lemmaly – skill Viết lách & Nội dung cho agent AI

lemmaly — Algorithm-First Proof

The model already knows Big-O, hash tables, divide-and-conquer, dynamic programming, sorting, graph algorithms, and amortized analysis. It just does not apply them spontaneously. lemmaly fixes the behavior, not the knowledge.

This skill is the gateway for an algorithm-discipline suite of four skills (lemmaly, mathguard, invariant-guard, complexity-cuts). It enforces the hard rules that every other guard in the suite assumes.

Violating the letter of these rules is violating the spirit of the skill. "Just this once" is how O(n²) ships to production.

When to Use This Skill

Use lemmaly when:

• Writing, editing, or reviewing code that involves loops, collections, lookups, searches, joins, recursion, graphs, queries, or any computation over more than a handful of items.
• About to write a for inside a for, .find / .includes / .indexOf inside a loop, await inside for / map / forEach over independent items, or one query per item in a collection.
• Auditing a codebase / PR for known anti-patterns (await-in-loop, .includes inside .filter, string-concat in loop, SELECT *, N+1, etc.).
• Reviewing AI-generated code that "looks idiomatic" but might hide O(n²) or N+1.

When in doubt, start at lemmaly — it is the gateway and will tell you when to escalate to its three sibling skills.

| If you are about to… | Use | Why |

| --- | --- | --- |

| Write *new* code that loops, queries, joins, recurses, or processes a collection | lemmaly | Forces complexity + data structure + algorithm family before code is written. |

| Refactor *existing* code that is already slow, OOMs, times out, or has nested loops / N+1 / repeated work | complexity-cuts | Corrective playbook for code that already shipped with bad Big-O. |

| Implement an algorithm where the obvious version is subtly wrong (binary search variants, in-place dedup, Boyer–Moore, QuickSelect partition, recursion with accumulators, fixed-point / termination concerns) | invariant-guard | Forces writing the function contract + loop invariant before code. The trap is in the contract, not the loop body. |

| Work with n ≥ 10⁶, similarity search, dedup at scale, top-K, streaming analytics, cardinality estimation, embeddings, FFT/NTT, dimensionality reduction, computational geometry, randomized algorithms | mathguard | Classical algorithms have hit their lower bound; an approximate or math-heavy technique (Bloom, HLL, Count-Min, MinHash/LSH, FFT, JL projection, sweep line, kd-tree) gives the asymptotic win. |

Routing flow

Are you writing new code?
├── yes → lemmaly (state complexity, structure, family BEFORE coding)
│         ├── classical algorithm at its lower bound AND n is large? → mathguard
│         └── subtle correctness trap (invariant, base case, off-by-one)? → invariant-guard
└── no, refactoring existing slow / OOM / timed-out code → complexity-cuts
          └── still slow after classical fixes? → mathguard

One-line mental model

• lemmaly = think first (prevention).
• complexity-cuts = clean up bad Big-O (correction).
• invariant-guard = prove it's correct (verification).
• mathguard = beat the classical floor (acceleration).

The Iron Law

NO NON-TRIVIAL CODE WITHOUT STATED COMPLEXITY, DATA STRUCTURE, AND ALGORITHM FAMILY

Before you write a loop, a recursion, a query, or any computation over more than a handful of items, three things must appear in your message — in this order:

1. time = O(?), space = O(?), with the dominant input dimension named.

2. The data structure you will use, with a one-phrase reason.

3. The algorithm family (one of: linear scan, two-pointer, sliding window, binary search, sort+sweep, hash join, BFS/DFS, topo sort, Dijkstra/A*, union-find, DP, greedy, recursion+memo, prefix sum, segment tree, monoid reduction).

If you cannot state all three, you do not understand the problem yet. Ask, or read more code. Do not write code.

Non-negotiable rules

1. State complexity before writing any non-trivial code. In one line:

• time = O(?), space = O(?)
• Dominant input dimension: n = what, with realistic magnitude (e.g. n ~ 10^6 rows)
• If you cannot state these, you do not yet understand the problem. Ask, or read more code.

2. Name the data structure with a one-phrase reason. Every collection-shaped value gets a deliberate choice from Array / List / Set / HashMap / TreeMap / Heap / Deque / Trie / Graph / BitSet / Counter / LinkedList — with the reason: "Set for expected/amortized constant-time membership under the runtime’s hash model", "Heap for top-K in O(n log k)", "Counter to fold the nested loop into a single pass". Default to hashed structures (Set, Map) for lookup inside loops. Default to streaming/iterator over materialized list when n is large.

3. Identify the algorithm family before writing. Name one of: linear scan, divide and conquer, two-pointer, sliding window, binary search, sort + sweep, hash join, BFS/DFS, topological sort, Dijkstra/A*, union-find, dynamic programming, greedy, recursion + memoization, prefix sum, segment tree, monoid reduction. If you cannot name a family, you are about to write brute force. Stop and reconsider.

4. Repeated work in loops is algorithmic waste. All of these are presumed wrong until justified:

• I/O inside a loop (database queries, HTTP calls, file reads) — consider bounded batches, bulk endpoints or streaming. Use a concurrency limit and service rate/memory budget; unbounded Promise.all over n remote calls can exhaust resources. Preserve sequential dependencies and transaction semantics.
• Recomputing the same value in a loop — hoist or memoize
• Re-sorting / re-grouping inside a loop — sort once outside
• Linear scan (.find, .indexOf, .includes, in list) inside a loop — precompute an index Map
• Allocating fresh structures per iteration when one can be safely reused — hoist only when values do not escape or alias across outputs/async tasks
• Materializing intermediate collections only to iterate again — fuse into one pass

If you must do any of these inside a loop, write one comment line explaining why.

5. No invented complexity or numbers. Never write "O(log n) on average" without an argument. Never write "10x faster" or "~3ms" without measuring. If you cannot derive the complexity, write <complexity: TBD>. If you have not measured, write <measured: TBD>. Move on.

The pre-write protocol

Before producing non-trivial code, your message must contain — in this order:

1. Problem shape — one sentence. ("Given n events with a timestamp, find the longest contiguous window where total weight ≤ K.")

2. Input dimensions — n = ?, realistic magnitude, whether hot path.

3. Target complexity — time = O(?), space = O(?).

4. Data structures — name them with a phrase each.

5. Algorithm family — one phrase.

6. Edge cases you will handle — empty, singleton, all-equal, n=1, n=max, overflow, duplicates. List the ones that apply.

7. The code.

If any of 1–6 is missing, do not emit code yet.

Canonical example — protocol vs no-protocol

The same problem with and without the seven-step protocol.

Problem. Given users: User[] and bannedIds: string[], return users whose id is not banned. Realistic n: 50k users, 5k banned.

Without the protocol — ships O(n·m)

// Looks idiomatic, ships O(n·m)
const active = users.filter((u) => !bannedIds.includes(u.id));

bannedIds.includes is O(m) per call. The filter runs it n times → up to 50k × 5k = 250M comparisons in the worst case; early matches reduce actual comparisons.

With the protocol — O(n + m)

// Protocol applied:
//   expected time = O(n + m), auxiliary index space = O(m), output space = O(n), n = 50k users, m = 5k banned
//   structure: Set<string> for expected/amortized constant-time membership
//   family: linear scan with hashed lookup
//   edge cases: empty users → [], empty bannedIds → users, duplicates in bannedIds → fine (Set dedupes)
const banned = new Set(bannedIds);
const active = users.filter((u) => !banned.has(u.id));

The first version is the default an AI ships when asked "filter the active users." The second is what the protocol forces — without changing how the code reads.

Rule catalog (the lemmaly scanner)

The upstream repo ships a deterministic CLI scanner with the same anti-patterns this skill enforces (a version-dependent rule catalog across multiple languages: JavaScript/TypeScript, Python, SQL, Java, C#, C++, Go, Rust, PHP, Ruby, Shell/Bash). Each rule has a documented why, an incorrect example, a correct example, and the sibling skill to escalate to.

The scanner is optional. Do not automatically clone and run the upstream

repository from its default branch, because that executes whatever code is

current in a third-part

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