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/*
* Copyright 2017 Google
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#import "Firestore/Source/Core/FSTTargetIDGenerator.h"
#import <libkern/OSAtomic.h>
NS_ASSUME_NONNULL_BEGIN
#pragma mark - FSTTargetIDGenerator
static const int kReservedBits = 1;
/** FSTTargetIDGeneratorID is the set of all valid generators. */
typedef NS_ENUM(NSInteger, FSTTargetIDGeneratorID) {
FSTTargetIDGeneratorIDLocalStore = 0,
FSTTargetIDGeneratorIDSyncEngine = 1
};
@interface FSTTargetIDGenerator () {
// This is volatile so it can be used with OSAtomicAdd32.
volatile FSTTargetID _previousID;
}
/**
* Initializes the generator.
*
* @param generatorID A unique ID indicating which generator this is.
* @param after Every call to nextID will return a number > @a after.
*/
- (instancetype)initWithGeneratorID:(FSTTargetIDGeneratorID)generatorID
startingAfterID:(FSTTargetID)after NS_DESIGNATED_INITIALIZER;
// This is typed as FSTTargetID because we need to do bitwise operations with them together.
@property(nonatomic, assign) FSTTargetID generatorID;
@end
@implementation FSTTargetIDGenerator
#pragma mark - Constructors
- (instancetype)initWithGeneratorID:(FSTTargetIDGeneratorID)generatorID
startingAfterID:(FSTTargetID)after {
self = [super init];
if (self) {
_generatorID = generatorID;
// Replace the generator part of |after| with this generator's ID.
FSTTargetID afterWithoutGenerator = (after >> kReservedBits) << kReservedBits;
FSTTargetID afterGenerator = after - afterWithoutGenerator;
if (afterGenerator >= _generatorID) {
// For example, if:
// self.generatorID = 0b0000
// after = 0b1011
// afterGenerator = 0b0001
// Then:
// previous = 0b1010
// next = 0b1100
_previousID = afterWithoutGenerator | self.generatorID;
} else {
// For example, if:
// self.generatorID = 0b0001
// after = 0b1010
// afterGenerator = 0b0000
// Then:
// previous = 0b1001
// next = 0b1011
_previousID = (afterWithoutGenerator | self.generatorID) - (1 << kReservedBits);
}
}
return self;
}
+ (instancetype)generatorForLocalStoreStartingAfterID:(FSTTargetID)after {
return [[FSTTargetIDGenerator alloc] initWithGeneratorID:FSTTargetIDGeneratorIDLocalStore
startingAfterID:after];
}
+ (instancetype)generatorForSyncEngineStartingAfterID:(FSTTargetID)after {
return [[FSTTargetIDGenerator alloc] initWithGeneratorID:FSTTargetIDGeneratorIDSyncEngine
startingAfterID:after];
}
#pragma mark - Public methods
- (FSTTargetID)nextID {
return OSAtomicAdd32(1 << kReservedBits, &_previousID);
}
@end
NS_ASSUME_NONNULL_END
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