From 70368a757e8d37b9f3dd96af4ca535275cb39580 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?V=C3=ADctor=20A=2E=20Rodr=C3=ADguez?= <victor@bit-man.guru>
Date: Thu, 28 Oct 2021 11:43:24 -0300
Subject: [PATCH] Implement Circular Queue using linked lists. Fixes
 TheAlgorithms#5361 (#5587)

* CircularQueueLinkedList: empty list, trivial implementation TheAlgorithms#5361

* CircularQueueLinkedList: single element list TheAlgorithms#5361

* CircularQueueLinkedList: refactor, no que empty attribute TheAlgorithms#5361

* CircularQueueLinkedList: refactor TheAlgorithms#5361

* CircularQueueLinkedList: changed internal data structure to use double linked list TheAlgorithms#5361

* CircularQueueLinkedList: enqueue test cases added TheAlgorithms#5361

* CircularQueueLinkedList: track full queue TheAlgorithms#5361

* CircularQueueLinkedList: adding functions description TheAlgorithms#5361

* CircularQueueLinkedList: type hints TheAlgorithms#5361

* CircularQueueLinkedList: algorithm explanation TheAlgorithms#5361

* CircularQueueLinkedList: missing type hints TheAlgorithms#5361

* CircularQueueLinkedList: more missing type hints TheAlgorithms#5361

* Update data_structures/queue/circular_queue_linked_list.py

Co-authored-by: John Law <johnlaw.po@gmail.com>
---
 .../queue/circular_queue_linked_list.py       | 150 ++++++++++++++++++
 1 file changed, 150 insertions(+)
 create mode 100644 data_structures/queue/circular_queue_linked_list.py

diff --git a/data_structures/queue/circular_queue_linked_list.py b/data_structures/queue/circular_queue_linked_list.py
new file mode 100644
index 000000000..1878403bd
--- /dev/null
+++ b/data_structures/queue/circular_queue_linked_list.py
@@ -0,0 +1,150 @@
+# Implementation of Circular Queue using linked lists
+# https://en.wikipedia.org/wiki/Circular_buffer
+
+from typing import Any
+
+
+class CircularQueueLinkedList:
+    """
+    Circular FIFO list with the given capacity (default queue length : 6)
+
+    >>> cq = CircularQueueLinkedList(2)
+    >>> cq.enqueue('a')
+    >>> cq.enqueue('b')
+    >>> cq.enqueue('c')
+    Traceback (most recent call last):
+       ...
+    Exception: Full Queue
+    """
+
+    def __init__(self, initial_capacity: int = 6) -> None:
+        self.front = None
+        self.rear = None
+        self.create_linked_list(initial_capacity)
+
+    def create_linked_list(self, initial_capacity: int) -> None:
+        current_node = Node()
+        self.front = current_node
+        self.rear = current_node
+        previous_node = current_node
+        for i in range(1, initial_capacity):
+            current_node = Node()
+            previous_node.next = current_node
+            current_node.prev = previous_node
+            previous_node = current_node
+        previous_node.next = self.front
+        self.front.prev = previous_node
+
+    def is_empty(self) -> bool:
+        """
+        Checks where the queue is empty or not
+        >>> cq = CircularQueueLinkedList()
+        >>> cq.is_empty()
+        True
+        >>> cq.enqueue('a')
+        >>> cq.is_empty()
+        False
+        >>> cq.dequeue()
+        'a'
+        >>> cq.is_empty()
+        True
+        """
+        return self.front == self.rear and self.front.data is None
+
+    def first(self) -> Any:
+        """
+        Returns the first element of the queue
+        >>> cq = CircularQueueLinkedList()
+        >>> cq.first()
+        Traceback (most recent call last):
+           ...
+        Exception: Empty Queue
+        >>> cq.enqueue('a')
+        >>> cq.first()
+        'a'
+        >>> cq.dequeue()
+        'a'
+        >>> cq.first()
+        Traceback (most recent call last):
+           ...
+        Exception: Empty Queue
+        >>> cq.enqueue('b')
+        >>> cq.enqueue('c')
+        >>> cq.first()
+        'b'
+        """
+        self.check_can_perform_operation()
+        return self.front.data
+
+    def enqueue(self, data: Any) -> None:
+        """
+        Saves data at the end of the queue
+
+        >>> cq = CircularQueueLinkedList()
+        >>> cq.enqueue('a')
+        >>> cq.enqueue('b')
+        >>> cq.dequeue()
+        'a'
+        >>> cq.dequeue()
+        'b'
+        >>> cq.dequeue()
+        Traceback (most recent call last):
+           ...
+        Exception: Empty Queue
+        """
+        self.check_is_full()
+        if self.is_empty():
+            self.rear.data = data
+        else:
+            self.rear = self.rear.next
+            self.rear.data = data
+
+    def dequeue(self) -> Any:
+        """
+        Removes and retrieves the first element of the queue
+
+        >>> cq = CircularQueueLinkedList()
+        >>> cq.dequeue()
+        Traceback (most recent call last):
+           ...
+        Exception: Empty Queue
+        >>> cq.enqueue('a')
+        >>> cq.dequeue()
+        'a'
+        >>> cq.dequeue()
+        Traceback (most recent call last):
+           ...
+        Exception: Empty Queue
+        """
+        self.check_can_perform_operation()
+        if self.front == self.rear:
+            data = self.front.data
+            self.front.data = None
+            return data
+
+        old_front = self.front
+        self.front = old_front.next
+        data = old_front.data
+        old_front.data = None
+        return data
+
+    def check_can_perform_operation(self) -> None:
+        if self.is_empty():
+            raise Exception("Empty Queue")
+
+    def check_is_full(self) -> None:
+        if self.rear.next == self.front:
+            raise Exception("Full Queue")
+
+
+class Node:
+    def __init__(self) -> None:
+        self.data = None
+        self.next = None
+        self.prev = None
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()