bloom-filters/src/bloom/bit-set.ts

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

/* file : BitSet.ts
MIT License

Copyright (c) 2021 David Leppik

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

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.
*/

import {encode, decode} from 'base64-arraybuffer'

const bitsPerWord = 8

/**
 * A memory-efficient Boolean array. Contains just the minimal operations needed for our Bloom filter implementation.
 *
 * @author David Leppik
 */
export default class BitSet {
  public readonly size: number

  // Uint32Array may be slightly faster due to memory alignment, but this avoids endianness when serializing
  public array: Uint8Array

  /**
   * Constructor. All bits are initially set to false.
   * @param size the number of bits that can be stored. (This is NOT required to be a multiple of 8.)
   */
  constructor(size: number) {
    const diff = bitsPerWord - (size % bitsPerWord)
    this.size = size + ([0, 8].includes(diff) ? 0 : diff)
    this.array = new Uint8Array(Math.ceil(this.size / bitsPerWord))
  }

  /**
   * Returns the value of the bit at the given index
   * @param index position of the bit, zero-indexed
   */
  public has(index: number): boolean {
    const wordIndex = Math.floor(index / bitsPerWord)
    const mask = 1 << index % bitsPerWord
    return (this.array[wordIndex] & mask) !== 0
  }

  /**
   * Set the bit to true
   * @param index position of the bit, zero-indexed
   */
  public add(index: number) {
    const wordIndex = Math.floor(index / bitsPerWord)
    const mask = 1 << index % bitsPerWord
    this.array[wordIndex] = this.array[wordIndex] | mask
  }

  /**
   * Returns the maximum true bit.
   */
  public max(): number {
    for (let i = this.array.length - 1; i >= 0; i--) {
      const bits = this.array[i]
      if (bits) {
        return BitSet.highBit(bits) + i * bitsPerWord
      }
    }
    return 0
  }

  /**
   * Returns the number of true bits.
   */
  public bitCount(): number {
    let result = 0
    for (let i = 0; i < this.array.length; i++) {
      result += BitSet.countBits(this.array[i]) // Assumes we never have bits set beyond the end
    }
    return result
  }

  /**
   * Returns true if the size and contents are identical.
   * @param other another BitSet
   */
  public equals(other: BitSet): boolean {
    if (other.size !== this.size) {
      return false
    }
    for (let i = 0; i < this.array.length; i++) {
      if (this.array[i] !== other.array[i]) {
        return false
      }
    }
    return true
  }

  /**
   * Returns a JSON-encodable object readable by {@link import}.
   */
  public export(): {size: number; content: string} {
    return {
      size: this.size,
      content: encode(this.array),
    }
  }

  /**
   * Returns an object written by {@link export}.
   * @param data an object written by {@link export}
   */
  public static import(data: {size: number; content: string}): BitSet {
    if (typeof data.size !== 'number') {
      throw Error('BitSet missing size')
    }
    if (typeof data.content !== 'string') {
      throw Error('BitSet: missing content')
    }
    const result = new BitSet(data.size)
    const buffer = decode(data.content)
    result.array = new Uint8Array(buffer)
    return result
  }

  /**
   * Returns the index of the maximum bit in the number, or -1 for 0
   * @bits an unsigned 8-bit number
   * ```js
   * @example
   * BitSet.highBit(0) // returns -1
   * BitSet.highBit(5) // returns 2
   * ```
   */
  public static highBit(bits: number): number {
    let result = bitsPerWord - 1
    let mask = 1 << result
    while (result >= 0 && (mask & bits) !== mask) {
      mask >>>= 1
      result--
    }
    return result
  }

  /**
   * Returns the number of true bits in the number
   * @bits an unsigned 8-bit number
   * @example
   * ```js
   * BitSet.countBits(0) // returns 0
   * BitSet.countBits(3) // returns 2
   * ```
   */
  public static countBits(bits: number): number {
    let result = bits & 1
    while (bits !== 0) {
      bits = bits >>> 1
      result += bits & 1
    }
    return result
  }
}