Fix escaping in JSON.stringify and add unit test

lib/json/json.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library("json");
 
 // Pure Dart implementation of JSON protocol.
 
 /**
  * Utility class to parse JSON and serialize objects to JSON.
@@ skipping 96 matching lines @@
   }
 }
 
 typedef bool Predicate(int c);
 
 class JsonTokenizer {
   static final int BACKSPACE = 8;  // '\b'.charCodeAt(0)
   static final int TAB = 9;  // '\t'.charCodeAt(0)
   static final int NEW_LINE = 10;  // '\n'.charCodeAt(0)
   static final int FORM_FEED = 12;  // '\f'.charCodeAt(0)
-  static final int LINE_FEED = 13;  // '\r'.charCodeAt(0)
+  static final int CARRIAGE_RETURN = 13;  // '\r'.charCodeAt(0)
   static final int SPACE = 32;  // ' '.charCodeAt(0)
   static final int QUOTE = 34;  // '"'.charCodeAt(0)
   static final int PLUS = 43;  // '+'.charCodeAt(0)
   static final int COMMA = 44;  // ','.charCodeAt(0)
   static final int MINUS = 45;  // '-'.charCodeAt(0)
   static final int DOT = 46;  // '.'.charCodeAt(0)
   static final int SLASH = 47;  // '/'.charCodeAt(0)
   static final int ZERO = 48;  // '0'.charCodeAt(0)
   static final int NINE = 57;  // '9'.charCodeAt(0)
   static final int COLON = 58;  // ':'.charCodeAt(0)
   static final int A_BIG = 65;  // 'A'.charCodeAt(0)
   static final int E_BIG = 69;  // 'E'.charCodeAt(0)
   static final int Z_BIG = 90;  // 'Z'.charCodeAt(0)
   static final int LBRACKET = 91;  // '['.charCodeAt(0)
   static final int BACKSLASH = 92;  // '\\'.charCodeAt(0)
   static final int RBRACKET = 93;  // ']'.charCodeAt(0)
   static final int A_SMALL = 97;  // 'a'.charCodeAt(0)
   static final int B_SMALL = 98;  // 'b'.charCodeAt(0)
   static final int E_SMALL = 101;  // 'e'.charCodeAt(0)
+  static final int F_SMALL = 102;  // 'f'.charCodeAt(0)
   static final int N_SMALL = 110;  // 'n'.charCodeAt(0)
   static final int R_SMALL = 114;  // 'r'.charCodeAt(0)
+  static final int T_SMALL = 116;  // 't'.charCodeAt(0)
+  static final int U_SMALL = 117;  // 'u'.charCodeAt(0)
   static final int Z_SMALL = 122;  // 'z'.charCodeAt(0)
   static final int LBRACE = 123;  // '{'.charCodeAt(0)
   static final int RBRACE = 125;  // '}'.charCodeAt(0)
 
   JsonTokenizer(String s) : _s = s + ' ', _pos = 0, _len = s.length + 1 {}
 
   /**
    * Fetches next token or [:null:] if the stream has been exhausted.
    */
   JsonToken next() {
@@ skipping 30 matching lines @@
               case '/':
                 c = SLASH;
                 break;
               case 'b':
                 c = BACKSPACE;
                 break;
               case 'n':
                 c = NEW_LINE;
                 break;
               case 'r':
-                c = LINE_FEED;
+                c = CARRIAGE_RETURN;
                 break;
               case 'f':
                 c = FORM_FEED;
                 break;
               case 't':
                 c = TAB;
                 break;
               case 'u':
                 if (_pos + 5 > _len) {
                   throw 'Invalid unicode esacape sequence: \\' +
@@ skipping 128 matching lines @@
     while (_pos < _len && predicate(_s.charCodeAt(_pos))) {
       _pos++;
     }
     if (_pos == _len) {
       throw errorMsg;
     }
   }
 
   // TODO other kind of whitespace.
   static bool isWhitespace(int c) {
-    return c == SPACE || c == TAB || c == NEW_LINE || c == LINE_FEED;
+    return c == SPACE || c == TAB || c == NEW_LINE || c == CARRIAGE_RETURN;
   }
   static bool isDigit(int c) {
     return (ZERO <= c) && (c <= NINE);
   }
   static bool isLetter(int c) {
      return ((A_SMALL <= c) && (c <= Z_SMALL)) || ((A_BIG <= c) && (c <= Z_BIG));
   }
 }
 
 class JsonParser {
@@ skipping 137 matching lines @@
         return x.toString();
 
       case x is double:
         return x.toString();
 
       default:
         return x.toDouble().toString();
     }
   }
 
-  // TODO: add others.
-  static bool _needsEscape(int charCode) {
-    return JsonTokenizer.QUOTE == charCode || JsonTokenizer.BACKSLASH == charCode
-      || JsonTokenizer.NEW_LINE == charCode || JsonTokenizer.LINE_FEED == charCode;
-  }
+  // ('0' + x) or ('a' + x - 10)
+  static int _hexDigit(int x) => x < 10 ? 48 + x : 87 + x;
 
   static void _escape(StringBuffer sb, String s) {
-    // TODO: support \u code points.
-    // TODO: use writeCodePoint when implemented.
-    // TODO: use for each if implemented.
     final int length = s.length;
     bool needsEscape = false;
     final charCodes = new List<int>();
     for (int i = 0; i < length; i++) {
       int charCode = s.charCodeAt(i);
-      if (_needsEscape(charCode)) {
+      if (charCode < 32) {
+        needsEscape = true;
         charCodes.add(JsonTokenizer.BACKSLASH);
+        switch (charCode) {
+        case JsonTokenizer.BACKSPACE:
+          charCodes.add(JsonTokenizer.B_SMALL);
+          break;
+        case JsonTokenizer.TAB:
+          charCodes.add(JsonTokenizer.T_SMALL);
+          break;
+        case JsonTokenizer.NEW_LINE:
+          charCodes.add(JsonTokenizer.N_SMALL);
+          break;
+        case JsonTokenizer.FORM_FEED:
+          charCodes.add(JsonTokenizer.F_SMALL);
+          break;
+        case JsonTokenizer.CARRIAGE_RETURN:
+          charCodes.add(JsonTokenizer.R_SMALL);
+          break;
+        default:
+          charCodes.add(JsonTokenizer.U_SMALL);
+          charCodes.add(_hexDigit((charCode >> 12) & 0xf));
+          charCodes.add(_hexDigit((charCode >> 8) & 0xf));
+          charCodes.add(_hexDigit((charCode >> 4) & 0xf));
+          charCodes.add(_hexDigit(charCode & 0xf));
+          break;
+        }
+      } else if (charCode == JsonTokenizer.QUOTE ||
+          charCode == JsonTokenizer.BACKSLASH) {
         needsEscape = true;
-
-        if (JsonTokenizer.NEW_LINE == charCode) {
-          charCode = JsonTokenizer.N_SMALL;
-        } else if (JsonTokenizer.LINE_FEED == charCode) {
-          charCode = JsonTokenizer.R_SMALL;
-        }
+        charCodes.add(JsonTokenizer.BACKSLASH);
+        charCodes.add(charCode);
+      } else {
+        charCodes.add(charCode);
       }
-      charCodes.add(charCode);
     }
     sb.add(needsEscape ? new String.fromCharCodes(charCodes) : s);
   }
 
   void _checkCycle(final object) {
     // TODO: use Iterables.
     for (int i = 0; i < _seen.length; i++) {
       if (_seen[i] === object) {
         throw 'Cyclic structure';
       }
@@ skipping 60 matching lines @@
         });
         _sb.add('}');
         _seen.removeLast();
         return;
 
       default:
         throw const JsonUnsupportedObjectType();
     }
   }
 }