Extensions

Small packages a math file imports. There is no plugin system, TS math can just import, so an extension is an ordinary package.

Generator<S> = (next) => Grid<S>          // makes a board
Selector<S>  = (grid, next) => Pos[]      // picks cells
Effect       = (grid, pos, next) => Grid  // changes a board

Three signatures carry all of it. next is always host.rng_next, nothing here makes randomness, everything receives it, and that is what lets one recorded seed replay a round.

New here? Build a slot walks the whole set end to end, from an empty grid to a running server.

The packages

• @open-rgs/grid: Shape is a height per column, not width by height.
• @open-rgs/weights: The number you read is the number you get.
• @open-rgs/strips: A fixed sequence, a stop, and the window it shows.
• @open-rgs/selectors: One primitive serving placement and hold-and-win targeting.
• @open-rgs/fill: Weighted draws, one cell at a time.
• @open-rgs/markov: Changes how clumpy reels look. Provably does not change frequency.
• @open-rgs/recipes: Every board comes from exactly one declared recipe.
• @open-rgs/paytable: One shared notion of payout, wild and scatter.
• @open-rgs/pay-lines: Longest run, paid once.
• @open-rgs/pay-ways: Paid by the product of per-column counts.
• @open-rgs/pay-anywhere: Total count, position irrelevant.
• @open-rgs/pay-cluster: A wild belongs to every cluster it touches.
• @open-rgs/cascade: Clear, fall, refill, repeat.
• @open-rgs/holdwin: A landing resets the counter.
• @open-rgs/freespins: A spin is paid at the multiplier that was showing.
• @open-rgs/picks: A pick round is priced by what it removes.
• @open-rgs/gamble: The interesting number is the edge, not the payout.
• @open-rgs/symbols: What a symbol turns into after the board is drawn.
• @open-rgs/meters: Collect five of these and something happens.
• @open-rgs/multiways: Each reel draws its own height, every spin.
• @open-rgs/big-symbols: One symbol across a block of cells.
• @open-rgs/scatters: The trigger rate is a parameter, not an emergent property.

Writing your own

Every extension point is one of the three signatures, so extending means writing a function. "Collect only from the same column" is a selector, not a new engine.

const sameColumn = (src) => (grid, next) =>
  coinCells()(grid, next).filter((p) => p.col === src.col);

const columnCollector = (src) => collector(sameColumn(src), JACKPOTS);

The one rule: take next, never make it. Code that reaches for Math.random() looks right, passes every test you would think to write, and silently voids any RTP measured from a replay.