let messageChannelYield;
let cooperativeTurn = 0;

export const COOPERATIVE_TIMER_TURN_INTERVAL = 8;

function createMessageChannelYield() {
  if (typeof globalThis.MessageChannel !== "function") return undefined;
  const channel = new globalThis.MessageChannel();
  const queue = [];
  channel.port1.onmessage = () => queue.shift()?.();
  channel.port1.unref?.();
  channel.port2.unref?.();
  return () => new Promise((resolve) => {
    queue.push(resolve);
    channel.port2.postMessage(0);
  });
}

/**
 * Yield one cooperative task without relying on nested zero-delay timers.
 * scheduler.yield() is preferred when the host implements it; MessageChannel
 * keeps the fallback task-based and avoids browser timer clamping.
 */
export function cooperativeYield() {
  cooperativeTurn += 1;
  if (cooperativeTurn % COOPERATIVE_TIMER_TURN_INTERVAL === 0) {
    return new Promise((resolve) => setTimeout(resolve, 0));
  }
  const hostScheduler = globalThis.scheduler;
  if (typeof hostScheduler?.yield === "function") {
    try {
      return hostScheduler.yield();
    } catch {
      // A partial scheduler implementation must not make computation unusable.
    }
  }
  messageChannelYield ??= createMessageChannelYield();
  if (messageChannelYield) return messageChannelYield();
  return new Promise((resolve) => setTimeout(resolve, 0));
}