Reverse-engineered implementation of -[NSString isEqual:] and -[NSString isEqualToString:]

NSString.m

/*
 * Most NSString instances will actually be __NSCFString instances, so here are both NSString and __NSCFString implementations.
 * If you know how to create an NSString instance whose class is actually NSString please let me know.
 * Other possible concrete subclasses of NSString are: NSConstantString, __NSCFConstantString, NSPathStore2, NSSimpleCString and __NSLocalizedString.
 */

// CoreFoundation.framework 635.19.0 (Mac OS X 10.7.3)
@implementation NSObject

- (BOOL) isNSString__
{
	return NO;
}

@end

@implementation __NSCFString

static CFStringEncoding enc = 0;

- (BOOL) isNSString__
{
	return YES;
}

- (BOOL) isEqual:(id)string
{
	if (self == string)
	{
		return YES;
	}
	else if (string == nil)
	{
		return NO
	}
	else
	{
		Class stringClass = [string class];
		Class nscfStringClass = objc_lookUpClass("__NSCFString");
		if (stringClass == nscfStringClass || class_getSuperclass(stringClass) == nscfStringClass)
		{
			if (enc == kCFStringEncodingInvalidId)
				enc = CFStringGetSystemEncoding();
			
			const char *selfCString = CFStringGetCStringPtr(self, enc);
			
			if (enc == kCFStringEncodingInvalidId)
				enc = CFStringGetSystemEncoding();
			
			const char *argCString = CFStringGetCStringPtr(string, enc);
			
			if (selfCString != NULL && argCString != NULL)
			{
				CFIndex length = _CFStringGetLength2(self);
				if (length == _CFStringGetLength2(string))
				{
					return memcmp(selfCString, argCString, length) == 0;
				}
				else
				{
					return NO;
				}
			}
		}
	}
	
	if (![string isNSString__])
		return NO;
	
	// Eventually calls __CFStringEqual whose implementation is available at http://opensource.apple.com/source/CF/CF-635.19/CFString.c
	return CFEqual(self, string);
}

- (BOOL) isEqualToString:(NSString *)string
{
	if (self == string)
	{
		return YES;
	}
	else if (string == nil)
	{
		return NO
	}
	else
	{
		Class stringClass = [string class];
		Class nscfStringClass = objc_lookUpClass("__NSCFString");
		if (stringClass == nscfStringClass || class_getSuperclass(stringClass) == nscfStringClass)
		{
			if (enc == kCFStringEncodingInvalidId)
				enc = CFStringGetSystemEncoding();
			
			const char *selfCString = CFStringGetCStringPtr(self, enc);
			
			if (enc == kCFStringEncodingInvalidId)
				enc = CFStringGetSystemEncoding();
			
			const char *argCString = CFStringGetCStringPtr(string, enc);
			
			if (selfCString != NULL && argCString != NULL)
			{
				CFIndex length = _CFStringGetLength2(self);
				if (length == _CFStringGetLength2(string))
				{
					return memcmp(selfCString, argCString, length) == 0;
				}
				else
				{
					return NO;
				}
			}
		}
	}
	
	// Eventually calls __CFStringEqual whose implementation is available at http://opensource.apple.com/source/CF/CF-635.19/CFString.c
	return CFEqual(self, string);
}

@end

// Foundation.framework 833.24.0 (Mac OS X 10.7.3)
@implementation NSString

- (BOOL) isNSString__
{
	return YES;
}

- (BOOL) isEqual:(id)string
{
	if ([string isNSString__])
		return [self isEqualToString:string];
	else
		return NO;
}

- (BOOL) isEqualToString:(NSString *)string
{
	if (self == string)
		return YES;
	else
		return [self compare:string options:NSLiteralSearch range:NSMakeRange(0, [self length])] == NSOrderedSame;
}

@end