Browsers offer five places to keep data, and they differ in size, lifetime, scope, speed, and whether the server sees them. A side-by-side guide to choosing — and the security mistakes, like tokens in localStorage, that come from choosing wrong.

The fewest inserts, deletes and substitutions to turn one word into another. How the Levenshtein distance is computed with a small DP table, how to recover the actual edits, and where it's used — from 'did you mean' to fuzzy search and DNA alignment.

Instead of overwriting the current state, store every change as an immutable event and derive state by replaying them. What event sourcing gives you — full history, audit, time travel, new views of old data — what it costs, and how CQRS fits alongside it.

Dijkstra answers 'how far from here to everywhere'. Floyd–Warshall answers 'how far from everywhere to everywhere' — with three nested loops and one table. How the via-vertex idea works, how to rebuild the paths, and when it beats running Dijkstra many times.