import { MultiSetCounter } from '../index'

/**
 * Implement this interface to support the use of different hash functions. Note that
 * your backend will need to be registered using `HasherFactory.register(..)` before
 * it can be used.
 */
export interface Hasher<V> {

  /**
   * Generate hash of the provided string.
   */
  hash(val: string): V

  /**
   * Returns the number of consecutive zeros in the provided hash value.
   * @param hashVal The hash value as computed by the `hash(..)` method.
   */
  runLength(hashVal: V): number

  /**
   * Determine the register index for a given hash value.
   * @param hashVal The hash value as computed by the `hash(..)` method.
   */
  regIdx(hashVal: V): number

  /**
   * Returns the length, in bits, of hash values generated by this `Hasher`.
   */
  readonly hashLen: number

  /**
   * The unique name of this `Hasher` implementation. Used to ensure counters 
   * are compatible before merging, as well as to instantiate the proper backend
   * when deserializing.
   */
  readonly hasherId: string

  /** Maximum number of register bits supported by a `Hasher` implementation */
  readonly maxPrecision: number
}

interface HasherFactoryImpl {
  new(counter: MultiSetCounter): Hasher<any>
}

export class HasherFactory {
  private static factories: Map<string, HasherFactoryImpl> = new Map()

  /**
   * Used to register custom backends so that they may be instantiated given their unique id
   * (see `Options.hasherId`). See the `Jenkins32` class for an example of how to register your backend.
   */
  static register(hasherId: string, f: HasherFactoryImpl) {
    if (HasherFactory.factories.has(hasherId))
      console.warn(`[HyperLogLog] HasherFactory '${hasherId}' is already registered and will be overwritten`)
    
    this.factories.set(hasherId, f)
  }

  /**
   * Builds a `Hasher` instance given it's `hasherId` and the counter it will be used with. This is
   * used internally by `HyperLogLog` during construction and deserialization.
   */
  static build(hasherId: string, counter: MultiSetCounter): Hasher<any> {
    if (!HasherFactory.factories.has(hasherId))
      console.warn(`[HyperLogLog] HasherFactory '${hasherId}' doesn't exist or hasn't been registered with the HasherFactory! See HasherFactory.register(..)`)

    const ctor = HasherFactory.factories.get(hasherId)
    return new ctor(counter)
  }
}