// Shared cache for RPC-backed page data (the "stale-while-revalidate" layer).
//
// Pages used to fetch-on-mount with a spinner on every navigation — Dashboard
// keys its router-view by route.path, so each visit unmounted and refetched
// everything. This store is the single place resource state lives instead:
// keyed entries survive navigation (Pinia) and reloads (sessionStorage
// snapshot), and `useCachedResource` renders them instantly while
// revalidating in the background.
//
// Semantics (generalized from homeStatus.ts / useFleetData.ts, the proven
// hand-rolled versions):
// - sticky-ready: once a key is 'ready' it never regresses to 'loading';
//   refreshes show as 'refreshing' so the UI keeps the data visible.
// - keep-last-known-value on error: a failed revalidate leaves data in place
//   (with `error` set and `fetchedAt` untouched → age badge shows staleness).
// - in-flight dedup per key: concurrent refreshes collapse into one fetch.

import { defineStore } from 'pinia'
import { reactive } from 'vue'

export type ResourceLoadState = 'idle' | 'loading' | 'ready' | 'refreshing' | 'error'

export interface ResourceEntry<T = unknown> {
  data: T | null
  loadState: ResourceLoadState
  /** Epoch ms of the last SUCCESSFUL fetch (drives TTL + stale badges). */
  fetchedAt: number | null
  error: string | null
}

const SNAPSHOT_PREFIX = 'resource:'

function readSnapshot<T>(key: string): { data: T; fetchedAt: number } | null {
  try {
    const raw = sessionStorage.getItem(SNAPSHOT_PREFIX + key)
    if (!raw) return null
    const parsed = JSON.parse(raw)
    if (parsed && typeof parsed.fetchedAt === 'number' && 'data' in parsed) return parsed
  } catch {
    /* corrupt/absent snapshot — fall through to a fresh fetch */
  }
  return null
}

function writeSnapshot(key: string, data: unknown, fetchedAt: number): void {
  try {
    sessionStorage.setItem(SNAPSHOT_PREFIX + key, JSON.stringify({ data, fetchedAt }))
  } catch {
    /* quota exceeded or unserializable — memory cache still works */
  }
}

export const useResourcesStore = defineStore('resources', () => {
  const entries = reactive(new Map<string, ResourceEntry>())
  // Non-reactive bookkeeping: in-flight fetches + active revalidators.
  const inflight = new Map<string, Promise<void>>()
  const revalidators = new Map<string, Set<() => void>>()
  const invalidateTimers = new Map<string, ReturnType<typeof setTimeout>>()

  /** Get (or create) the reactive entry for a key, hydrating from the
   * sessionStorage snapshot on first sight so revisits after a reload paint
   * before any RPC completes. Pass `persist: false` to skip snapshots. */
  function entry<T>(key: string, persist = true): ResourceEntry<T> {
    let e = entries.get(key)
    if (!e) {
      const snap = persist ? readSnapshot<T>(key) : null
      e = reactive<ResourceEntry>({
        data: snap ? snap.data : null,
        loadState: snap ? 'ready' : 'idle',
        fetchedAt: snap ? snap.fetchedAt : null,
        error: null,
      })
      entries.set(key, e)
    }
    return e as ResourceEntry<T>
  }

  /** Run `fetcher` for `key` with sticky-ready + keep-last-value semantics.
   * Concurrent calls for the same key share one in-flight fetch. */
  function refresh<T>(
    key: string,
    fetcher: () => Promise<T>,
    opts: { persist?: boolean } = {},
  ): Promise<void> {
    const existing = inflight.get(key)
    if (existing) return existing
    const e = entry<T>(key, opts.persist ?? true)
    e.loadState = e.loadState === 'ready' || e.loadState === 'refreshing' ? 'refreshing' : 'loading'
    const p = (async () => {
      try {
        const data = await fetcher()
        e.data = data
        e.error = null
        e.fetchedAt = Date.now()
        e.loadState = 'ready'
        if (opts.persist ?? true) writeSnapshot(key, data, e.fetchedAt)
      } catch (err) {
        e.error = err instanceof Error ? err.message : String(err)
        // Keep last-known data visible; only 'error' when we have nothing.
        e.loadState = e.data !== null ? 'ready' : 'error'
      } finally {
        inflight.delete(key)
      }
    })()
    inflight.set(key, p)
    return p
  }

  /** Mark a key stale and (debounced) re-run every mounted subscriber's
   * fetcher. Call after a mutation or on a relevant WS push. */
  function invalidate(key: string, opts: { debounceMs?: number } = {}): void {
    const e = entries.get(key)
    if (e) e.fetchedAt = null
    const subs = revalidators.get(key)
    if (!subs || subs.size === 0) return
    const t = invalidateTimers.get(key)
    if (t) clearTimeout(t)
    invalidateTimers.set(
      key,
      setTimeout(() => {
        invalidateTimers.delete(key)
        for (const fn of subs) fn()
      }, opts.debounceMs ?? 800),
    )
  }

  /** Register a live revalidator for a key (used by useCachedResource);
   * returns an unsubscribe fn. */
  function subscribe(key: string, revalidate: () => void): () => void {
    let subs = revalidators.get(key)
    if (!subs) {
      subs = new Set()
      revalidators.set(key, subs)
    }
    subs.add(revalidate)
    return () => {
      subs.delete(revalidate)
    }
  }

  /** Optimistically apply `update` to the cached value; returns a rollback.
   * Pattern: rollback on RPC failure (generalized TransportPrefsCard). */
  function optimistic<T>(key: string, update: (current: T | null) => T): () => void {
    const e = entry<T>(key)
    const before = e.data
    const beforeState = e.loadState
    e.data = update(before)
    if (e.loadState === 'idle' || e.loadState === 'error') e.loadState = 'ready'
    return () => {
      e.data = before
      e.loadState = beforeState
    }
  }

  /** Drop a key entirely (memory + snapshot). */
  function evict(key: string): void {
    entries.delete(key)
    try {
      sessionStorage.removeItem(SNAPSHOT_PREFIX + key)
    } catch {
      /* noop */
    }
  }

  return { entries, entry, refresh, invalidate, subscribe, optimistic, evict }
})