Saturday, February 11, 2012

implement a thread-safe queue using 2 stacks

Your idea is basically

void enqueue(Element e){
    this->stack1.push(e);
}

Element dequeue(){
    if (this->stack2.size()){
      return this->stack2.pop();
    }
    this->transferFromStack1to2();
    if ( ! this->stack2.size()){
        //throw exception or return a special value indicating QUEUE_EMPTY;
    }
    return this->stack2.pop();
}
void transferFromStack1to2(){
    assert this->stack2.size() == 0;
    //get locks on Stack1 and Stack2
    while(this->stack1.size()){
       this->stack2.push(this->stack1.pop()); 
    }
    //release locks
}

We can improve performance a bit with a proactively transfer -- as soon as stack2 becomes empty. However, this must be done on another thread.

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