The CIDR matching itself isn't the hard part.
Clarify the rules and edge cases, then propose an efficient algorithm that parses CIDR blocks and compares prefix lengths. Discuss trade-offs between sorting rules and using a trie, and walk through an example to validate the logic.
Pro tip: Mention that you would preprocess rules into a trie for O(32) lookup time, which is crucial for high-throughput systems like AWS security groups. Also, highlight the importance of handling overlapping rules correctly by considering both prefix length and rule order.
Ask about input size, rule ordering, and whether rules can overlap. Confirm that longer prefixes take precedence and later rules win on ties, with default deny.
Decide between sorting rules by prefix length and iterating, or building a trie for efficient lookup. Discuss time/space trade-offs.
For each rule, check if the target IP falls within the CIDR block. Track the best match based on prefix length and rule order.
If multiple rules have the same prefix length, select the one that appears later. If no rule matches, return deny.
Walk through a sample list and target IP to verify correctness, including edge cases like exact match and no match.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.