Implement an expiring counter class

Expiring Counter (sliding time window)

Design and implement an expiring counter that counts events in a recent time window.

Requirements

• You are given a fixed window size W in seconds when constructing the counter.
• Support two operations:
  1. inc(timestamp, delta=1) : record delta events occurring at integer time timestamp (in seconds).
  2. get(timestamp) -> int : return the total number of events recorded in the inclusive time range:

$[timestamp - W + 1,\; timestamp]$

Assumptions / clarifications (state and use in your solution)

• Timestamps are integer seconds.
• You may assume calls arrive with non-decreasing timestamps (typical for streaming counters). If you choose not to assume this, explain how you would handle out-of-order inputs.

Constraints / goals

• Aim for efficient time and space usage: roughly O(1) amortized per operation and memory proportional to the window.

Follow-up

If inc() can be called up to 1,000,000 times per second, how would you modify the design to reduce memory and CPU overhead (e.g., batching/aggregating within each second)?