string_decoder: reimplement in C++

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: https://github.com/nodejs/node/pull/18537
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

1 files changed, 47 insertions, 238 deletions

diff --git a/lib/string_decoder.js b/lib/string_decoder.js
index 1e569ba6b2..d955a66330 100644
--- a/lib/string_decoder.js
+++ b/lib/string_decoder.js
@@ -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) {
@@ -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;