// Component instance cache for state.cache().
// LRU ported from nanolru 1.0.0 (MIT) — https://github.com/s3ththompson/nanolru
// Cache wrapper ported from choo 7.1.0 component/cache.js (MIT)

import { ok, equal } from './assert.js'

export class LRU {
  constructor (opts) {
    if (typeof opts === 'number') opts = { max: opts }
    if (!opts) opts = {}
    this.cache = {}
    this.head = this.tail = null
    this.length = 0
    this.max = opts.max || 1000
  }

  get keys () {
    return Object.keys(this.cache)
  }

  clear () {
    this.cache = {}
    this.head = this.tail = null
    this.length = 0
  }

  remove (key) {
    if (typeof key !== 'string') key = '' + key
    if (!Object.hasOwn(this.cache, key)) return

    const element = this.cache[key]
    delete this.cache[key]
    this._unlink(key, element.prev, element.next)
    return element.value
  }

  _unlink (key, prev, next) {
    this.length--

    if (this.length === 0) {
      this.head = this.tail = null
    } else {
      if (this.head === key) {
        this.head = prev
        this.cache[this.head].next = null
      } else if (this.tail === key) {
        this.tail = next
        this.cache[this.tail].prev = null
      } else {
        this.cache[prev].next = next
        this.cache[next].prev = prev
      }
    }
  }

  peek (key) {
    if (!Object.hasOwn(this.cache, key)) return
    return this.cache[key].value
  }

  set (key, value) {
    if (typeof key !== 'string') key = '' + key

    let element

    if (Object.hasOwn(this.cache, key)) {
      element = this.cache[key]
      element.value = value

      // If it's already the head, there's nothing more to do:
      if (key === this.head) return value
      this._unlink(key, element.prev, element.next)
    } else {
      element = { value, next: null, prev: null }
      this.cache[key] = element

      // Eviction is only possible if the key didn't already exist:
      if (this.length === this.max) this.evict()
    }

    this.length++
    element.next = null
    element.prev = this.head

    if (this.head) this.cache[this.head].next = key
    this.head = key

    if (!this.tail) this.tail = key
    return value
  }

  get (key) {
    if (typeof key !== 'string') key = '' + key
    if (!Object.hasOwn(this.cache, key)) return

    const element = this.cache[key]

    if (this.head !== key) {
      if (key === this.tail) {
        this.tail = element.next
        this.cache[this.tail].prev = null
      } else {
        // Set prev.next -> element.next:
        this.cache[element.prev].next = element.next
      }

      // Set element.next.prev -> element.prev:
      this.cache[element.next].prev = element.prev

      // Element is the new head
      this.cache[this.head].next = key
      element.prev = this.head
      element.next = null
      this.head = key
    }

    return element.value
  }

  evict () {
    if (!this.tail) return
    this.remove(this.tail)
  }
}

export default class ComponentCache {
  constructor (state, emit, lru) {
    equal(typeof state, 'object', 'ComponentCache: state should be type object')
    equal(typeof emit, 'function', 'ComponentCache: emit should be type function')

    if (typeof lru === 'number') this.cache = new LRU(lru)
    else this.cache = lru || new LRU(100)
    this.state = state
    this.emit = emit
  }

  // Get & create component instances.
  render (Component, id, ...args) {
    equal(typeof Component, 'function', 'ComponentCache.render: Component should be type function')
    ok(typeof id === 'string' || typeof id === 'number', 'ComponentCache.render: id should be type string or type number')

    let el = this.cache.get(id)
    if (!el) {
      el = new Component(id, this.state, this.emit, ...args)
      this.cache.set(id, el)
    }

    return el
  }
}