Design Bounded Blocking Queue

# System & Data Structure Design Design problems in DSA interviews test your ability to translate requirements into a functional, efficient, and maintainable class structure. Unlike standard algorithmic problems, the focus here is on **State Management** and **API Design**. ### Core Principles 1. **Encapsulation:** Keep data private and expose functionality through well-defined methods. 2. **Trade-offs:** Every design choice has a cost. Is it better to have $O(1)$ read and $O(N)$ write, or vice versa? 3. **State Consistency:** Ensure that your internal data structures (e.g., a Map and a List) stay in sync after every operation. ### Common Design Patterns #### 1. HashMap + Doubly Linked List (DLL) The "Gold Standard" for $O(1)$ caching (LRU/LFU). ```text [Head] <-> [Node A] <-> [Node B] <-> [Node C] <-> [Tail] ^ ^ ^ ^ ^ (MRU) (Data) (Data) (Data) (LRU) ``` - **HashMap:** Provides $O(1)$ lookups for keys to their corresponding nodes. - **DLL:** Provides $O(1)$ addition/removal of nodes at both ends, maintaining the order of access. #### 2. Amortized Analysis (Rebalancing) Commonly used in **Queue using Stacks** or **Dynamic Arrays**. - Instead of doing heavy work on every call, we batch it. Pushing to a stack is $O(1)$, and "flipping" elements to another stack happens only when necessary, averaging $O(1)$ per operation. #### 3. Ring Buffers (Circular Arrays) Used for fixed-size memory management (e.g., **Circular Queue**, **Hit Counter**). ```text [0] [1] [2] [3] [4] [5] ^ ^ ^ Head (Data) Tail (Pops) (Next Push) ``` - Use `(index + 1) % capacity` to wrap around the array. #### 4. Concurrency & Thread Safety For "Hard" design problems (e.g., **Bounded Blocking Queue**). - Use **Mutexes** (Locks) to prevent data races. - Use **Condition Variables** (`wait`/`notify`) to manage producer-consumer logic efficiently without busy-waiting. ### How to Approach a Design Problem 1. **Identify the API:** What methods do you need to implement? (`get`, `put`, `push`, etc.) 2. **Define the State:** What variables represent the current state? (Size, Capacity, Pointers). 3. **Choose the Data Structures:** Select the combination that minimizes time complexity for the most frequent operations. 4. **Dry Run:** Trace the state changes through a sequence of operations based on your chosen structure.

Design Bounded Blocking Queue

Implement a thread-safe bounded blocking queue. Submitting a task to a full queue should block the calling thread until a slot becomes available. Fetching a task from an empty queue should block the calling thread until a task becomes available.

Hard

Examples

Input: BoundedBlockingQueue(2), enqueue(1), enqueue(2), dequeue() (blocks if empty)
Output: 1

Approach 1

Level I: Synchronized Methods with wait/notify

Intuition

Use synchronized on enqueue and dequeue. Inside each, use a while loop to wait() if the condition (full or empty) isn't met, and notifyAll() after making a change. This is the classic Java threading building block.

⏱ O(1) excluding wait time.💾 O(Capacity).

Detailed Dry Run

Thread 1: Enqueue(1). Thread 2: Enqueue(2). Thread 3: Enqueue(3) -> waits (full). Thread 4: Dequeue() -> returns 1, signals threads.

java

class BoundedBlockingQueue {
    private Queue<Integer> q = new LinkedList<>();
    private int cap;
    public BoundedBlockingQueue(int capacity) { cap = capacity; }
    public synchronized void enqueue(int e) throws InterruptedException {
        while(q.size() == cap) wait();
        q.add(e); notifyAll();
    }
    public synchronized int dequeue() throws InterruptedException {
        while(q.isEmpty()) wait();
        int res = q.poll(); notifyAll(); return res;
    }
}

Approach 2

Level II: Semaphores

Intuition

Use Semaphore fill = new Semaphore(0) and Semaphore drain = new Semaphore(cap). enqueue calls drain.acquire() and fill.release(). dequeue does the reverse.

⏱ O(1) excluding wait time.💾 O(Capacity).

java

class BoundedBlockingQueue {
    Semaphore fill, drain; Queue<Integer> q = new LinkedList<>();
    public BoundedBlockingQueue(int cap) {
        fill = new Semaphore(0); drain = new Semaphore(cap);
    }
    public void enqueue(int el) throws InterruptedException {
        drain.acquire(); synchronized(q) { q.add(el); } fill.release();
    }
    public int dequeue() throws InterruptedException {
        fill.acquire(); int res; synchronized(q) { res = q.poll(); } drain.release();
        return res;
    }
}

Approach 3

Level III: Condition Variables (Producer-Consumer)

Intuition

Use a ReentrantLock with two Condition variables (e.g., notEmpty, notFull). Threads wait on these conditions when the queue is empty or at capacity, and signal each other when appropriate.

⏱ O(1) excluding wait time.💾 O(Capacity).

java

class BoundedBlockingQueue {
    private final Lock lock = new ReentrantLock();
    private final Condition notFull = lock.newCondition();
    private final Condition notEmpty = lock.newCondition();
    private final Queue<Integer> queue = new LinkedList<>();
    private final int capacity;
    public BoundedBlockingQueue(int capacity) { this.capacity = capacity; }
    public void enqueue(int element) throws InterruptedException {
        lock.lock();
        try {
            while (queue.size() == capacity) notFull.await();
            queue.add(element); notEmpty.signal();
        } finally { lock.unlock(); }
    }
    public int dequeue() throws InterruptedException {
        lock.lock();
        try {
            while (queue.isEmpty()) notEmpty.await();
            int element = queue.poll(); notFull.signal(); return element;
        } finally { lock.unlock(); }
    }
}

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