Codebreaker

Fill the row with symbols and submit it. You are answered with two counts, and reading them correctly is the whole game.

• In place — the right symbol in the right slot.
• Misplaced — a symbol that is in the code, but not in that slot.

Each symbol in the code can be counted only once, and the in-place matches are counted first. So if the code holds a single circle and you guess four circles, you are told one — not four. That one rule is where most confusion in this kind of puzzle comes from, so it is worth reading twice.

Nothing in the feedback tells you which slot was right. Working that out is the puzzle.

Tap a symbol to place it in the highlighted slot, or press its letter — every symbol carries its letter, so the key to press is written on the thing. Arrow keys move between slots, Backspace clears one, and Enter submits.

Notes lets you cross a symbol off a slot as you reason. The marks are yours rather than the game’s — a crossed-off symbol can still be placed, so a mistake in your notes never locks you out of the right answer.

Each one is a distinct shape, carries a letter, and is coloured — in that order of importance. The puzzle is unchanged in greyscale, and unchanged for anybody who cannot tell two of the colours apart.

• A — Circle
• B — Triangle
• C — Square
• D — Diamond
• E — Hexagon
• F — Star
• G — Cross
• H — Chevron

• Classic — Four slots, six symbols, repeats allowed. Ten guesses. (1,296 possible codes)
• No repeats — Four slots, six symbols, and the code never repeats one. A different kind of thinking, in eight guesses. (360 possible codes)
• Expert — Five slots, eight symbols, repeats allowed. Twelve guesses. (32,768 possible codes)
• Daily code — The same code for everybody today. Ten guesses. (1,296 possible codes)
• Practice — Unlimited guesses, hints, and take-backs. Never sets a best, because it does not have to. (1,296 possible codes)

No repeats is a different game rather than an easier one: knowing the code cannot repeat a symbol collapses 1,296 possibilities to 360, and it changes the reasoning entirely, because every symbol you place rules itself out everywhere else.

Donald Knuth showed in 1976 that a perfect player never needs more than five guesses at the four-slot, six-symbol game. Our solver reproduces that: it cracks all 1,296 codes in an average of about 4.5 guesses and never needs a sixth.

Its opening is two pairs, and the reason is worth knowing. Of the 1,296 codes, no answer to that probe leaves more than 256 still standing — no other opening does better in the worst case. Opening with four different symbols feels more informative and is not.

The number to think about is not how many symbols you got right, it is how many codes are still standing. A guess that can only be answered two ways has told you almost nothing whatever the answer is; a guess whose answers split the field evenly has told you a great deal. Turn on how many codes still fit in the deductions panel and you can watch that number fall.

A guess you already know is wrong can still be the best question. When only a handful of codes remain, a probe that cannot possibly be the answer often separates them faster than one that could.

The whole game runs in your browser. Ordinary codes come from your browser’s cryptographic random generator; the Daily comes from the UTC date, so everybody gets the same one. Nothing about your game is uploaded, and no account is needed.

Your saved puzzle stores the seed it was generated from rather than the answer, so the code is not sitting in your browser storage next to the puzzle. That is a courtesy, not encryption — somebody determined enough to run the generator with that seed would get the answer.

Your records, your saved puzzle, your Daily history and your sound and assist settings are stored in this browser only. Nothing here asks for a name or an email, and the button on this page removes all of it — and only this game’s share of it.

Is it really free, and do I need an account?

Yes, and no. There is no sign-up, no trial and nothing to buy. The game runs in your browser and never sends your guesses anywhere.

Is this that boxed game with the coloured pegs?

It is the same kind of deduction puzzle and it is not that product. The symbols here are shapes carrying letters rather than coloured pegs, the feedback is two counted numbers rather than a scatter of markers, and the deduction panel has no counterpart in the boxed game at all.

What do the two numbers mean?

The filled marker counts symbols that are the right symbol in the right slot. The ring counts symbols that are in the code but in a different slot. A symbol in the code can only be counted once, and the right-slot matches are counted first — so four of one symbol against a code holding one of it scores one, not four.

Can a hint ever be wrong?

It cannot contradict what you have been told, and that is structural rather than a promise: the hint system is given your guesses and their feedback and nothing else. It has never seen the code. If a hint says a slot is settled, that is because every code still fitting your clues agrees about it.

Is the answer sitting in my browser storage?

No — the save carries the seed the puzzle was generated from, and the code is worked out again when you come back. That stops it being one glance away in developer tools. It is not encryption, and anybody determined enough to run the generator with that seed would get the answer.

What is the Daily code?

One code for everybody, seeded from the UTC date. It changes at midnight UTC rather than at your local midnight, and your run of solved days is kept in this browser.

• NonogramDeduce a picture from the numbers along its edges.
• SudokuNine by nine, with notes, hints and a real difficulty ladder.
• Word SearchFind every word in the grid, in any direction.