived = charReceived;
return charStr;
}
}
// determine and set charLength / charReceived
if (this.detectIncompleteChar(buffer))
charLength = this.charLength;
charReceived = this.charReceived;
var end = buflen;
if (charLength) {
// buffer the incomplete character bytes we got
buffer.copy(charBuffer, 0, buflen - charReceived, end);
end -= charReceived;
}
this.charLength = charLength;
charStr += buffer.toString(encoding, 0, end);
var end = charStr.length - 1;
var charCode = charStr.charCodeAt(end);
// CESU-8: lead surrogate (D800-DBFF) is also the incomplete character
if (charCode >= 0xD800 && charCode <= 0xDBFF) {
charLength += surrogateSize;
charReceived += surrogateSize;
charBuffer.copy(charBuffer, surrogateSize, 0, surrogateSize);
buffer.copy(charBuffer, 0, 0, surrogateSize);
this.charLength = charLength;
this.charReceived = charReceived;
return charStr.substring(0, end);
}
// or just emit the charStr
return charStr;
};
// detectIncompleteChar determines if there is an incomplete UTF-8 character at
// the end of the given buffer. If so, it sets this.charLength to the byte
// length that character, and sets this.charReceived to the number of bytes
// that are available for this character.
StringDecoder.prototype.detectIncompleteChar = function(buffer) {
var buflen = buffer.length;
// determine how many bytes we have to check at the end of this buffer
var i = (buflen >= 3) ? 3 : buflen;
var newlen = false;
// Figure out if one of the last i bytes of our buffer announces an
// incomplete char.
for (; i > 0; i--) {
var c = buffer[buflen - i];
// See http://en.wikipedia.org/wiki/UTF-8#Description
// 110XXXXX
if (i === 1 && c >> 5 === 0x06) {
this.charLength = 2;
newlen = true;
break;
}
// 1110XXXX
if (i <= 2 && c >> 4 === 0x0E) {
this.charLength = 3;
newlen = true;
break;
}
// 11110XXX
if (i <= 3 && c >> 3 === 0x1E) {
this.charLength = 4;
newlen = true;
break;
}
}
this.charReceived = i;
return newlen;
};
StringDecoder.prototype.end = function(buffer) {
var res = '';
if (buffer && buffer.length)
res = this.write(buffer);
var charReceived = this.charReceived;
if (charReceived) {
var cr = charReceived;
var buf = this.charBuffer;
var enc = this.encoding;
res += buf.toString(enc, 0, cr);
}
return res;
};
function passThroughWrite(buffer) {
return buffer.toString(this.encoding);
}
function utf16DetectIncompleteChar(buffer) {
var charReceived = this.charReceived = buffer.length % 2;
this.charLength = charReceived ? 2 : 0;
return true;
}
function base64DetectIncompleteChar(buffer) {
var charReceived = this.charReceived = buffer.length % 3;
this.charLength = charReceived ? 3 : 0;
return true;
}