# Implementation of the Queue ADT using a circular array.
from array204 import Array

class Queue :
  # Creates an empty queue.
  def __init__( self, max_size = 16) :
    self._count = 0
    self._front = 0
    self._back = max_size - 1
    self._qarray = Array( max_size )

  # Returns True if the queue is empty.
  def is_empty( self ) :
    return self._count == 0
    
  # Returns True if the queue is full.
  def is_full( 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.is_full(), "Cannot enqueue to a full queue."
    max_size = len(self._qarray)
    self._back = (self._back + 1) % max_size 
    self._qarray[self._back] = item
    self._count += 1                 

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

  # Examine the first item without removing it
  def peek( self ):
    assert not self.is_empty(), "Cannot peek from an empty queue."
    item = self._qarray[ self._front ]
    return item