NSJSONSerialization: Implement NSJSONWritingSortedKeys

Author: hmelder

Headers/Foundation/NSJSONSerialization.h

@@ -50,7 +50,13 @@ enum
    * If this is not set, then the writer will not generate any superfluous
    * whitespace, producing space-efficient but not very human-friendly JSON.
    */
-  NSJSONWritingPrettyPrinted = (1UL << 0)
+  NSJSONWritingPrettyPrinted = (1UL << 0),
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_13, GS_API_LATEST)
+  /**
+   * When writing JSON, sort keys in lexicographic order.
+   */
+  NSJSONWritingSortedKeys = (1UL << 1)
+#endif
 };
 /**
  * A bitmask containing flags from the NSJSONWriting* set, specifying options

Source/NSJSONSerialization.m

@@ -860,7 +860,7 @@
 }
 
 static BOOL
-writeObject(id obj, NSMutableString *output, NSInteger tabs)
+writeObject(id obj, NSMutableString *output, NSInteger tabs, NSJSONWritingOptions opt)
 {
   if ([obj isKindOfClass: NSArrayClass])
     {
@@ -874,7 +874,7 @@
         writeComma = YES;
         writeNewline(output, tabs);
         writeTabs(output, tabs);
-        writeObject(o, output, tabs + 1);
+        writeObject(o, output, tabs + 1, opt);
       END_FOR_IN(obj)
       writeNewline(output, tabs);
       writeTabs(output, tabs);
@@ -883,8 +883,15 @@
   else if ([obj isKindOfClass: NSDictionaryClass])
     {
       BOOL writeComma = NO;
+      NSArray *keys = [obj allKeys];
       [output appendString: @"{"];
-      FOR_IN(id, o, obj)
+
+      if ((opt & NSJSONWritingSortedKeys) == NSJSONWritingSortedKeys)
+        {
+          keys = [keys sortedArrayUsingSelector: @selector(compare:)];
+        }
+
+      FOR_IN(id, o, keys)
         // Keys in dictionaries must be strings
         if (![o isKindOfClass: NSStringClass]) { return NO; }
         if (writeComma)
@@ -894,10 +901,11 @@
         writeComma = YES;
         writeNewline(output, tabs);
         writeTabs(output, tabs);
-        writeObject(o, output, tabs + 1);
-        [output appendString: @": "];
-        writeObject([obj objectForKey: o], output, tabs + 1);
-      END_FOR_IN(obj)
+        writeObject(o, output, tabs + 1, opt);
+        [output appendString: @":"];
+        writeObject([obj objectForKey: o], output, tabs + 1, opt);
+      END_FOR_IN(keys)
+
       writeNewline(output, tabs);
       writeTabs(output, tabs);
       [output appendString: @"}"];
@@ -1062,7 +1070,7 @@ + (NSData*) dataWithJSONObject: (id)obj
 
   tabs = ((opt & NSJSONWritingPrettyPrinted) == NSJSONWritingPrettyPrinted) ?
     0 : NSIntegerMin;
-  if (writeObject(obj, str, tabs))
+  if (writeObject(obj, str, tabs, opt))
     {
       data = [str dataUsingEncoding: NSUTF8StringEncoding];
       if (NULL != error)
@@ -1089,7 +1097,7 @@ + (NSData*) dataWithJSONObject: (id)obj
 
 + (BOOL) isValidJSONObject: (id)obj
 {
-  return writeObject(obj, nil, NSIntegerMin);
+  return writeObject(obj, nil, NSIntegerMin, 0);
 }
 
 + (id) JSONObjectWithData: (NSData *)data

Tests/base/NSJSONSerialization/sorted.m

@@ -0,0 +1,34 @@
+#import <Foundation/Foundation.h>
+#import "ObjectTesting.h"
+
+void testLexicographicalOrder() {
+  NSArray *objects =
+      [NSArray arrayWithObjects:@"a", @"b", @"c", @"d", @"e", nil];
+  NSArray *keys = [NSArray
+      arrayWithObjects:@"c_ab", @"a_ab", @"d_ab", @"f_cb", @"f_ab", nil];
+  NSDictionary *dict = [NSDictionary dictionaryWithObjects:objects
+                                                   forKeys:keys];
+
+  NSError *error = nil;
+  NSData *actualData =
+      [NSJSONSerialization dataWithJSONObject:dict
+                                      options:NSJSONWritingSortedKeys
+                                        error:&error];
+  PASS_EQUAL(error, nil, "no error occurred during serialisation");
+
+
+  NSString *actual = [[NSString alloc] initWithData:actualData
+                                           encoding:NSUTF8StringEncoding];
+  NSString *expected = @"{\"a_ab\":\"b\",\"c_ab\":\"a\",\"d_ab\":\"c\",\"f_"
+                        "ab\":\"e\",\"f_cb\":\"d\"}";
+  PASS_EQUAL(actual, expected, "JSON is correctly sorted");
+  NSLog(@"%@", actual);
+}
+
+int main(void) {
+  NSAutoreleasePool *arp = [NSAutoreleasePool new];
+
+  testLexicographicalOrder();
+
+  [arp release];
+}