import nntpConfig from '#config/nntp'
import indexerConfig from '#config/indexer'
import { XoverClient } from '#lib/nntp/xover_client'

interface Slot {
  client: XoverClient
  group?: string
  usedAt: number
}

/** A pooled connection unused this long is replaced: the news server may have dropped it. */
const IDLE_MS = 2 * 60 * 1000

/**
 * NNTP connections in the seed API for relay nodes (CONCEPT.md D38): the
 * indexer fetches overview chunks and passes the gzip stream on unopened, so
 * nodes need no news server account and every connection comes from this
 * machine's IP. SEED_NNTP_CONNECTIONS (0: relay off).
 */
class SeedFetchPool {
  private idle: Slot[] = []
  private open = 0
  private waiters: (() => void)[] = []

  get size() {
    return indexerConfig.seed.nntpConnections
  }

  async fetch(group: string, from: number, to: number): Promise<Buffer> {
    const slot = await this.acquire()
    let ok = false
    try {
      if (slot.group !== group) {
        await slot.client.group(group)
        slot.group = group
      }
      const raw = await slot.client.xoverRaw(from, to)
      ok = true
      return raw
    } finally {
      this.release(slot, ok)
    }
  }

  private async acquire(): Promise<Slot> {
    while (true) {
      const slot = this.idle.pop()
      if (slot) {
        if (Date.now() - slot.usedAt < IDLE_MS) return slot
        slot.client.close()
        this.open--
        continue
      }
      if (this.open < this.size) {
        this.open++
        try {
          const client = await XoverClient.connect({
            host: nntpConfig.host,
            port: nntpConfig.port,
            secure: nntpConfig.secure,
            servername: nntpConfig.tlsServername,
            user: nntpConfig.user,
            password: nntpConfig.password?.release(),
            timeoutMs: nntpConfig.commandTimeoutMs,
            compress: true,
          })
          return { client, usedAt: Date.now() }
        } catch (error) {
          this.open--
          this.waiters.shift()?.()
          throw error
        }
      }
      await new Promise<void>((resolve) => this.waiters.push(resolve))
    }
  }

  private release(slot: Slot, ok: boolean) {
    if (ok) {
      slot.usedAt = Date.now()
      this.idle.push(slot)
    } else {
      // A failed command can leave the connection mid-reply: start afresh.
      slot.client.close()
      this.open--
    }
    this.waiters.shift()?.()
  }
}

export default new SeedFetchPool()