string_decoder: Fix failures from new test cases

This patch simplifies the implementation of StringDecoder, fixes the
failures from the new test cases, and also no longer relies on v8's
WriteUtf8 function to encode individual surrogates.

lib/string_decoder.js

@@ -57,29 +57,29 @@ var StringDecoder = exports.StringDecoder = function(encoding) {
 
 StringDecoder.prototype.write = function(buffer) {
   var charStr = '';
-  var offset = 0;
-
   // if our last write ended with an incomplete multibyte character
   while (this.charLength) {
     // determine how many remaining bytes this buffer has to offer for this char
-    var i = (buffer.length >= this.charLength - this.charReceived) ?
+    var available = (buffer.length >= this.charLength - this.charReceived) ?
                 this.charLength - this.charReceived :
                 buffer.length;
 
     // add the new bytes to the char buffer
-    buffer.copy(this.charBuffer, this.charReceived, offset, i);
-    this.charReceived += (i - offset);
-    offset = i;
+    buffer.copy(this.charBuffer, this.charReceived, 0, available);
+    this.charReceived += available;
 
     if (this.charReceived < this.charLength) {
       // still not enough chars in this buffer? wait for more ...
       return '';
     }
 
+    // remove bytes belonging to the current character from the buffer
+    buffer = buffer.slice(available, buffer.length);
+
     // get the character that was split
     charStr = this.charBuffer.slice(0, this.charLength).toString(this.encoding);
 
-    // lead surrogate (D800-DBFF) is also the incomplete character
+    // CESU-8: lead surrogate (D800-DBFF) is also the incomplete character
     var charCode = charStr.charCodeAt(charStr.length - 1);
     if (charCode >= 0xD800 && charCode <= 0xDBFF) {
       this.charLength += this.surrogateSize;
@@ -89,34 +89,33 @@ StringDecoder.prototype.write = function(buffer) {
     this.charReceived = this.charLength = 0;
 
     // if there are no more bytes in this buffer, just emit our char
-    if (i == buffer.length) return charStr;
-
-    // otherwise cut off the characters end from the beginning of this buffer
-    buffer = buffer.slice(i, buffer.length);
+    if (buffer.length === 0) {
+      return charStr;
+    }
     break;
   }
 
-  var lenIncomplete = this.detectIncompleteChar(buffer);
+  // determine and set charLength / charReceived
+  this.detectIncompleteChar(buffer);
 
   var end = buffer.length;
   if (this.charLength) {
     // buffer the incomplete character bytes we got
-    buffer.copy(this.charBuffer, 0, buffer.length - lenIncomplete, end);
-    this.charReceived = lenIncomplete;
-    end -= lenIncomplete;
+    buffer.copy(this.charBuffer, 0, buffer.length - this.charReceived, end);
+    end -= this.charReceived;
   }
 
   charStr += buffer.toString(this.encoding, 0, end);
 
   var end = charStr.length - 1;
   var charCode = charStr.charCodeAt(end);
-  // lead surrogate (D800-DBFF) is also the incomplete character
+  // CESU-8: lead surrogate (D800-DBFF) is also the incomplete character
   if (charCode >= 0xD800 && charCode <= 0xDBFF) {
     var size = this.surrogateSize;
     this.charLength += size;
     this.charReceived += size;
     this.charBuffer.copy(this.charBuffer, size, 0, size);
-    this.charBuffer.write(charStr.charAt(charStr.length - 1), this.encoding);
+    buffer.copy(this.charBuffer, 0, 0, size);
     return charStr.substring(0, end);
   }
 
@@ -153,8 +152,7 @@ StringDecoder.prototype.detectIncompleteChar = function(buffer) {
       break;
     }
   }
-
-  return i;
+  this.charReceived = i;
 };
 
 StringDecoder.prototype.end = function(buffer) {
@@ -177,13 +175,11 @@ function passThroughWrite(buffer) {
 }
 
 function utf16DetectIncompleteChar(buffer) {
-  var incomplete = this.charReceived = buffer.length % 2;
-  this.charLength = incomplete ? 2 : 0;
-  return incomplete;
+  this.charReceived = buffer.length % 2;
+  this.charLength = this.charReceived ? 2 : 0;
 }
 
 function base64DetectIncompleteChar(buffer) {
-  var incomplete = this.charReceived = buffer.length % 3;
-  this.charLength = incomplete ? 3 : 0;
-  return incomplete;
+  this.charReceived = buffer.length % 3;
+  this.charLength = this.charReceived ? 3 : 0;
 }