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README.md

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gno.land/p/moul/gno4/v0

Connect Four, as rules and nothing else: New, Drop, Winner, Encode, Decode, SVG.

1import "gno.land/p/moul/gno4/v0"
2
3b, _ := gno4.Decode("1122334") // replay a whole game from 7 bytes
4b.Winner()                     // -> gno4.Red
5b.Line()                       // -> the four cells that won
6b.Image("board")               // -> ![board](data:image/svg+xml;base64,...)

A realm that wants a game needs three different things: the rules, the authority to decide who may move, and a way to show the result. Only the first is reusable, and only the first can be tested without a chain. This package is the first, alone: it imports no chain package, takes no address, reads no height, and has no idea a block exists.

A game is its move list, deliberately. Encode returns one digit per move and nothing else; Decode replays them. The position, the winner and the winning line are all derived, never stored, so there is exactly one representation and no second encoder that can drift from it. A whole game is at most 42 bytes, which is small enough that a realm stores the string and replays it rather than persisting a board.

Four behaviours worth knowing before you use it:

  • Drop is the only mutator. Every illegal position this package could reach is refused in one place, which is why Decode can be trusted: it replays through Drop, so an encoding describing an impossible game (a column overflowing, a move after the win) is rejected rather than becoming a board no rule allows.
  • At returns Empty out of range instead of panicking. Render loops walk past the edge on purpose, and a Render that panics is a realm page that is unreadable forever.
  • The winner is computed once, by the Drop that creates it. A finished game is rendered far more often than it is played, and rescanning 69 lines per read is gas spent to learn something the board already knew.
  • WinningMove is one ply deep, on purpose. It answers "can I win now" and "must I block now" for seven scans and no allocation. Real search belongs in the front-end, where nobody pays gas to think.

SVG draws the board with p/nt/svg and Image wraps it as a data:image/svg+xml;base64 markdown image, which is the one inline data URI gnoweb allows. It is generated at render time, so it costs no storage and there is no asset to go missing later.

Live demo: r/moul/gno4.