Circular Queue

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```# 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
```