import {readable, derived, type Readable} from 'svelte/store'
import {indexBy, type Maybe, now} from '@welshman/lib'
import {withGetter} from '@welshman/store'
import {getFreshness, setFreshnessThrottled} from './freshness'

export const collection = <T, LoadArgs extends any[]>({
  name,
  store,
  getKey,
  load,
}: {
  name: string
  store: Readable<T[]>
  getKey: (item: T) => string
  load?: (key: string, ...args: LoadArgs) => Promise<any>
}) => {
  const indexStore = withGetter(derived(store, $items => indexBy(getKey, $items)))
  const pending = new Map<string, Promise<Maybe<T>>>()
  const loadAttempts = new Map<string, number>()

  const loadItem = async (key: string, ...args: LoadArgs) => {
    const stale = indexStore.get().get(key)

    // If we have no loader function, nothing we can do
    if (!load) {
      return stale
    }

    const freshness = getFreshness(name, key)

    // If we have an item, reload if it's stale
    if (stale && freshness > now() - 3600) {
      return stale
    }

    // If we already are loading, await and return
    if (pending.has(key)) {
      return pending.get(key)!.then(() => indexStore.get().get(key))
    }

    const attempt = loadAttempts.get(key) || 0

    // Use exponential backoff to throttle attempts
    if (freshness > now() - Math.pow(2, attempt)) {
      return stale
    }

    loadAttempts.set(key, attempt + 1)

    setFreshnessThrottled({ns: name, key, ts: now()})

    const promise = load(key, ...args)

    pending.set(key, promise)

    await promise

    pending.delete(key)

    const fresh = indexStore.get().get(key)

    if (fresh) {
      loadAttempts.delete(key)
    }

    return fresh
  }

  const deriveItem = (key: Maybe<string>, ...args: LoadArgs) => {
    if (!key) {
      return readable(undefined)
    }

    // If we don't yet have the item, or it's stale, trigger a request for it. The derived
    // store will update when it arrives
    loadItem(key, ...args)

    return derived(indexStore, $index => $index.get(key))
  }

  return {indexStore, deriveItem, loadItem}
}