I knew what CIDR was but converting the prefix length to a bitmask on the fly under pressure is a different thing.
Clarify the problem constraints and edge cases first, then propose a solution that converts IP addresses to integers and checks rules in order. Discuss trade-offs between linear scan and optimized approaches like interval trees or prefix tries, and handle edge cases explicitly.
Pro tip: Mention that you would preprocess rules into a data structure for O(log n) or O(1) lookups if the ruleset is large, but start with a simple linear scan for correctness. This shows you balance simplicity with scalability.
Ask about input format, rule ordering, default behavior when no rule matches, and whether rules can overlap. Confirm if IPv4 only or IPv6 too.
Convert IP addresses and CIDR ranges to 32-bit integers for efficient comparison. Represent each rule as a start and end integer with an allow/deny action.
For each rule in order, check if the IP falls within the range; return the action of the first match. If no match, return the default (e.g., deny).
If rules are numerous, discuss using a trie of prefixes or sorting rules and binary search, but note that first-match semantics may require careful handling.
Walk through edge cases: IP at range boundaries, overlapping rules, no match, and invalid input. Write unit tests to verify correctness.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.