index.js

'use strict'

// A linked list to keep track of recently-used-ness
const Yallist = require('yallist')

const MAX = Symbol('max')
const LENGTH = Symbol('length')
const LENGTH_CALCULATOR = Symbol('lengthCalculator')
const ALLOW_STALE = Symbol('allowStale')
const MAX_AGE = Symbol('maxAge')
const DISPOSE = Symbol('dispose')
const NO_DISPOSE_ON_SET = Symbol('noDisposeOnSet')
const LRU_LIST = Symbol('lruList')
const CACHE = Symbol('cache')
const UPDATE_AGE_ON_GET = Symbol('updateAgeOnGet')

const naiveLength = () => 1

// lruList is a yallist where the head is the youngest
// item, and the tail is the oldest.  the list contains the Hit
// objects as the entries.
// Each Hit object has a reference to its Yallist.Node.  This
// never changes.
//
// cache is a Map (or PseudoMap) that matches the keys to
// the Yallist.Node object.
class LRUCache {
  constructor (options) {
    if (typeof options === 'number')
      options = { max: options }

    if (!options)
      options = {}

    if (options.max && (typeof options.max !== 'number' || options.max < 0))
      throw new TypeError('max must be a non-negative number')
    // Kind of weird to have a default max of Infinity, but oh well.
    const max = this[MAX] = options.max || Infinity

    const lc = options.length || naiveLength
    this[LENGTH_CALCULATOR] = (typeof lc !== 'function') ? naiveLength : lc
    this[ALLOW_STALE] = options.stale || false
    if (options.maxAge && typeof options.maxAge !== 'number')
      throw new TypeError('maxAge must be a number')
    this[MAX_AGE] = options.maxAge || 0
    this[DISPOSE] = options.dispose
    this[NO_DISPOSE_ON_SET] = options.noDisposeOnSet || false
    this[UPDATE_AGE_ON_GET] = options.updateAgeOnGet || false
    this.reset()
  }

  // resize the cache when the max changes.
  set max (mL) {
    if (typeof mL !== 'number' || mL < 0)
      throw new TypeError('max must be a non-negative number')

    this[MAX] = mL || Infinity
    trim(this)
  }
  get max () {
    return this[MAX]
  }

  set allowStale (allowStale) {
    this[ALLOW_STALE] = !!allowStale
  }
  get allowStale () {
    return this[ALLOW_STALE]
  }