/*
Drift: a server whose config is no longer what the hub says it should be, usually because someone
changed it in that server's own console. Three kinds:

  app       an app's config is not the baseline plus the server's own local values
  settings  the shared settings fields differ from the settings baseline
  lists     the custom Blocked or Allowed list differs from the lists baseline

`check` reads every server with the same rules as the compare views and keeps the result 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. An entry carries names and field names only, never a value.
Two checks never run at once. The timer lives in server.ts, so the tests drive `check` themselves.

When a webhook is given, a CHANGE of state is sent to it: `drift.found` for what newly differs and
`drift.cleared` for what no longer does (an entry that became unreadable is not cleared: it is not
known). The first state is the one loaded from drift.json, so a restart does not repeat notices.
*/
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 { SettingsConfig } from './settings-config.ts'
import { listBaselineApps, readBaseline, readOverrides, writeAtomic } from './store.ts'
import type { Upstream } from './upstream.ts'
import type { Webhook } from './webhook.ts'

export type DriftStatus = 'differs' | 'unreadable' | 'not-installed'
export type DriftType = 'app' | 'settings' | 'lists'

export interface DriftEntry {
  kind: DriftType
  /** The app's name; absent for settings and lists. */
  name?: string
  /** Same as `name` for an app (the console's matrix reads it). */
  app?: string
  server: string
  since: string
  status: DriftStatus
  /** Setting names, or `blocked` / `allowed`. Never values. */
  fields?: string[]
}

export interface DriftState {
  checkedAt: string | null
  /** Every entry, whatever its kind (the key is kept from stage 4). */
  apps: DriftEntry[]
}

const STATUSES: DriftStatus[] = ['differs', 'unreadable', 'not-installed']
const TYPES: DriftType[] = ['app', 'settings', 'lists']

export const keyOf = (e: Pick<DriftEntry, 'kind' | 'name' | 'server'>): string => `${e.kind}\n${e.name ?? ''}\n${e.server}`

/* An entry from drift.json. Before stage 8 `kind` held the status and `app` the name. */
export function readEntry(raw: unknown): DriftEntry | null {
  if (raw === null || typeof raw !== 'object') return null
  const e = raw as Record<string, unknown>
  if (typeof e.server !== 'string' || typeof e.since !== 'string') return null
  const oldKind = typeof e.kind === 'string' && STATUSES.includes(e.kind as DriftStatus)
  const status = (oldKind ? e.kind : e.status) as DriftStatus
  if (!STATUSES.includes(status)) return null
  const kind = (oldKind || e.kind === undefined ? 'app' : e.kind) as DriftType
  if (!TYPES.includes(kind)) return null
  const name = typeof e.name === 'string' ? e.name : typeof e.app === 'string' ? e.app : undefined
  if (kind === 'app' && name === undefined) return null
  const fields = Array.isArray(e.fields) ? e.fields.filter((f): f is string => typeof f === 'string').slice(0, 100) : undefined
  return {
    kind,
    ...(name === undefined ? {} : { name }),
    ...(kind === 'app' ? { app: name } : {}),
    server: e.server,
    since: e.since,
    status,
    ...(fields === undefined || fields.length === 0 ? {} : { fields }),
  }
}

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

  constructor(config: HubConfig, upstream: Upstream, now: () => number = Date.now, options: { webhook?: Webhook } = {}) {
    this.#config = config
    this.#upstream = upstream
    this.#now = now
    this.#settings = new SettingsConfig(config, upstream, now)
    this.#webhook = options.webhook ?? null
    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 : []).map(readEntry).filter((e): e is DriftEntry => e !== null)
      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 entries differ from their baseline, of any kind (unreadable ones are listed, not counted). */
  count(): number {
    return this.#state.apps.filter((e) => e.status === '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: Omit<DriftEntry, 'since'>[] = []

    const appChecks = 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({ kind: 'app', name: a.app, app: a.app, server: server.id, status: 'unreadable' })
          return
        }
        await Promise.all(
          apps.map(async (a) => {
            if (a.baseline === null) return
            const entry = { kind: 'app' as const, name: a.app, app: a.app, server: server.id }
            if (!installed.has(a.app)) return void found.push({ ...entry, status: '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({ ...entry, status: 'differs' })
            } catch {
              found.push({ ...entry, status: 'unreadable' })
            }
          }),
        )
      }),
    )
    const otherChecks = this.#settings.driftFindings().then(
      (list) => {
        for (const f of list) found.push({ kind: f.kind, server: f.server, status: f.status, ...(f.fields.length > 0 ? { fields: f.fields } : {}) })
      },
      () => undefined,
    )
    await Promise.all([appChecks, otherChecks])

    const at = new Date(this.#now()).toISOString()
    const before = new Map(this.#state.apps.map((e) => [`${keyOf(e)}\n${e.status}`, e.since]))
    const entries = found
      .map((f): DriftEntry => ({ ...f, since: before.get(`${keyOf(f)}\n${f.status}`) ?? at }))
      .sort((x, y) => x.kind.localeCompare(y.kind) || (x.name ?? '').localeCompare(y.name ?? '') || x.server.localeCompare(y.server))
    const previous = this.#state.apps
    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)}).`)
    }
    this.#notify(previous, entries)
    return this.#state
  }

  /* A notice for what newly differs and for what is back in line. Names and field names only. */
  #notify(previous: DriftEntry[], now: DriftEntry[]): void {
    if (this.#webhook === null || !this.#webhook.configured) return
    const item = (e: DriftEntry): Record<string, unknown> => ({
      kind: e.kind,
      ...(e.name === undefined ? {} : { name: e.name }),
      server: e.server,
      since: e.since,
      ...(e.fields === undefined ? {} : { fields: e.fields }),
    })
    const was = new Set(previous.filter((e) => e.status === 'differs').map(keyOf))
    const nowDiffers = new Set(now.filter((e) => e.status === 'differs').map(keyOf))
    const unknown = new Set(now.filter((e) => e.status === 'unreadable').map(keyOf))
    const newly = now.filter((e) => e.status === 'differs' && !was.has(keyOf(e)))
    const cleared = previous.filter((e) => e.status === 'differs' && !nowDiffers.has(keyOf(e)) && !unknown.has(keyOf(e)))
    if (newly.length > 0) this.#webhook.send('drift.found', newly.map(item))
    if (cleared.length > 0) this.#webhook.send('drift.cleared', cleared.map(item))
  }
}