/**
   NSKVOSupport.m

   Copyright (C) 2024 Free Software Foundation, Inc.

   Written by: Hugo Melder <hugo@algoriddim.com>
   Date: June 2024

   Based on WinObjC KVO tests by Microsoft Corporation.

   This file is part of GNUStep-base

   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2 of the License, or (at your option) any later version.

   This library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   If you are interested in a warranty or support for this source code,
   contact Scott Christley <scottc@net-community.com> for more information.

   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, write to the Free
   Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
   Boston, MA 02110 USA.
*/
/**
  Copyright (c) Microsoft. All rights reserved.

  This code is licensed under the MIT License (MIT).

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  THE SOFTWARE.
*/

/* This Key Value Observing Implementation is tied to libobjc2 */

#import "common.h"
#import "NSKVOInternal.h"
#import <objc/objc-arc.h>
#import <stdatomic.h>

#import <Foundation/Foundation.h>

typedef void (^DispatchChangeBlock)(_NSKVOKeyObserver *);

static NSString *
_NSKVCSplitKeypath(NSString *keyPath, NSString *__autoreleasing *pRemainder)
{
  NSRange result = [keyPath rangeOfString:@"."];
  if (keyPath.length > 0 && result.location != NSNotFound)
    {
      *pRemainder = [keyPath substringFromIndex:result.location + 1];
      return [keyPath substringToIndex:result.location];
    }
  *pRemainder = nil;
  return keyPath;
}

#pragma region Key Observer
@interface
_NSKVOKeyObserver ()
{
  _Atomic(BOOL) _isRemoved;
}
@end

@implementation _NSKVOKeyObserver
- (instancetype)initWithObject: (id)object
               keypathObserver: (_NSKVOKeypathObserver *)keypathObserver
                           key: (NSString *)key
                 restOfKeypath: (NSString *)restOfKeypath
             affectedObservers: (NSArray *)affectedObservers
{
  if (self = [super init])
    {
      _object = object;
      _keypathObserver = [keypathObserver retain];
      _key = [key copy];
      _restOfKeypath = [restOfKeypath copy];
      _affectedObservers = [affectedObservers copy];
    }
  return self;
}

- (void)dealloc
{
  [_keypathObserver release];
  [_key release];
  [_restOfKeypath release];
  [_dependentObservers release];
  [_restOfKeypathObserver release];
  [_affectedObservers release];
  [super dealloc];
}

- (BOOL)isRemoved
{
  return _isRemoved;
}

- (void)setIsRemoved: (BOOL)removed
{
  _isRemoved = removed;
}
@end
#pragma endregion

#pragma region Keypath Observer
@interface
_NSKVOKeypathObserver ()
{
  _Atomic(int) _changeDepth;
}
@end

@implementation _NSKVOKeypathObserver
- (instancetype) initWithObject: (id)object
                       observer: (id)observer
                        keyPath: (NSString *)keypath
                        options: (NSKeyValueObservingOptions)options
                        context: (void *)context
{
  if (self = [super init])
    {
      _object = object;
      _observer = observer;
      _keypath = [keypath copy];
      _options = options;
      _context = context;
    }
  return self;
}

- (void) dealloc
{
  [_keypath release];
  [_pendingChange release];
  [super dealloc];
}

- (id) observer
{
  return _observer;
}

- (BOOL) pushWillChange
{
  return atomic_fetch_add(&_changeDepth, 1) == 0;
}

- (BOOL) popDidChange
{
  return atomic_fetch_sub(&_changeDepth, 1) == 1;
}
@end
#pragma endregion

#pragma region Object - level Observation Info
@implementation _NSKVOObservationInfo
- (instancetype) init
{
  if (self = [super init])
    {
      _keyObserverMap = [[NSMutableDictionary alloc] initWithCapacity:1];
      GS_MUTEX_INIT(_lock);
    }
  return self;
}

- (void) dealloc
{
  if (![self isEmpty])
    {
      // We only want to flag for root observers: anything we created internally
      // is fair game to be destroyed.
      for (NSString *key in [_keyObserverMap keyEnumerator])
        {
          for (_NSKVOKeyObserver *keyObserver in
               [_keyObserverMap objectForKey:key])
            {
              if (keyObserver.root)
                {
                  [NSException
                     raise:NSInvalidArgumentException
                    format:
                      @"Object %@ deallocated with observers still registered.",
                      keyObserver.object];
                }
            }
        }
    }
  [_keyObserverMap release];
  [_existingDependentKeys release];

  GS_MUTEX_DESTROY(_lock);

  [super dealloc];
}

- (void) pushDependencyStack
{
  GS_MUTEX_LOCK(_lock);
  if (_dependencyDepth == 0)
    {
      _existingDependentKeys = [NSMutableSet new];
    }
  ++_dependencyDepth;
  GS_MUTEX_UNLOCK(_lock);
}

- (BOOL) lockDependentKeypath: (NSString *)keypath
{
  GS_MUTEX_LOCK(_lock);
  if ([_existingDependentKeys containsObject:keypath])
    {
      GS_MUTEX_UNLOCK(_lock);
      return NO;
    }
  [_existingDependentKeys addObject:keypath];
  GS_MUTEX_UNLOCK(_lock);
  return YES;
}

- (void) popDependencyStack
{
  GS_MUTEX_LOCK(_lock);
  --_dependencyDepth;
  if (_dependencyDepth == 0)
    {
      [_existingDependentKeys release];
      _existingDependentKeys = nil;
    }
  GS_MUTEX_UNLOCK(_lock);
}

- (void) addObserver: (_NSKVOKeyObserver *)observer
{
  NSString       *key = observer.key;
  NSMutableArray *observersForKey = nil;

  GS_MUTEX_LOCK(_lock);
  observersForKey = [_keyObserverMap objectForKey:key];
  if (!observersForKey)
    {
      observersForKey = [NSMutableArray array];
      [_keyObserverMap setObject:observersForKey forKey:key];
    }
  [observersForKey addObject:observer];
  GS_MUTEX_UNLOCK(_lock);
}

- (void) removeObserver: (_NSKVOKeyObserver *)observer
{
  NSString      	*key;
  NSMutableArray	*observersForKey;

  GS_MUTEX_LOCK(_lock);
  key = observer.key;
  observersForKey = [_keyObserverMap objectForKey:key];
  [observersForKey removeObject:observer];
  observer.isRemoved = true;
  if (observersForKey.count == 0)
    {
      [_keyObserverMap removeObjectForKey:key];
    }
  GS_MUTEX_UNLOCK(_lock);
}

- (NSArray *) observersForKey: (NSString *)key
{
  NSArray	*result;

  GS_MUTEX_LOCK(_lock);
  result = [[[_keyObserverMap objectForKey:key] copy] autorelease];
  GS_MUTEX_UNLOCK(_lock);
  return result;
}

- (bool) isEmpty
{
  BOOL result;

  GS_MUTEX_LOCK(_lock);
  result = (_keyObserverMap.count == 0);
  GS_MUTEX_UNLOCK(_lock);
  return result;
}
@end

static _NSKVOObservationInfo *
_createObservationInfoForObject(id object)
{
  _NSKVOObservationInfo *observationInfo = [_NSKVOObservationInfo new];
  [object setObservationInfo:observationInfo];
  [observationInfo release];
  return observationInfo;
}
#pragma endregion

#pragma region Observer / Key Registration
static _NSKVOKeyObserver *
_addKeypathObserver(id object, NSString *keypath,
                    _NSKVOKeypathObserver *keyPathObserver,
                    NSArray               *affectedObservers);
static void
_removeKeyObserver(_NSKVOKeyObserver *keyObserver);

// Add all observers with declared dependencies on this one:
// * All keypaths that could trigger a change (keypaths for values affecting
// us).
// * The head of the remaining keypath.
static void
_addNestedObserversAndOptionallyDependents(_NSKVOKeyObserver *keyObserver,
                                           bool               dependents)
{
  id                     object = keyObserver.object;
  NSString              *key = keyObserver.key;
  _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
  _NSKVOObservationInfo *observationInfo
    = (__bridge _NSKVOObservationInfo *) [object observationInfo]
        ?: _createObservationInfoForObject(object);

  // Aggregate all keys whose values will affect us.
  if (dependents)
    {
      // Make sure to retrieve the underlying class of the observee.
      // This is just [object class] for an NSObject derived class.
      // When observing an object through a proxy, we instead use KVC
      // to optain the underlying class.
      Class cls = [object _underlyingClass];
      NSSet *valueInfluencingKeys = [cls keyPathsForValuesAffectingValueForKey: key];
      if (valueInfluencingKeys.count > 0)
        {
          NSArray 		*affectedKeyObservers;
          NSMutableArray 	*dependentObservers;

          /* affectedKeyObservers is the list of observers that must be notified
           * of changes. If we have descendants, we have to add ourselves to the
           * growing list of affected keys. If not, we must pass it along
           * unmodified. (This is a minor optimization: we don't need to signal
           * for our own reconstruction
           *  if we have no subpath observers.)
	   */
          affectedKeyObservers = (keyObserver.restOfKeypath
	    ? ([keyObserver.affectedObservers arrayByAddingObject:keyObserver]
	    ?: [NSArray arrayWithObject:keyObserver])
	    : keyObserver.affectedObservers);

          [observationInfo pushDependencyStack];
          /* Don't allow our own key to be recreated.
	   */
          [observationInfo lockDependentKeypath:keyObserver.key];

          dependentObservers =
            [NSMutableArray arrayWithCapacity:[valueInfluencingKeys count]];
          for (NSString *dependentKeypath in valueInfluencingKeys)
            {
              if ([observationInfo lockDependentKeypath:dependentKeypath])
                {
                  _NSKVOKeyObserver *dependentObserver
                    = _addKeypathObserver(object, dependentKeypath,
                                          keypathObserver,
                                          affectedKeyObservers);
                  if (dependentObserver)
                    {
                      [dependentObservers addObject:dependentObserver];
                    }
                }
            }
          keyObserver.dependentObservers = dependentObservers;

          [observationInfo popDependencyStack];
        }
    }
  else
    {
      // Our dependents still exist, but their leaves have been pruned. Give
      // them the same treatment as us: recreate their leaves.
      for (_NSKVOKeyObserver *dependentKeyObserver in keyObserver
             .dependentObservers)
        {
          _addNestedObserversAndOptionallyDependents(dependentKeyObserver,
                                                     false);
        }
    }

  // If restOfKeypath is non-nil, we have to chain on further observers.
  if (keyObserver.restOfKeypath && !keyObserver.restOfKeypathObserver)
    {
      keyObserver.restOfKeypathObserver
        = _addKeypathObserver([object valueForKey:key],
                              keyObserver.restOfKeypath, keypathObserver,
                              keyObserver.affectedObservers);
    }

  // Back-propagation of changes.
  // This is where a value-affecting key signals to its dependent that it should
  // be reconstructed.
  for (_NSKVOKeyObserver *affectedObserver in keyObserver.affectedObservers)
    {
      if (!affectedObserver.restOfKeypathObserver)
        {
          affectedObserver.restOfKeypathObserver
            = _addKeypathObserver([affectedObserver.object
                                    valueForKey:affectedObserver.key],
                                  affectedObserver.restOfKeypath,
                                  affectedObserver.keypathObserver,
                                  affectedObserver.affectedObservers);
        }
    }
}

static void
_addKeyObserver(_NSKVOKeyObserver *keyObserver)
{
  _NSKVOObservationInfo	*observationInfo;
  id 			object = keyObserver.object;

  _NSKVOEnsureKeyWillNotify(object, keyObserver.key);
  observationInfo
    = (__bridge _NSKVOObservationInfo *) [object observationInfo]
      ?: _createObservationInfoForObject(object);
  [observationInfo addObserver:keyObserver];
}

static _NSKVOKeyObserver *
_addKeypathObserver(id object, NSString *keypath,
  _NSKVOKeypathObserver *keyPathObserver, NSArray *affectedObservers)
{
  _NSKVOKeyObserver	*keyObserver;
  NSString 		*key;
  NSString 		*restOfKeypath;

  if (!object)
    {
      return nil;
    }
  key = _NSKVCSplitKeypath(keypath, &restOfKeypath);

  keyObserver =
    [[[_NSKVOKeyObserver alloc] initWithObject:object
                               keypathObserver:keyPathObserver
                                           key:key
                                 restOfKeypath:restOfKeypath
                             affectedObservers:affectedObservers] autorelease];

  if (object)
    {
      _addNestedObserversAndOptionallyDependents(keyObserver, true);
      _addKeyObserver(keyObserver);
    }

  return keyObserver;
}
#pragma endregion

#pragma region Observer / Key Deregistration
static void
_removeNestedObserversAndOptionallyDependents(_NSKVOKeyObserver *keyObserver,
  bool dependents)
{
  if (keyObserver.restOfKeypathObserver)
    {
      // Destroy the subpath observer recursively.
      _removeKeyObserver(keyObserver.restOfKeypathObserver);
      keyObserver.restOfKeypathObserver = nil;
    }

  if (dependents)
    {
      // Destroy each observer whose value affects ours, recursively.
      for (_NSKVOKeyObserver *dependentKeyObserver in keyObserver
             .dependentObservers)
        {
          _removeKeyObserver(dependentKeyObserver);
        }

      keyObserver.dependentObservers = nil;
    }
  else
    {
      // Our dependents must be kept alive but pruned.
      for (_NSKVOKeyObserver *dependentKeyObserver in keyObserver
             .dependentObservers)
        {
          _removeNestedObserversAndOptionallyDependents(dependentKeyObserver,
                                                        false);
        }
    }

  if (keyObserver.affectedObservers)
    {
      // Begin to reconstruct each observer that depends on our key's value
      // (triggers in _addDependentAndNestedObservers).
      for (_NSKVOKeyObserver *affectedObserver in keyObserver.affectedObservers)
        {
          _removeKeyObserver(affectedObserver.restOfKeypathObserver);
          affectedObserver.restOfKeypathObserver = nil;
        }
    }
}

static void
_removeKeyObserver(_NSKVOKeyObserver *keyObserver)
{
  _NSKVOObservationInfo *observationInfo;

  if (!keyObserver)
    {
      return;
    }

  observationInfo
    = (_NSKVOObservationInfo *) [keyObserver.object observationInfo];

  [keyObserver retain];

  _removeNestedObserversAndOptionallyDependents(keyObserver, true);

  // These are removed elsewhere; we're probably being cleared as a result of
  // their deletion anyway.
  keyObserver.affectedObservers = nil;

  [observationInfo removeObserver:keyObserver];

  [keyObserver release];
}

static void
_removeKeypathObserver(id object, NSString *keypath, id observer, void *context)
{
  NSString 		*key;
  NSString 		*restOfKeypath;
  _NSKVOObservationInfo	*observationInfo;

  key = _NSKVCSplitKeypath(keypath, &restOfKeypath);

  observationInfo = (_NSKVOObservationInfo *) [object observationInfo];
  for (_NSKVOKeyObserver *keyObserver in [observationInfo observersForKey:key])
    {
      _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;

      if (keypathObserver.observer == observer
        && keypathObserver.object == object
	&& [keypathObserver.keypath isEqual:keypath]
        && (!context || keypathObserver.context == context))
        {
          _removeKeyObserver(keyObserver);
          return;
        }
    }

  [NSException raise: NSInvalidArgumentException
              format: @"Cannot remove observer %@ for keypath \"%@\" from %@"
    @" as it is not a registered observer.",
    observer, keypath, object];
}
#pragma endregion

#pragma region KVO Core Implementation - NSObject category

static const char *const KVO_MAP = "_NSKVOMap";

@implementation
NSObject (NSKeyValueObserving)

+ (void) setKeys: (NSArray *) triggerKeys 
triggerChangeNotificationsForDependentKey: (NSString *) dependentKey
{
  NSMutableDictionary<NSString *, NSSet *> *affectingKeys;
  NSSet *triggerKeySet;

  affectingKeys = objc_getAssociatedObject(self, KVO_MAP);
  if (nil == affectingKeys)
    {
      affectingKeys = [NSMutableDictionary dictionaryWithCapacity: 10];
      objc_setAssociatedObject(self, KVO_MAP, affectingKeys,
                               OBJC_ASSOCIATION_RETAIN);
    }

  triggerKeySet = [NSSet setWithArray: triggerKeys];
  [affectingKeys setValue: triggerKeySet forKey: dependentKey];
}

- (void) observeValueForKeyPath: (NSString *)keyPath
                       ofObject: (id)object
                         change: (NSDictionary<NSString *, id> *)change
                        context: (void *)context
{
  [NSException raise: NSInternalInconsistencyException
              format: @"A key-value observation notification fired, but nobody "
    @"responded to it: object %@, keypath %@, change %@.",
    object, keyPath, change];
}

static void *s_kvoObservationInfoAssociationKey; // has no value; pointer used
                                                 // as an association key.

- (void *) observationInfo
{
  return (__bridge void *)
    objc_getAssociatedObject(self, &s_kvoObservationInfoAssociationKey);
}

- (void) setObservationInfo: (void *)observationInfo
{
  objc_setAssociatedObject(self, &s_kvoObservationInfoAssociationKey,
    (__bridge id) observationInfo,
    OBJC_ASSOCIATION_RETAIN);
}

+ (BOOL) automaticallyNotifiesObserversForKey: (NSString *)key
{
  if ([key length] > 0)
    {
      static const char *const	sc_prefix = "automaticallyNotifiesObserversOf";
      static const size_t     	sc_prefixLength = 32; // strlen(sc_prefix)
      const char               	*rawKey = [key UTF8String];
      size_t                   	keyLength = strlen(rawKey);
      size_t                   	bufferSize = sc_prefixLength + keyLength + 1;
      char                    	*selectorName = (char *) malloc(bufferSize);
      SEL 			sel;

      memcpy(selectorName, sc_prefix, sc_prefixLength);
      selectorName[sc_prefixLength] = toupper(rawKey[0]);
      memcpy(&selectorName[sc_prefixLength + 1], &rawKey[1],
             keyLength); // copy keyLength characters to include terminating
                         // NULL from rawKey
      sel = sel_registerName(selectorName);
      free(selectorName);
      if ([self respondsToSelector:sel])
        {
          return ((BOOL(*)(id, SEL)) objc_msgSend)(self, sel);
        }
    }
  return YES;
}

+ (NSSet *) keyPathsForValuesAffectingValueForKey: (NSString *)key
{
  static NSSet		*emptySet = nil;
  static gs_mutex_t	lock = GS_MUTEX_INIT_STATIC;
  NSUInteger 		keyLength;
  NSDictionary *affectingKeys;

  if (nil == emptySet)
    {
      GS_MUTEX_LOCK(lock);
      if (nil == emptySet)
        {
          emptySet = [[NSSet alloc] init];
          [NSObject leakAt: &emptySet];
        }
      GS_MUTEX_UNLOCK(lock);
    }

  // This function can be a KVO bottleneck, so it will prefer to use c string
  // manipulation when safe
  keyLength = [key length];
  if (keyLength > 0)
    {
      static const char *const sc_prefix = "keyPathsForValuesAffecting";
      static const size_t      sc_prefixLength = 26; // strlen(sc_prefix)
      static const size_t      sc_bufferSize = 128;

      // max length of a key that can guaranteed fit in the char buffer,
      // even if UTF16->UTF8 conversion causes length to double, or a null
      // terminator is needed
      static const size_t sc_safeKeyLength
        = (sc_bufferSize - sc_prefixLength) / 2 - 1; // 50

      const char *rawKey;
      size_t      rawKeyLength;
      SEL         sel;

      rawKey = [key UTF8String];
      rawKeyLength = strlen(rawKey);

      if (keyLength <= sc_safeKeyLength)
        {
          // fast path using c string manipulation, will cover most cases, as
          // most keyPaths are short
          char selectorName[sc_bufferSize];

	  strncpy(selectorName, "keyPathsForValuesAffecting", 26);

          selectorName[sc_prefixLength] = toupper(rawKey[0]);
          // Copy the rest of the key, including the null terminator
          memcpy(&selectorName[sc_prefixLength + 1], &rawKey[1], rawKeyLength);
          sel = sel_registerName(selectorName);
        }
      else // Guaranteed path for long keyPaths
        {
          size_t keyLength;
          size_t bufferSize;
          char  *selectorName;

          keyLength = strlen(rawKey);
          bufferSize = sc_prefixLength + keyLength + 1;
          selectorName = (char *) malloc(bufferSize);
          memcpy(selectorName, sc_prefix, sc_prefixLength);

          selectorName[sc_prefixLength] = toupper(rawKey[0]);
          // Copy the rest of the key, including the null terminator
          memcpy(&selectorName[sc_prefixLength + 1], &rawKey[1], keyLength);

          sel = sel_registerName(selectorName);
          free(selectorName);
        }

      if ([self respondsToSelector:sel])
        {
          return [self performSelector:sel];
        }

      // We compute an NSSet from information provided by previous invocations
      // of the now-deprecated setKeys:triggerChangeNotificationsForDependentKey:
      // if the original imp returns an empty set.
      // This aligns with Apple's backwards compatibility.
      affectingKeys = (NSDictionary *)objc_getAssociatedObject(self, KVO_MAP);
      if (unlikely(nil != affectingKeys))
        {
          NSSet *set = [affectingKeys objectForKey:key];
          if (set != nil)
            {
            return set;
            }
        }
    }
  return emptySet;
}

- (void) addObserver: (id)observer
          forKeyPath: (NSString *)keyPath
             options: (NSKeyValueObservingOptions)options
             context: (void *)context
{
  _NSKVOKeypathObserver *keypathObserver =
    [[[_NSKVOKeypathObserver alloc] initWithObject:self
                                          observer:observer
                                           keyPath:keyPath
                                           options:options
                                           context:context] autorelease];
  _NSKVOKeyObserver *rootObserver
    = _addKeypathObserver(self, keyPath, keypathObserver, nil);
  rootObserver.root = true;

  if ((options & NSKeyValueObservingOptionInitial))
    {
      NSMutableDictionary *change = [NSMutableDictionary
        dictionaryWithObjectsAndKeys:@(NSKeyValueChangeSetting),
                                     NSKeyValueChangeKindKey, nil];

      if ((options & NSKeyValueObservingOptionNew))
        {
          id newValue = [self valueForKeyPath:keyPath] ?: [NSNull null];
          [change setObject:newValue forKey:NSKeyValueChangeNewKey];
        }

      [observer observeValueForKeyPath:keyPath
                              ofObject:self
                                change:change
                               context:context];
    }
}

- (void) removeObserver: (id)observer
             forKeyPath: (NSString *)keyPath
                context: (void *)context
{
  _NSKVOObservationInfo *observationInfo;

  _removeKeypathObserver(self, keyPath, observer, context);
  observationInfo = (__bridge _NSKVOObservationInfo *) [self observationInfo];
  if ([observationInfo isEmpty])
    {
      // TODO: was nullptr prior
      [self setObservationInfo:nil];
    }
}

- (void) removeObserver: (id)observer forKeyPath:(NSString *)keyPath
{
  [self removeObserver:observer forKeyPath:keyPath context:NULL];
}

// Reference platform does not provide the Set Mutation Kind in the changes
// dictionary, just shows which elements were inserted/removed/replaced
static inline NSKeyValueChange
_changeFromSetMutationKind(NSKeyValueSetMutationKind kind)
{
  switch (kind)
    {
    case NSKeyValueUnionSetMutation:
      return NSKeyValueChangeInsertion;
    case NSKeyValueMinusSetMutation:
    case NSKeyValueIntersectSetMutation:
      return NSKeyValueChangeRemoval;
    default:
      return NSKeyValueChangeReplacement;
    }
}

static inline id
_valueForPendingChangeAtIndexes(id notifyingObject, NSString *key,
                                NSString *keypath, id rootObject,
                                _NSKVOKeyObserver *keyObserver,
                                NSDictionary      *pendingChange)
{
  id          value = nil;
  NSIndexSet *indexes = pendingChange[NSKeyValueChangeIndexesKey];
  if (indexes)
    {
      NSArray  *collection = [notifyingObject valueForKey:key];
      NSString *restOfKeypath = keyObserver.restOfKeypath;
      value = restOfKeypath.length > 0
                ? [collection valueForKeyPath:restOfKeypath]
                : collection;
      if ([value respondsToSelector:@selector(objectsAtIndexes:)])
        {
          value = [value objectsAtIndexes:indexes];
        }
    }
  else
    {
      value = [rootObject valueForKeyPath:keypath];
    }

  return value ?: [NSNull null];
}

// void TFunc(_NSKVOKeyObserver* keyObserver);
inline static void
_dispatchWillChange(id notifyingObject, NSString *key,
                    DispatchChangeBlock block)
{
  _NSKVOObservationInfo *observationInfo
    = (__bridge _NSKVOObservationInfo *) [notifyingObject observationInfo];
  for (_NSKVOKeyObserver *keyObserver in [observationInfo observersForKey:key])
    {
      _NSKVOKeypathObserver *keypathObserver;

      if (keyObserver.isRemoved)
        {
          continue;
        }

      // Skip any keypaths that are in the process of changing.
      keypathObserver = keyObserver.keypathObserver;
      if ([keypathObserver pushWillChange])
        {
          NSKeyValueObservingOptions options;

          // Call into the lambda function, which will do the actual set-up for
          // pendingChanges
          block(keyObserver);

          options = keypathObserver.options;
          if (options & NSKeyValueObservingOptionPrior)
            {
              NSMutableDictionary *change = keypathObserver.pendingChange;

              [change setObject:@(YES)
                         forKey:NSKeyValueChangeNotificationIsPriorKey];
              [keypathObserver.observer
                observeValueForKeyPath:keypathObserver.keypath
                              ofObject:keypathObserver.object
                                change:change
                               context:keypathObserver.context];
              [change
                removeObjectForKey:NSKeyValueChangeNotificationIsPriorKey];
            }
        }

      // This must happen regardless of whether we are currently notifying.
      _removeNestedObserversAndOptionallyDependents(keyObserver, false);
    }
}

static void
_dispatchDidChange(id notifyingObject, NSString *key, DispatchChangeBlock block)
{
  _NSKVOObservationInfo *observationInfo
    = (__bridge _NSKVOObservationInfo *) [notifyingObject observationInfo];
  NSArray<_NSKVOKeyObserver *> *observers =
    [observationInfo observersForKey:key];
  for (_NSKVOKeyObserver *keyObserver in [observers reverseObjectEnumerator])
    {
      _NSKVOKeypathObserver *keypathObserver;

      if (keyObserver.isRemoved)
        {
          continue;
        }

      // This must happen regardless of whether we are currently notifying.
      _addNestedObserversAndOptionallyDependents(keyObserver, false);

      // Skip any keypaths that are in the process of changing.
      keypathObserver = keyObserver.keypathObserver;
      if ([keypathObserver popDidChange])
        {
          id			observer;
          NSString            	*keypath;
          id                   	rootObject;
          NSMutableDictionary 	*change;
          void                	*context;

          // Call into lambda, which will do set-up for finalizing changes
          // dictionary
          block(keyObserver);

          observer = keypathObserver.observer;
          keypath = keypathObserver.keypath;
          rootObject = keypathObserver.object;
          change = keypathObserver.pendingChange;
          context = keypathObserver.context;
          [observer observeValueForKeyPath:keypath
                                  ofObject:rootObject
                                    change:change
                                   context:context];
          keypathObserver.pendingChange = nil;
        }
    }
}

- (void) willChangeValueForKey: (NSString *)key
{
  if ([self observationInfo])
    {
      _dispatchWillChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        NSMutableDictionary *change =
          [NSMutableDictionary dictionaryWithObject:@(NSKeyValueChangeSetting)
                                             forKey:NSKeyValueChangeKindKey];
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueObservingOptions options = keypathObserver.options;

        if (options & NSKeyValueObservingOptionOld)
          {
            // For to-many mutations, we can't get the old values at indexes
            // that have not yet been inserted.
            id        rootObject = keypathObserver.object;
            NSString *keypath = keypathObserver.keypath;
            id oldValue = [rootObject valueForKeyPath:keypath] ?: [NSNull null];
            change[NSKeyValueChangeOldKey] = oldValue;
          }

        keypathObserver.pendingChange = change;
      });
    }
}

- (void) didChangeValueForKey: (NSString *)key
{
  if ([self observationInfo])
    {
      _dispatchDidChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueObservingOptions options = keypathObserver.options;
        NSMutableDictionary       *change = keypathObserver.pendingChange;
        if ((options & NSKeyValueObservingOptionNew) &&
            [change[NSKeyValueChangeKindKey] integerValue]
              != NSKeyValueChangeRemoval)
          {
            NSString *keypath = keypathObserver.keypath;
            id        rootObject = keypathObserver.object;
            id newValue = [rootObject valueForKeyPath:keypath] ?: [NSNull null];

            change[NSKeyValueChangeNewKey] = newValue;
          }
      });
    }
}

- (void) willChange: (NSKeyValueChange)changeKind
    valuesAtIndexes: (NSIndexSet *)indexes
             forKey: (NSString *)key
{
  __block NSKeyValueChange kind = changeKind;
  if ([self observationInfo])
    {
      _dispatchWillChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        NSMutableDictionary   *change = [NSMutableDictionary
          dictionaryWithObjectsAndKeys:@(kind), NSKeyValueChangeKindKey,
                                       indexes, NSKeyValueChangeIndexesKey,
                                       nil];
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueObservingOptions options = keypathObserver.options;
        id                         rootObject = keypathObserver.object;

        // The reference platform does not support to-many mutations on nested
        // keypaths. We have to treat them as to-one mutations to support
        // aggregate functions.
        if (kind != NSKeyValueChangeSetting
            && keyObserver.restOfKeypathObserver)
          {
            // This only needs to be done in willChange because didChange
            // derives from the existing changeset.
            change[NSKeyValueChangeKindKey] = @(kind = NSKeyValueChangeSetting);

            // Make change Old/New values the entire collection rather than a
            // to-many change with objectsAtIndexes:
            [change removeObjectForKey:NSKeyValueChangeIndexesKey];
          }

        if ((options & NSKeyValueObservingOptionOld)
            && kind != NSKeyValueChangeInsertion)
          {
            // For to-many mutations, we can't get the old values at indexes
            // that have not yet been inserted.
            NSString *keypath = keypathObserver.keypath;
            change[NSKeyValueChangeOldKey]
              = _valueForPendingChangeAtIndexes(self, key, keypath, rootObject,
                                                keyObserver, change);
          }

        keypathObserver.pendingChange = change;
      });
    }
}

- (void) didChange: (NSKeyValueChange)changeKind
   valuesAtIndexes: (NSIndexSet *)indexes
            forKey: (NSString *)key
{
  if ([self observationInfo])
    {
      _dispatchDidChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueObservingOptions options = keypathObserver.options;
        NSMutableDictionary       *change = keypathObserver.pendingChange;
        if ((options & NSKeyValueObservingOptionNew) &&
            [change[NSKeyValueChangeKindKey] integerValue]
              != NSKeyValueChangeRemoval)
          {
            // For to-many mutations, we can't get the new values at indexes
            // that have been deleted.
            id        rootObject = keypathObserver.object;
            NSString *keypath = keypathObserver.keypath;
            id        newValue
              = _valueForPendingChangeAtIndexes(self, key, keypath, rootObject,
                                                keyObserver, change);

            change[NSKeyValueChangeNewKey] = newValue;
          }
      });
    }
}

// Need to know the previous value for the set if we need to find the values
// added
static const NSString *_NSKeyValueChangeOldSetValue
  = @"_NSKeyValueChangeOldSetValue";

- (void)willChangeValueForKey: (NSString *)key
              withSetMutation: (NSKeyValueSetMutationKind)mutationKind
                 usingObjects: (NSSet *)objects
{
  if ([self observationInfo])
    {
      NSKeyValueChange changeKind = _changeFromSetMutationKind(mutationKind);
      _dispatchWillChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        NSMutableDictionary *change =
          [NSMutableDictionary dictionaryWithObject:@(changeKind)
                                             forKey:NSKeyValueChangeKindKey];
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueObservingOptions options = keypathObserver.options;
        id                         rootObject = keypathObserver.object;
        NSString                  *keypath = keypathObserver.keypath;

        NSSet *oldValues = [rootObject valueForKeyPath:keypath];
        if ((options & NSKeyValueObservingOptionOld)
            && changeKind != NSKeyValueChangeInsertion)
          {
            // The old value should only contain values which are removed from
            // the original dictionary
            switch (mutationKind)
              {
              case NSKeyValueMinusSetMutation:
                // The only objects which were removed are those both in
                // oldValues and objects
                change[NSKeyValueChangeOldKey] =
                  [oldValues objectsPassingTest:^(id obj, BOOL *stop) {
                    return [objects containsObject:obj];
                  }];
                break;
              case NSKeyValueIntersectSetMutation:
              case NSKeyValueSetSetMutation:
              default:
                // The only objects which were removed are those in oldValues
                // and NOT in objects
                change[NSKeyValueChangeOldKey] =
                  [oldValues objectsPassingTest:^BOOL(id obj, BOOL *stop) {
                    return [objects member:obj] ? NO : YES;
                  }];
                break;
              }
          }

        if (options & NSKeyValueObservingOptionNew)
          {
            // Save old value in change dictionary for
            // didChangeValueForKey:withSetMutation:usingObjects: to use for
            // determining added objects Only needed if observer wants New
            // value
            change[_NSKeyValueChangeOldSetValue] =
              [[oldValues copy] autorelease];
          }

        keypathObserver.pendingChange = change;
      });
    }
}

- (void)didChangeValueForKey: (NSString *)key
             withSetMutation: (NSKeyValueSetMutationKind)mutationKind
                usingObjects: (NSSet *)objects
{
  if ([self observationInfo])
    {
      _dispatchDidChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueChange changeKind = _changeFromSetMutationKind(mutationKind);
        NSKeyValueObservingOptions options = keypathObserver.options;

        if ((options & NSKeyValueObservingOptionNew)
            && changeKind != NSKeyValueChangeRemoval)
          {
            // New values only exist for inserting or replacing, not removing
            NSMutableDictionary *change = keypathObserver.pendingChange;
            NSSet *oldValues = change[_NSKeyValueChangeOldSetValue];
            // The new value should only contain values which are added to the
            // original set The only objects added are those in objects but
            // NOT in oldValues
            NSSet *newValue =
              [objects objectsPassingTest:^BOOL(id obj, BOOL *stop) {
                return [oldValues member:obj] ? NO : YES;
              }];

            change[NSKeyValueChangeNewKey] = newValue;
            [change removeObjectForKey:_NSKeyValueChangeOldSetValue];
          }
      });
    }
}
@end

#pragma endregion

#pragma region KVO Core Implementation - Private Access

@implementation
NSObject (NSKeyValueObservingPrivate)

- (Class)_underlyingClass
{
  return [self class];
}

- (void)_notifyObserversOfChangeForKey: (NSString *)key
                              oldValue: (id)oldValue
                              newValue: (id)newValue
{
  if ([self observationInfo])
    {
      _dispatchWillChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        NSMutableDictionary *change =
          [NSMutableDictionary dictionaryWithObject:@(NSKeyValueChangeSetting)
                                             forKey:NSKeyValueChangeKindKey];
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueObservingOptions options = keypathObserver.options;

        if (options & NSKeyValueObservingOptionOld)
          {
            change[NSKeyValueChangeOldKey] = oldValue ? oldValue : [NSNull null];
          }

        keypathObserver.pendingChange = change;
      });
      _dispatchDidChange(self, key, ^(_NSKVOKeyObserver *keyObserver) {
        _NSKVOKeypathObserver *keypathObserver = keyObserver.keypathObserver;
        NSKeyValueObservingOptions options = keypathObserver.options;
        NSMutableDictionary       *change = keypathObserver.pendingChange;
        if ((options & NSKeyValueObservingOptionNew) &&
            [change[NSKeyValueChangeKindKey] integerValue]
              != NSKeyValueChangeRemoval)
          {
            change[NSKeyValueChangeNewKey] = newValue ? newValue : [NSNull null];
          }
      });
    }
}

@end

#pragma endregion

#pragma region KVO Core Implementation - NSArray category

@implementation
NSArray (NSKeyValueObserving)

- (void)addObserver: (id)observer
         forKeyPath: (NSString *)keyPath
            options: (NSKeyValueObservingOptions)options
            context: (void *)context
{
  NS_COLLECTION_THROW_ILLEGAL_KVO(keyPath);
}

- (void)removeObserver: (id)observer
            forKeyPath: (NSString *)keyPath
               context: (void *)context
{
  NS_COLLECTION_THROW_ILLEGAL_KVO(keyPath);
}

- (void)removeObserver: (id)observer forKeyPath:(NSString *)keyPath
{
  NS_COLLECTION_THROW_ILLEGAL_KVO(keyPath);
}

- (void)addObserver: (id)observer
  toObjectsAtIndexes: (NSIndexSet *)indexes
          forKeyPath: (NSString *)keyPath
             options: (NSKeyValueObservingOptions)options
             context: (void *)context
{
  [indexes enumerateIndexesUsingBlock:^(NSUInteger index, BOOL *stop) {
    [[self objectAtIndex:index] addObserver:observer
                                 forKeyPath:keyPath
                                    options:options
                                    context:context];
  }];
}

- (void)removeObserver: (id)observer
  fromObjectsAtIndexes: (NSIndexSet *)indexes
            forKeyPath: (NSString *)keyPath
               context: (void *)context
{
  [indexes enumerateIndexesUsingBlock:^(NSUInteger index, BOOL *stop) {
    [[self objectAtIndex:index] removeObserver:observer
                                    forKeyPath:keyPath
                                       context:context];
  }];
}

- (void)removeObserver: (NSObject *)observer
  fromObjectsAtIndexes: (NSIndexSet *)indexes
            forKeyPath: (NSString *)keyPath
{
  [self removeObserver:observer
    fromObjectsAtIndexes:indexes
              forKeyPath:keyPath
                 context:NULL];
}

@end

#pragma endregion

#pragma region KVO Core Implementation - NSSet category

@implementation
NSSet (NSKeyValueObserving)

- (void)addObserver: (id)observer
         forKeyPath: (NSString *)keyPath
            options: (NSKeyValueObservingOptions)options
            context: (void *)context
{
  NS_COLLECTION_THROW_ILLEGAL_KVO(keyPath);
}

- (void)removeObserver: (id)observer
            forKeyPath: (NSString *)keyPath
               context: (void *)context
{
  NS_COLLECTION_THROW_ILLEGAL_KVO(keyPath);
}

- (void)removeObserver: (id)observer forKeyPath:(NSString *)keyPath
{
  NS_COLLECTION_THROW_ILLEGAL_KVO(keyPath);
}

@end

#pragma endregion

#pragma region KVO forwarding - NSProxy category

@implementation
NSProxy (NSKeyValueObserving)

- (Class)_underlyingClass
{
  // Retrieve the underlying class via KVC
  // Note that we assume that the class is KVC-compliant, when KVO is used
  return [(NSObject *)self valueForKey: @"class"];
}

@end

#pragma endregion