Simplified Grep

Implement a simplified `grep`: scan a file line by line and emit lines containing a target substring, with extensibility for additional flags (case-insensitive, invert match, line numbers). Graded for clean parsing and extensible code shape, not raw algorithm. Requirements Implement a function or small class: List<String> grep(Iterator<String> lines, String pattern, GrepFlags flags) that returns the lines containing pattern as a substring. The interviewer cares about: Clean parsing. Separate the line reader, the matcher, and the formatter. Flag extensibility. The API must accept a flags object that can later be extended without breaking callers (caseInsensitive, invertMatch, lineNumbers, wholeWord, etc.). Clarifying questions first. Confirm: case sensitivity default, line-number formatting, what counts as a line boundary, behavior on empty pattern. Follow-ups: Add caseInsensitive and invertMatch. Implement and show how the matcher is parameterized. Add lineNumbers: prefix output with <n>:. Discuss why this is a formatter concern, not a matcher concern. Switch the matcher to a regex. Now what changes about the API contract? Notes The algorithmic core is String.contains (or a regex match) — this round is not about KMP, Aho-Corasick, or any other clever string matcher unless the interviewer steers there. The grade is on code structure. A passing answer cleanly separates: source (line iterator), matcher (predicate on a line), formatter (line → output string). Each is independently testable. Don't read the whole file into memory; the iterator pattern makes this explicit. Common stumble: hard-coding the case-insensitive comparison inside the line loop instead of building it once into the matcher. Refactor before the interviewer points it out. An optional sub-discussion: how would you parallelize this across multiple files? (Thread per file, output queue ordered by file id; mention but don't implement unless asked.) Preparation Sketch a 30-line skeleton that already has the matcher factored as a Predicate<String> and the formatter as a function of (lineNumber, line) -> String. Reuse this shape for any extension the interviewer asks for. Drill clarifying questions out loud: this round explicitly grades whether the candidate enumerates ambiguities before writing. Be ready to add a flag in 30 seconds of editing — pattern of if (flags.foo) ... checks inside grep() is the wrong shape; pre-composing the predicate is the right shape.