/**
 * Helpers that let a harness adapter surface custom events emitted by BRIDGED
 * tools (via {@link ToolBridgeCoreOptions.emitCustomEvent}) on its live output
 * stream.
 *
 * The bridge runs out-of-band from the harness's own event stream, so a bridged
 * tool's progress/console events have no path to the client on their own. An
 * adapter creates a {@link BridgeEventChannel}, hands its `emitCustomEvent` to
 * the bridge provisioner, and {@link mergeChunkStreams | merges} the channel's
 * stream into its translated output — so events interleave live while the agent
 * runs (e.g. code mode's `code_mode:console` logs during a long execution).
 */
import { EventType, withTanstackMetadata } from '@tanstack/ai'
import type { StreamChunk } from '@tanstack/ai'

export interface BridgeEventChannel {
  /** Pass as the bridge's `emitCustomEvent`; buffers a CUSTOM chunk for the stream. */
  emitCustomEvent: (eventName: string, value: Record<string, unknown>) => void
  /** Live CUSTOM-chunk stream; ends after {@link close} once drained. */
  stream: AsyncIterable<StreamChunk>
  /** Stop the stream (call when the run's main output is done). */
  close: () => void
}

/** Create a channel whose emitted events become CUSTOM {@link StreamChunk}s. */
export function createBridgeEventChannel(meta: {
  model: string
  threadId?: string
  runId?: string
}): BridgeEventChannel {
  const buffer: Array<StreamChunk> = []
  let notify: (() => void) | null = null
  let closed = false

  async function* stream(): AsyncIterable<StreamChunk> {
    for (;;) {
      const next = buffer.shift()
      if (next !== undefined) {
        yield next
        continue
      }
      if (closed) return
      await new Promise<void>((resolve) => {
        notify = resolve
      })
      notify = null
    }
  }

  return {
    emitCustomEvent(eventName, value) {
      if (closed) return
      buffer.push(
        withTanstackMetadata(
          {
            type: EventType.CUSTOM,
            name: eventName,
            value,
            timestamp: Date.now(),
          },
          {
            model: meta.model,
            ...(meta.threadId !== undefined ? { threadId: meta.threadId } : {}),
            ...(meta.runId !== undefined ? { runId: meta.runId } : {}),
          },
        ) as StreamChunk,
      )
      notify?.()
    },
    close() {
      closed = true
      notify?.()
    },
    stream: stream(),
  }
}

/**
 * Merge a `side` chunk stream into a `base` chunk stream, yielding from whichever
 * settles first. Terminates when `base` ends (the run is over), then releases the
 * side iterator — so a never-ending channel (until closed) doesn't hang the merge.
 */
export async function* mergeChunkStreams(
  base: AsyncIterable<StreamChunk>,
  side: AsyncIterable<StreamChunk>,
): AsyncIterable<StreamChunk> {
  const baseIt = base[Symbol.asyncIterator]()
  const sideIt = side[Symbol.asyncIterator]()
  let baseNext = baseIt.next().then((r) => ({ from: 'base' as const, r }))
  let sideNext = sideIt.next().then((r) => ({ from: 'side' as const, r }))
  let sideLive = true
  try {
    for (;;) {
      const winner = await Promise.race(
        sideLive ? [baseNext, sideNext] : [baseNext],
      )
      if (winner.from === 'base') {
        if (winner.r.done) return
        yield winner.r.value
        baseNext = baseIt.next().then((r) => ({ from: 'base' as const, r }))
      } else if (winner.r.done) {
        sideLive = false
      } else {
        yield winner.r.value
        sideNext = sideIt.next().then((r) => ({ from: 'side' as const, r }))
      }
    }
  } finally {
    // Fire-and-forget: do NOT await the side return. The channel generator is
    // suspended on a promise that only `close()` resolves, and `close()` runs in
    // the adapter's `finally` AFTER this merge completes — awaiting here would
    // deadlock. The adapter's `close()` lets the generator unwind afterwards.
    const baseReturn = baseIt.return?.(undefined)
    if (baseReturn) void baseReturn.catch(() => {})
    const sideReturn = sideIt.return?.(undefined)
    if (sideReturn) void sideReturn.catch(() => {})
  }
}