All files / core target_id_generator.ts

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/**
 * Copyright 2017 Google Inc.
 *
 * 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 { TargetId } from './types';
 
const RESERVED_BITS = 1;
 
enum GeneratorIds {
  LocalStore = 0,
  SyncEngine = 1
}
 
/**
 * TargetIdGenerator generates monotonically increasing integer IDs. There are
 * separate generators for different scopes. While these generators will operate
 * independently of each other, they are scoped, such that no two generators
 * will ever produce the same ID. This is useful, because sometimes the backend
 * may group IDs from separate parts of the client into the same ID space.
 */
export class TargetIdGenerator {
  private previousId: TargetId;
 
  constructor(private generatorId: number, initAfter: TargetId = 0) {
    // Replace the generator part of initAfter with this generator's ID.
    const afterWithoutGenerator = (initAfter >> RESERVED_BITS) << RESERVED_BITS;
    const afterGenerator = initAfter - afterWithoutGenerator;
    if (afterGenerator >= generatorId) {
      // For example, if:
      //   this.generatorId = 0b0000
      //   after = 0b1011
      //   afterGenerator = 0b0001
      // Then:
      //   previous = 0b1010
      //   next = 0b1100
      this.previousId = afterWithoutGenerator | this.generatorId;
    } else {
      // For example, if:
      //   this.generatorId = 0b0001
      //   after = 0b1010
      //   afterGenerator = 0b0000
      // Then:
      //   previous = 0b1001
      //   next = 0b1011
      this.previousId =
        (afterWithoutGenerator | this.generatorId) - (1 << RESERVED_BITS);
    }
  }
 
  next(): TargetId {
    this.previousId += 1 << RESERVED_BITS;
    return this.previousId;
  }
 
  static forLocalStore(initAfter: TargetId = 0): TargetIdGenerator {
    return new TargetIdGenerator(GeneratorIds.LocalStore, initAfter);
  }
 
  static forSyncEngine(): TargetIdGenerator {
    return new TargetIdGenerator(GeneratorIds.SyncEngine);
  }
}