import { promises as fs } from 'node:fs'
import { assertActive } from './policy'

interface Waiting { workspace: string; start(): void; cancel(): void }
const queue: Waiting[] = [], busy = new Set<string>()
const maximumActive = 4, maximumQueued = 64
function drain(): void {
  for (let index = 0; index < queue.length && busy.size < maximumActive;) {
    const item = queue[index]
    if (busy.has(item.workspace)) { index++; continue }
    queue.splice(index, 1); busy.add(item.workspace); item.start()
  }
}
/** A workspace snapshot observes the previous approved commit, never a concurrent stale copy. */
export async function withSandboxExecution<T>(root: string, signal: AbortSignal | undefined, operation: (canonicalRoot: string) => Promise<T>): Promise<T> {
  assertActive(signal)
  const real = await fs.realpath(root), workspace = process.platform === 'win32' ? real.toLowerCase() : real
  if (queue.length >= maximumQueued || queue.filter(item => item.workspace === workspace).length >= 12) throw new Error('沙盒执行队列已满，请稍后重试')
  return new Promise<T>((done, reject) => {
    const item: Waiting = {
      workspace,
      start() {
        signal?.removeEventListener('abort', item.cancel)
        Promise.resolve().then(() => { assertActive(signal); return operation(real) }).then(done, reject).finally(() => { busy.delete(workspace); drain() })
      },
      cancel() {
        const index = queue.indexOf(item)
        if (index < 0) return
        queue.splice(index, 1); signal?.removeEventListener('abort', item.cancel)
        reject(Object.assign(new Error('排队中的沙盒执行已取消'), { name: 'AbortError' }))
      }
    }
    queue.push(item)
    signal?.addEventListener('abort', item.cancel, { once: true })
    if (signal?.aborted) item.cancel()
    drain()
  })
}