simple-semaphore.ts

type Semaphore = {
  acquire: () => Promise<void>;
  release: () => void;
};

export const simpleSemaphore = (size: number): Semaphore => {
  const waiting: (() => void)[] = [];
  let available = size;

  const acquire = async (): Promise<void> => {
    if (--available < 0) {
      await new Promise<void>((r) => waiting.unshift(r));
    }
  };

  const release = () => {
    if (available >= size) {
      // this would ALWAYS be a bug in the calling code
      throw new Error('lock released when no lock was held')
    }
    available++;
    waiting.pop()?.();
  };

  return { acquire, release };
}