Async queue with concurrency limit

interface QueueTask<T> {
  task: () => Promise<T>;
  resolve: (value: T) => void;
  reject: (error: unknown) => void;
}

class AsyncQueue<T = unknown> {
  private readonly concurrency: number;
  private running: number = 0;
  private readonly queue: QueueTask<T>[] = [];

  constructor(concurrency: number) {
    if (concurrency < 1) {
      throw new Error('Concurrency must be at least 1');
    }
    this.concurrency = concurrency;
  }

  async add(task: () => Promise<T>): Promise<T> {
    return new Promise<T>((resolve, reject) => {
      this.queue.push({ task, resolve, reject });
      this.processNext();
    });
  }

  private async processNext(): Promise<void> {
    if (this.running >= this.concurrency || this.queue.length === 0) {
      return;
    }

    const item = this.queue.shift();
    if (!item) return;

    this.running++;

    try {
      const result = await item.task();
      item.resolve(result);
    } catch (error) {
      item.reject(error);
    } finally {
      this.running--;
      this.processNext();
    }
  }

  get pending(): number {
    return this.queue.length;
  }

  get active(): number {
    return this.running;
  }

  get size(): number {
    return this.queue.length + this.running;
  }
}

// Example: Processing 10 tasks with concurrency 3
const simulateApiCall = (id: number, duration: number): Promise<string> => {
  return new Promise((resolve) => {
    console.log(`[${new Date().toISOString()}] Task ${id} started (will take ${duration}ms)`);
    setTimeout(() => {
      console.log(`[${new Date().toISOString()}] Task ${id} completed`);
      resolve(`Result from task ${id}`);
    }, duration);
  });
};

const runExample = async (): Promise<void> => {
  const queue = new AsyncQueue<string>(3);

  console.log('Starting 10 tasks with concurrency limit of 3\n');

  const tasks = Array.from({ length: 10 }, (_, i) => {
    const taskId = i + 1;
    const duration = Math.floor(Math.random() * 1000) + 500;
    return queue.add(() => simulateApiCall(taskId, duration));
  });

  const results = await Promise.all(tasks);

  console.log('\nAll tasks completed!');
  console.log('Results:', results);
};

runExample();