使用循环数组实现队列

Question

I am reading about implementation of DynaArrayQueue (queue that doubles in size when there are no enough elements)

I have certain questions regarding two methods in it.

Let capacity be the capacity of the queue.

getQueueSize() Method

public int getQueueSize()
 {
if(front == -1) return 0;

//Here why can't the size by  simply [(rear -front +1) %capacity ]
int size = (capacity - front + rear +1) % capacity;

if(size == 0) return capacity;
else return size;
 }

When calculating the size why are we using

size = (capacity - front + rear +1 ) % capacity , instead of simply (rear - front +1) % capacity. ?

Question 2:

resizeQueue()

This is the method that resizes the queue

  private void resizeQueue()
 {
int initCapacity = capacity;
capacity *=2;

int[] oldArray = array;

array = new init[this.capacity];

//this is fine
for(int i=0; i<oldArray.length;i++)
{
    array[i] =oldArray[i];
}

//why do we need this ?
if(rear < front)
{
    for(int i =0 ; i<front ; i++)
    {
        array[i+initCapacity] = this.array[i];
        array[i]= null;

    }

    rear = rear + initCapacity;

}


  }

The first for loop in the method is straight forward, but why do we need the second if loop for ? . What conditions it is taking care of ?

Answer 1

1. The reason capacity is there is because

(rear - front) 

can be negative, negative modulus vary by language implementations.

2. The reason you have the 2nd loop is because

In this situation

[#####000000####]
     ^      ^
    rear   front

# is array item
0 is empty space

When you copy to a larger array, you get a disjointed array

[#####000000####000000000000000]

The second loop moves

[#####000000####000000000000000]
 ^---^

to

[00000000000#########0000000000]
                ^---^

Answer 2

The second loop is making sure that items are stored in the proper order in the new array. Imagine a circular buffer where front (the place you're taking from) is greater than rear (where you're adding things). So capacity is 50, front is 40, and rear is 10. Items will be removed from the queue in the order 40, 41, 42, 43...0, 1, 2, ... 9.

Now, when you resize the queue, items are copied to the new array in the same order. But removing items from the queue, which now has capacity of 100, will be 40, 41, 42, ... 49, 50, 51. But there's nothing at position 50!

So those 10 items are moved from positions 0 through 9 to positions 50 through 59. That's what the second loop does.

Answer 3

Not familiar with this class, but I guess if the queue has wrapped around, so that new items are being written at the start of the array, then "rear" could be less than "front". This kind of answers both of your questions, because in this case, (rear - front +1) % capacity would be negative, which is not what you want (Q1), and also the new part of the queue would have to be transplanted to the end, which the capacity increases (Q2).