JSQuestions/solutions/async-scheduler.test.js

const TaskScheduler = require('./async-scheduler');

// Suppress unhandled rejection warnings in tests
process.on('unhandledRejection', () => {});

describe('TaskScheduler', () => {
  describe('constructor', () => {
    it('should create a scheduler with custom concurrency', () => {
      const scheduler = new TaskScheduler(5);
      expect(scheduler.concurrencyLimit).toBe(5);
    });

    it('should start in paused mode by default', () => {
      const scheduler = new TaskScheduler();
      expect(scheduler.isProcessing).toBe(false);
    });
  });

  describe('addTask', () => {
    it('should return a promise', () => {
      const scheduler = new TaskScheduler();
      const promise = scheduler.addTask(() => Promise.resolve('result'));
      expect(promise).toBeInstanceOf(Promise);
    });

    it('should resolve when task completes successfully', async () => {
      const scheduler = new TaskScheduler();
      scheduler.play();
      const promise = scheduler.addTask(() => Promise.resolve('success'));
      await expect(promise).resolves.toBe('success');
    });

    it('should reject when task fails', async () => {
      const scheduler = new TaskScheduler();
      scheduler.play();
      const promise = scheduler.addTask(() => Promise.reject(new Error('task failed')));
      await expect(promise).rejects.toThrow('task failed');
    });

    it('should capture synchronous throws as rejections', async () => {
      const scheduler = new TaskScheduler();
      scheduler.play();
      const promise = scheduler.addTask(() => {
        throw new Error('sync error');
      });
      await expect(promise).rejects.toThrow('sync error');
    });

    it('should queue task if scheduler is paused', async () => {
      const scheduler = new TaskScheduler();
      const mockFn = jest.fn(() => Promise.resolve());
      scheduler.addTask(mockFn);
      await new Promise(r => setTimeout(r, 50));
      expect(mockFn).not.toHaveBeenCalled();
    });
  });

  describe('play and pause', () => {
    it('should start processing when play() is called', async () => {
      const scheduler = new TaskScheduler();
      const mockFn = jest.fn(() => Promise.resolve());
      scheduler.addTask(mockFn);
      
      expect(mockFn).not.toHaveBeenCalled();
      
      scheduler.play();
      await new Promise(r => setTimeout(r, 10));
      
      expect(mockFn).toHaveBeenCalled();
    });

    it('should stop starting new tasks when pause() is called', async () => {
      const scheduler = new TaskScheduler(1);
      const mockFn1 = jest.fn(() => new Promise(r => setTimeout(r, 50)));
      const mockFn2 = jest.fn(() => Promise.resolve());
      
      scheduler.play();
      scheduler.addTask(mockFn1);
      scheduler.addTask(mockFn2);
      scheduler.pause();
      
      await new Promise(r => setTimeout(r, 100));
      expect(mockFn2).not.toHaveBeenCalled();
    });

    it('should resume processing when play() is called again', async () => {
      const scheduler = new TaskScheduler(1);
      const mockFn1 = jest.fn(() => new Promise(r => setTimeout(() => r(), 50)));
      const mockFn2 = jest.fn(() => Promise.resolve());
      
      scheduler.play();
      scheduler.addTask(mockFn1);
      scheduler.pause();
      scheduler.addTask(mockFn2);
      
      await new Promise(r => setTimeout(r, 60));
      expect(mockFn2).not.toHaveBeenCalled();
      
      scheduler.play();
      await new Promise(r => setTimeout(r, 60));
      expect(mockFn2).toHaveBeenCalled();
    });
  });

  describe('concurrency', () => {
    it('should execute up to concurrency limit simultaneously', async () => {
      const scheduler = new TaskScheduler(2);
      const executionTimes = [];
      
      const createTask = (id) => async () => {
        executionTimes.push({ id, event: "start" });
        await new Promise(r => setTimeout(r, 50));
        executionTimes.push({ id, event: "end" });
      };
      
      scheduler.play();
      await Promise.all([
        scheduler.addTask(createTask(1)),
        scheduler.addTask(createTask(2)),
        scheduler.addTask(createTask(3)),
      ]);
      
      // Expected order:
      // 1. Task 1 starts
      // 2. Task 2 starts (parallel, within limit)
      // 3. Task 1 or 2 ends
      // 4. Task 3 starts (frees up a slot)
      // 5. Task 3 ends
      expect(executionTimes[0]).toEqual({id: 1, event: "start"});
      expect(executionTimes[1]).toEqual({id: 2, event: "start"});
      expect(executionTimes[2]).toEqual({id: 1, event: "end"});
      expect(executionTimes[3]).toEqual({id: 3, event: "start"});
      expect(executionTimes[4]).toEqual({id: 2, event: "end"});
      expect(executionTimes[5]).toEqual({id: 3, event: "end"});
    });

    it('should queue tasks when concurrency limit is reached', async () => {
      const scheduler = new TaskScheduler(1);
      const execOrder = [];
      
      const createTask = (id) => async () => {
        execOrder.push(`start-${id}`);
        await new Promise(r => setTimeout(r, 10));
        execOrder.push(`end-${id}`);
      };
      
      scheduler.play();
      await Promise.all([
        scheduler.addTask(createTask(1)),
        scheduler.addTask(createTask(2)),
        scheduler.addTask(createTask(3)),
      ]);
      
      expect(execOrder).toEqual([
        'start-1', 'end-1',
        'start-2', 'end-2',
        'start-3', 'end-3',
      ]);
    });
  });

  describe('FIFO ordering', () => {
    it('should execute tasks in FIFO order', async () => {
      const scheduler = new TaskScheduler(1);
      const execOrder = [];
      
      const createTask = (id) => () => {
        execOrder.push(id);
        return Promise.resolve();
      };
      
      scheduler.play();
      await Promise.all([
        scheduler.addTask(createTask(1)),
        scheduler.addTask(createTask(2)),
        scheduler.addTask(createTask(3)),
        scheduler.addTask(createTask(4)),
      ]);
      
      expect(execOrder).toEqual([1, 2, 3, 4]);
    });

    it('should maintain FIFO order even with variable task durations', async () => {
      const scheduler = new TaskScheduler(1);
      const execOrder = [];
      
      const createTask = (id, duration) => async () => {
        execOrder.push(`start-${id}`);
        await new Promise(r => setTimeout(r, duration));
        execOrder.push(`end-${id}`);
      };
      
      scheduler.play();
      await Promise.all([
        scheduler.addTask(createTask(1, 30)),
        scheduler.addTask(createTask(2, 10)),
        scheduler.addTask(createTask(3, 20)),
      ]);
      
      expect(execOrder).toEqual([
        'start-1', 'end-1',
        'start-2', 'end-2',
        'start-3', 'end-3',
      ]);
    });
  });

  describe('automatic task pickup after completion', () => {
    it('should pick next task from queue when running task completes', async () => {
      const scheduler = new TaskScheduler(1);
      const mockFn2 = jest.fn(() => Promise.resolve());
      
      scheduler.play();
      await scheduler.addTask(() => new Promise(r => setTimeout(r, 20)));
      const p2 = scheduler.addTask(mockFn2);
      
      await p2;
      expect(mockFn2).toHaveBeenCalled();
    });

    // it('should pick next task even if current task rejects', async () => {
    //   const scheduler = new TaskScheduler(1);
    //   const mockFn2 = jest.fn(() => Promise.resolve());
      
    //   const p1 = scheduler.addTask(() => Promise.reject(new Error("this should fail")));
    //   p1.catch(() => {}); // suppress unhandled rejection
    //   scheduler.play();
      
    //   try {
    //     await p1;
    //   } catch (e) {
    //     // expected
    //   }
    //   const p2 = scheduler.addTask(mockFn2);
      
    //   await p2;
    //   expect(mockFn2).toHaveBeenCalled();
    // });
  });

  describe('edge cases', () => {
    it('should handle concurrency of 1', async () => {
      const scheduler = new TaskScheduler(1);
      const execOrder = [];
      
      scheduler.play();
      await Promise.all([
        scheduler.addTask(() => { execOrder.push(1); return Promise.resolve(); }),
        scheduler.addTask(() => { execOrder.push(2); return Promise.resolve(); }),
      ]);
      
      expect(execOrder).toEqual([1, 2]);
    });

    it('should handle large concurrency', async () => {
      const scheduler = new TaskScheduler(100);
      const promises = [];
      
      scheduler.play();
      for (let i = 0; i < 50; i++) {
        promises.push(scheduler.addTask(() => Promise.resolve(i)));
      }
      
      const results = await Promise.all(promises);
      expect(results.length).toBe(50);
    });

    it('should handle empty queue gracefully', () => {
      const scheduler = new TaskScheduler();
      scheduler.play();
      // Should not throw
      expect(() => scheduler._processQueue()).not.toThrow();
    });
  });
});