Circular Queue
A fixed-size queue that reuses empty slots at the beginning when the end is reached.
See the Logic in Motion
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A standard array-based queue has a major flaw: as you dequeue items, the 'front' moves forward, leaving unused empty space at the beginning. A Circular Queue connects the end back to the beginning, forming a circle. This ensures no space is wasted.
Think of it as a Ring Buffer used in traffic light controllers or CPU scheduling.
Concept
It utilizes modular arithmetic logic: next_index = (current_index + 1) % capacity.
- Front: Index of the first element.
- Rear: Index of the last element.
- Full Condition:
(rear + 1) % capacity == front - Empty Condition:
front == -1
How it Works
When you seek to enqueue an element:
- Check if full. If not, calculate new rear:
(rear + 1) % size. - Insert data.
When you dequeue:
- Check if empty. If not, retrieve data at front.
- Move front:
(front + 1) % size. - If front passes rear (queue becomes empty), reset both to -1.
Step-by-Step Breakdown
- Enqueue 10: Rear moves from -1 to 0. Arr[0] = 10.
- Enqueue 20: Rear moves to 1. Arr[1] = 20.
- Dequeue: Front moves to 1. Space at 0 is potentially reusable.
- Wrap Around: If Rear is at index N-1 and index 0 is free, Rear wraps to 0.
When to Use
- Memory Management: Buffering data streams (e.g., video buffering).
- Traffic Systems: Cyclic round-robin scheduling.
- Fixed Resource Pools: When total capacity is strictly limited.
When NOT to Use
- When you need dynamically growing storage (use a Linked List or dynamic Deque).
- When random access to the middle of the queue is required.
How to Identify
Problems asking for 'design a buffer', 'round robin', or 'moving average' over a fixed window size usually imply a Circular Queue/Buffer.
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