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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)
        >>> cq.enqueue("A")  # doctest: +ELLIPSIS
        <data_structures.queue.circular_queue.CircularQueue object at ...
        >>> len(cq)
        """
        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

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