async-sema.ts

interface SemaHandle {
  release(): void;
}

function createSema(concurrency: number): { acquire(): Promise<SemaHandle> } {
  let handles: string[] = [];
  const queue: ((value: unknown) => void)[] = [];

  function createHandle(): string {
    const handle = nanoid();
    handles.push(handle);
    return handle;
  }

  function destroyHandle(handle: string) {
    handles = handles.filter((h) => h !== handle);
  }

  function acquire(): Promise<string> {
    return new Promise((resolve, reject) => {
      if (handles.length < concurrency) {
        resolve(createHandle());
      } else {
        queue.push(() => {
          resolve(createHandle());
        });
      }
    });
  }

  function release(handle: string) {
    if (handles.includes(handle)) {
      destroyHandle(handle);

      if (queue.length) {
        const item = queue.shift();
        item?.(void 0);
      }
    }
  }

  return {
    async acquire() {
      const handle = await acquire();

      return {
        release: () => release(handle),
      };
    },
  };
}