What is a circular buffer (ring buffer) and when would you use one?

Question

Accepted Answer

A **circular buffer** is a fixed-size array treated as if its ends are joined into a ring. Two indices — **head** (read) and **tail** (write) — advance and wrap around using modulo arithmetic, giving an O(1) FIFO queue with **no allocation** after creation.

## How wrapping works

```text
capacity 5, after writing A,B,C,D and reading A,B:
index:  0   1   2   3   4
      [ . ][ . ][ C ][ D ][ . ]
              head^       tail^   (tail wraps to 0 next write)
```

## Code

```python
class RingBuffer:
    def __init__(self, n):
        self.buf = [None]*n
        self.head = self.tail = self.size = 0
        self.cap = n
    def enqueue(self, x):                  # O(1)
        if self.size == self.cap: raise Exception("full")
        self.buf[self.tail] = x
        self.tail = (self.tail + 1) % self.cap   # wrap
        self.size += 1
    def dequeue(self):                     # O(1)
        x = self.buf[self.head]
        self.head = (self.head + 1) % self.cap    # wrap
        self.size -= 1
        return x
```

| Operation | Time |
|---|---|
| enqueue / dequeue | O(1) |
| space | O(n) fixed |

## When to use

- **Streaming / audio / video buffers** — bounded, steady throughput.
- **Producer–consumer** pipelines with backpressure.
- **Logging the last N events** (overwrite oldest).

## Why it matters

A ring buffer provides predictable, allocation-free FIFO behavior, which is essential in real-time and embedded systems where dynamic allocation is too slow or risky.

Its bounded size also acts as natural backpressure, preventing fast producers from exhausting memory.