Circular Buffer Queue

Implement a class RingBuffer(capacity), a fixed-capacity first-in first-out queue built with head and tail index arithmetic.

A fixed-size queue sits at the thread boundaries of a trading system: between the feed handler and the strategy, and between the strategy and the order gateway. Production versions are ring buffers over pre-allocated memory. Here you build the single-threaded core.

It is backed by a single Python list of fixed length capacity, allocated once in the constructor. You may not use collections.deque (or anything from queue); the point is the wrap-around bookkeeping.

• enqueue(item): append at the back and report success, or report failure if the buffer is full (the buffer is unchanged).
• dequeue(): remove and return the oldest item, or report that the buffer is empty.
• peek(): return the oldest item without removing it, or report that the buffer is empty.
• size(): return the number of items currently held.

In Python enqueue returns True on success and False on a full buffer, and an empty buffer is reported as None.

class RingBuffer:
    def __init__(self, capacity: int) -> None: ...
    def enqueue(self, item: int) -> bool: ...
    def dequeue(self) -> int | None: ...
    def peek(self) -> int | None: ...
    def size(self) -> int: ...

The tests drive your class through the run_ops(capacity, ops) driver already in the scaffold (do not modify it): ops are ("enqueue", x), ("dequeue",), ("peek",) and ("size",), one recorded output per op.

Examples

run_ops(2, [("enqueue", 7), ("enqueue", 8), ("enqueue", 9), ("size",), ("peek",), ("dequeue",), ("dequeue",), ("dequeue",)])
# [True, True, False, 2, 7, 7, 8, None]
# the third enqueue is rejected: capacity 2, and FIFO order is preserved

run_ops(3, [("dequeue",), ("peek",), ("enqueue", 1), ("dequeue",), ("enqueue", 2), ("size",), ("peek",)])
# [None, None, True, 1, True, 1, 2]
# dequeue and peek on an empty buffer return None

Constraints

• 1 <= capacity <= 30000.
• Up to \(2 \times 10^5\) operations; items are integers (which are never confused with None or booleans in the outputs).
• Every operation must be \(O(1)\). A plain list used as a queue via pop(0) shifts every element on each dequeue; the fixed-length backing list with indices never moves an element.