/**
 * A gate letting at most `limit` tasks run at once, first in first out, or
 * newest first with `lifo` (the last request is the one the user waits on).
 */
export function createLimiter(limit, { lifo = false } = {}) {
  let running = 0;
  const queue = [];
  function next() {
    while (running < limit && queue.length) {
      const { task, resolve, reject, signal } = lifo
        ? queue.pop()
        : queue.shift();
      if (signal?.aborted) {
        reject(signal.reason);
        continue;
      }
      running++;
      Promise.resolve()
        .then(task)
        .then(resolve, reject)
        .finally(() => {
          running--;
          next();
        });
    }
  }
  return {
    run(task, { signal } = {}) {
      return new Promise((resolve, reject) => {
        const item = { task, resolve, reject, signal };
        queue.push(item);
        signal?.addEventListener(
          'abort',
          () => {
            const at = queue.indexOf(item);
            if (at === -1) return;
            queue.splice(at, 1);
            reject(signal.reason);
          },
          { once: true },
        );
        next();
      });
    },
    get pending() {
      return running + queue.length;
    },
  };
}