Build a small in-memory document indexer and retriever from scratch (inverted index + BM25), then add a vector option.

A bridge between classic DSA and modern RAG. The signal is building a working inverted index and a correct BM25 score by hand, reasoning about its complexity, and then knowing exactly when you would reach for embeddings and an ANN index instead.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.