# Implementation of Circular Queue (using Python lists)


class CircularQueue:
 """Circular FIFO queue with a fixed capacity"""

 def __init__(self, n: int):
 self.n = n
 self.array = [None] * self.n
 self.front = 0 # index of the first element
 self.rear = 0
 self.size = 0

 def __len__(self) -> int:
 """
 >>> cq = CircularQueue(5)
 >>> len(cq)
 0
 >>> cq.enqueue("A") # doctest: +ELLIPSIS
 <data_structures.queue.circular_queue.CircularQueue object at ...
 >>> len(cq)
 1
 """
 return self.size

 def is_empty(self) -> bool:
 """
 >>> cq = CircularQueue(5)
 >>> cq.is_empty()
 True
 >>> cq.enqueue("A").is_empty()
 False
 """
 return self.size == 0

 def first(self):
 """
 >>> cq = CircularQueue(5)
 >>> cq.first()
 False
 >>> cq.enqueue("A").first()
 'A'
 """
 return False if self.is_empty() else self.array[self.front]

 def enqueue(self, data):
 """
 This function insert an element in the queue using self.rear value as an index
 >>> cq = CircularQueue(5)
 >>> cq.enqueue("A") # doctest: +ELLIPSIS
 <data_structures.queue.circular_queue.CircularQueue object at ...
 >>> (cq.size, cq.first())
 (1, 'A')
 >>> cq.enqueue("B") # doctest: +ELLIPSIS
 <data_structures.queue.circular_queue.CircularQueue object at ...
 >>> (cq.size, cq.first())
 (2, 'A')
 """
 if self.size >= self.n:
 raise Exception("QUEUE IS FULL")

 self.array[self.rear] = data
 self.rear = (self.rear + 1) % self.n
 self.size += 1
 return self

 def dequeue(self):
 """
 This function removes an element from the queue using on self.front value as an
 index
 >>> cq = CircularQueue(5)
 >>> cq.dequeue()
 Traceback (most recent call last):
 ...
 Exception: UNDERFLOW
 >>> cq.enqueue("A").enqueue("B").dequeue()
 'A'
 >>> (cq.size, cq.first())
 (1, 'B')
 >>> cq.dequeue()
 'B'
 >>> cq.dequeue()
 Traceback (most recent call last):
 ...
 Exception: UNDERFLOW
 """
 if self.size == 0:
 raise Exception("UNDERFLOW")

 temp = self.array[self.front]
 self.array[self.front] = None
 self.front = (self.front + 1) % self.n
 self.size -= 1
 return temp