/*
Drift: a server whose config is no longer what the hub says it should be (the baseline plus the
server's own local values), usually because someone changed it in that server's own console.

`check` reads every server's config for every app that has a baseline, with the read token, and
compares it with the effective config as the sync does. The result is kept in memory and in
`${HUB_DATA}/drift.json`; `since` is the first check that saw the same problem, and it is dropped
once the problem is gone. Two checks never run at once. The timer lives in server.ts, so the
tests drive `check` themselves.
*/
import { existsSync, readFileSync } from 'node:fs'
import path from 'node:path'
import type { HubConfig } from './config.ts'
import { effectiveConfig, sameConfig } from './apps-sync.ts'
import { readConfigText } from './apps-config.ts'
import { listBaselineApps, readBaseline, readOverrides, writeAtomic } from './store.ts'
import type { Upstream } from './upstream.ts'

export type DriftKind = 'differs' | 'unreadable' | 'not-installed'

export interface DriftEntry {
  app: string
  server: string
  since: string
  kind: DriftKind
}

export interface DriftState {
  checkedAt: string | null
  apps: DriftEntry[]
}

const KINDS: DriftKind[] = ['differs', 'unreadable', 'not-installed']

export class Drift {
  #config: HubConfig
  #upstream: Upstream
  #now: () => number
  #state: DriftState
  #running: Promise<DriftState> | null = null

  constructor(config: HubConfig, upstream: Upstream, now: () => number = Date.now) {
    this.#config = config
    this.#upstream = upstream
    this.#now = now
    this.#state = this.#load()
  }

  #file(): string {
    return path.join(this.#config.dataDir, 'drift.json')
  }

  #load(): DriftState {
    try {
      if (!existsSync(this.#file())) return { checkedAt: null, apps: [] }
      const data = JSON.parse(readFileSync(this.#file(), 'utf8')) as Partial<DriftState>
      const apps = (Array.isArray(data.apps) ? data.apps : []).filter(
        (e): e is DriftEntry => typeof e?.app === 'string' && typeof e.server === 'string' && typeof e.since === 'string' && KINDS.includes(e.kind),
      )
      return { checkedAt: typeof data.checkedAt === 'string' ? data.checkedAt : null, apps }
    } catch {
      return { checkedAt: null, apps: [] }
    }
  }

  get(): DriftState & { checking: boolean } {
    return { ...this.#state, checking: this.#running !== null }
  }

  /** How many servers differ from their baseline (the other kinds are listed, not counted). */
  count(): number {
    return this.#state.apps.filter((e) => e.kind === 'differs').length
  }

  /* Runs a check, or joins the one that is already running. */
  check(): Promise<DriftState> {
    if (this.#running !== null) return this.#running
    const run = this.#run().finally(() => {
      this.#running = null
    })
    this.#running = run
    return run
  }

  async #run(): Promise<DriftState> {
    const apps = listBaselineApps(this.#config.dataDir).map((app) => ({
      app,
      baseline: readBaseline(this.#config.dataDir, app),
      overrides: readOverrides(this.#config.dataDir, app),
    }))
    const found: { app: string; server: string; kind: DriftKind }[] = []

    await Promise.all(
      this.#config.servers.map(async (server) => {
        if (apps.length === 0) return
        this.#upstream.invalidate(server.id)
        let installed: Set<string>
        try {
          const list = (await this.#upstream.call(server, 'apps/list')) as { apps?: { name: string }[] } | null
          installed = new Set((list?.apps ?? []).map((a) => a.name))
        } catch {
          for (const a of apps) found.push({ app: a.app, server: server.id, kind: 'unreadable' })
          return
        }
        await Promise.all(
          apps.map(async (a) => {
            if (a.baseline === null) return
            if (!installed.has(a.app)) return void found.push({ app: a.app, server: server.id, kind: 'not-installed' })
            try {
              const text = (await readConfigText(this.#upstream, server, a.app)) ?? ''
              const effective = effectiveConfig(a.baseline.text, a.overrides, server.id).text
              if (!sameConfig(text, effective)) found.push({ app: a.app, server: server.id, kind: 'differs' })
            } catch {
              found.push({ app: a.app, server: server.id, kind: 'unreadable' })
            }
          }),
        )
      }),
    )

    const at = new Date(this.#now()).toISOString()
    const before = new Map(this.#state.apps.map((e) => [`${e.app}\n${e.server}\n${e.kind}`, e.since]))
    const entries = found
      .map((f): DriftEntry => ({ ...f, since: before.get(`${f.app}\n${f.server}\n${f.kind}`) ?? at }))
      .sort((x, y) => x.app.localeCompare(y.app) || x.server.localeCompare(y.server))
    this.#state = { checkedAt: at, apps: entries }
    try {
      writeAtomic(this.#file(), JSON.stringify(this.#state, null, 2) + '\n')
    } catch (e) {
      console.error(`hub: drift.json could not be written (${e instanceof Error ? e.message : String(e)}).`)
    }
    return this.#state
  }
}