Competitive programming goldmines: Halim and the deep archive

The starter resources (CPH, CSES, USACO Guide) are in my earlier CP post. This is the layer beneath — the material strong competitors actually keep open, which almost never appears on beginner lists.

The Halim universe

• Competitive Programming 4 — Steven Halim, Felix Halim, Suhendry Effendy. Subtitled “the lower bound of programming contests in the 2020s,” and that’s the right frame: two volumes that are less a tutorial than a map of everything examinable, with thousands of classified problems. Where CPH teaches you techniques, CP4 tells you what exists — its taxonomy is how you discover that your gap is, say, DP on broken profiles before a contest exposes it.
• Methods to Solve — Halim’s companion site: thousands of judge problems, each labeled with the technique that solves it. Reverse it for training: pick a technique, solve five problems knowing the tag, then five mixed ones where you must recognize it cold. Recognition is the contest skill.
• VisuAlgo — Halim’s algorithm visualizations: segment trees, max flow, suffix arrays, animated step by step with your own inputs. The fastest way to repair a wrong mental model — watching lazy propagation actually propagate beats re-reading any proof.

The free book nobody assigns

• Principles of Algorithmic Problem Solving — Johan Sannemo. Free PDF, written by an IOI/ICPC coach, and unusual in the best way: it teaches the problem-solving process (simplify, look for invariants, solve a smaller case) as explicitly as the algorithms themselves. The chapters between “intro” and “here’s a segment tree” — the ones most books skip — are the reason to read it.

The reference layer

• KACTL — KTH’s ICPC team notebook: the most battle-tested algorithm library in the community, every routine fuzz-tested, sized to the 25-page ICPC limit. Read it even if you never compete in teams — seeing HLD or min-cost flow expressed in 40 tight, correct lines recalibrates what implementations should look like.
• cp-algorithms — already in my starter post, but it belongs in any goldmine list too: the translated-and-maintained e-maxx wiki is still the single best technique reference.
• Codeforces Catalog — the curated index over fifteen years of community blogs. The best explanations of advanced techniques (slope trick, Aliens trick, suffix automata) were never written as articles anywhere else — they’re Codeforces blogs, and this is how you find them.

The masterclass tier

• Petr Mitrichev’s blog — a legend’s contest diaries, going back to 2009. The value isn’t solutions; it’s watching how one of the strongest solvers alive narrates his own reasoning, including the wrong turns. Read the posts where he struggles.
• The AtCoder DP contest plus Errichto’s and Colin Galen’s deep-dive videos — the modern oral tradition. Watching a strong solver think in real time is deliberate-practice feedback you cannot get from editorials.

The pattern across all of these: the goldmines aren’t hidden because they’re obscure — they’re hidden because they sit one layer past where most people stop.