bloom-filters/src/hashing/deprecated_hashing.ts

JS implementation of probabilistic data structures: Bloom Filter (and its derived), HyperLogLog, Count-Min Sketch, Top-K and MinHash

import {getDefaultSeed, HashableInput} from '../utils'
import Hashing from './hashing'

/**
 * @deprecated
 * Hashing class to use before v1.3.7
 */
export default class DeprecatedHashing extends Hashing {
  /**
   * Apply Double Hashing to produce a n-hash
   * @param  n - The indice of the hash function we want to produce
   * @param  hashA - The result of the first hash function applied to a value.
   * @param  hashB - The result of the second hash function applied to a value.
   * @param  size - The size of the datastructures associated to the hash context (ex: the size of a Bloom Filter)
   * @return The result of hash_n applied to a value.
   * @returns
   */
  public doubleHashing = (
    n: number,
    hashA: number,
    hashB: number,
    size: number
  ): number => {
    return Math.abs((hashA + n * hashB) % size)
  }

  /**
   * Generate N indexes on range [0, size)
   * It uses the double hashing technique to generate the indexes.
   * It hash twice the value only once before generating the indexes.
   * Warning: you can have a lot of modulo collisions.
   * @param  element    - The element to hash
   * @param  size       - The range on which we can generate the index, exclusive
   * @param  hashCount  - The number of indexes we want
   * @return An array of indexes on range [0, size)
   */
  public getIndexes(
    element: HashableInput,
    size: number,
    hashCount: number,
    seed?: number
  ) {
    return this.getDistinctIndexes(element, size, hashCount, seed)
  }

  /**
   * Generate a set of distinct indexes on interval [0, size) using the double hashing technique
   * This function is the old method called by a lot of filters.
   * To work in the current version, replace, the getIndexes function of the filters by this one
   * @param  element  - The element to hash
   * @param  size     - the range on which we can generate an index [0, size) = size
   * @param  number   - The number of indexes desired
   * @param  seed     - The seed used
   * @return A array of indexes
   * @author Arnaud Grall
   */
  public getDistinctIndexes(
    element: HashableInput,
    size: number,
    number: number,
    seed?: number
  ): Array<number> {
    if (seed === undefined) {
      seed = getDefaultSeed()
    }

    const getDistinctIndicesBis = (
      n: number,
      elem: HashableInput,
      size: number,
      count: number,
      indexes: Array<number> = []
    ): Array<number> => {
      if (indexes.length === count) {
        return indexes
      } else {
        const hashes = this.hashTwice(elem, seed! + (size % n)) // eslint-disable-line @typescript-eslint/no-non-null-assertion
        const ind = this.doubleHashing(n, hashes.first, hashes.second, size)
        if (indexes.includes(ind)) {
          // console.log('generate index: %d for %s', ind, elem)
          return getDistinctIndicesBis(n + 1, elem, size, count, indexes)
        } else {
          // console.log('already found: %d for %s', ind, elem)
          indexes.push(ind)
          return getDistinctIndicesBis(n + 1, elem, size, count, indexes)
        }
      }
    }
    return getDistinctIndicesBis(1, element, size, number)
  }
}