fix: enforce must-use-both-dice and must-use-larger-die rules#129
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A player could complete a turn using fewer dice than the rules require (e.g. roll [5,2], play only the 5 and strand the 2 even though playing the 2 first then the 5 uses both). pureMove validated against the post-move state, where the stranded die was already consumed, so the alternative- sequence search always found nothing. Compute the maximum dice any legal ordering can use from the turn-start board in Play.initialize, persist it on the play (maxDiceUsable, dieValuesPlayableAtStart), and enforce it in pureMove once no ready moves remain: throw MustUseBothDiceError when fewer dice were used than possible, and MustUseLargerDieError for the either-but-not-both case. Also: - Make the Sim engine rule-compliant (try candidates best-first, fall back on rule rejection) so simulations only play legal moves. - Fix the Sim LCG normalization: (s & 0xffffffff) is signed in JS and produced negative values, yielding invalid die values once the seeded RNG was wired into Dice for deterministic runs. - Add an optional injectable random source to Dice (default unchanged) so simulations are deterministic per seed.
The must-use-both-dice enforcement reads play.maxDiceUsable / dieValuesPlayableAtStart, added in backgammon-types 1.0.3. Lockfile regeneration is deferred until 1.0.3 is published to npm.
….0.4 Resolves the pre-existing npm ci drift (lock pinned rc.1/rc.3 against ^1.0.0 ranges) and points at the published backgammon-types 1.0.3 that carries the maxDiceUsable fields.
This was referenced Jun 7, 2026
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Problem
In production game
01727ded-7a6a-4125-8729-02f44a6b60db, a player rolled[5,2]and was allowed to play only the 5 (cc7→cc2), recording the 2 as ano-move— illegal. A legal sequence using both dice existed: play the 2 first (cc7→cc5), then the 5 (cc23→cc18). The rules require using as many dice as legally possible.Root cause:
Play.pureMovevalidated against the post-move state (result.newBoard/result.newPlay), where the stranded die was already consumed. The alternative-sequence explorer only inspectsreadymoves and found none, so it always reported the single-die play as valid.Fix
Play.initializecomputes the maximum dice any legal ordering can use from the turn-start board (reusingtestSequenceDiceUsage) and persists it on the play (maxDiceUsable,dieValuesPlayableAtStart). Covers doubles.Play.pureMoveenforces it once no ready moves remain:MustUseBothDiceErrorwhen fewer dice were used than possible;MustUseLargerDieErrorfor the either-but-not-both case.Incidental fixes surfaced by the enforcement
(s & 0xffffffff)is signed in JS and produced negative values → invalid die values once the seeded RNG was wired intoDice. Fixed normalization tos / 0x100000000.Dicegained an optional injectable random source (default = cryptorandomInt, unchanged) so simulations are deterministic per seed; this makes the previously-flakysimulate-until-winreliable.Tests
must-use-both-dice-rule.test.ts: the exact production position (both directions), doubles, and larger-die — 9 tests.Depends on
nodots/backgammon-types PR #66 (adds the two
BackgammonPlayfields).