# Implementation of the Queue ADT using a circular array.
from array import Array

class Queue :
 # Creates an empty queue.
 def __init__( self, maxSize ) :
 self._count = 0
 self._front = 0
 self._back = maxSize - 1
 self._qArray = Array( maxSize )

 # Returns True if the queue is empty.
 def isEmpty( self ) :
 return self._count == 0

 # Returns True if the queue is full.
 def isFull( self ) :
 return self._count == len(self._qArray)

 # Returns the number of items in the queue.
 def __len__( self ) :
 return self._count

 # Adds the given item to the queue.
 def enqueue( self, item ):
 assert not self.isFull(), "Cannot enqueue to a full queue."
 maxSize = len(self._qArray)
 self._back = (self._back + 1) % maxSize
 self._qArray[self._back] = item
 self._count += 1

 # Removes and returns the first item in the queue.
 def dequeue( self ):
 assert not self.isEmpty(), "Cannot dequeue from an empty queue."
 item = self._qArray[ self._front ]
 maxSize = len(self._qArray)
 self._front = (self._front + 1) % maxSize
 self._count -= 1
 return item