feat!: split NoBnbSolution into infeasible vs round-limit variants#55
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evanlinjin merged 1 commit intoJul 8, 2026
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`run_bnb` failing conflated three different cases behind one struct. Make
`NoBnbSolution` an enum that says which happened:
- `InsufficientFunds` — the candidates can't cover the target value.
- `MaxWeightExceeded` — the value is coverable, but every selection busts
`max_weight` (mirrors `SelectError`).
- `RoundLimit { max_rounds, rounds }` — hit the round cap; a solution may
still exist with more rounds.
The distinction is derivable in `run_bnb` without giving `BnbMetric` an error
channel: if the search runs to completion (rounds < max_rounds, or a final
pull drains the iterator) then no selection satisfies the target — a genuine
infeasibility — and a cheap `is_fundable` value check splits it into value vs
weight. Otherwise we merely ran out of rounds.
Breaking: `NoBnbSolution` is now an enum; its `max_rounds`/`rounds` fields move
under the `RoundLimit` variant.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Description
run_bnb's failure is currently a singleNoBnbSolution { max_rounds, rounds }, which conflates three different situations. This makes it an enum:InsufficientFunds— the candidates can't cover the target value.MaxWeightExceeded— the value is coverable, but every selection bustsmax_weight(mirrors theSelectErrorfromselect_until_target_met).RoundLimit { max_rounds, rounds }— hit the round cap; a solution may still exist with more rounds.How — without touching
BnbMetricThe metric returning
Nonejust means "prune"; it never needs an error channel. All the information already lives inrun_bnb: if the search runs to completion (rounds < max_rounds, or a final pull drains the iterator) then the tree was fully explored and no selection satisfies the target — a genuine infeasibility. A cheapis_fundablevalue check then splits that into value (InsufficientFunds) vs weight (MaxWeightExceeded). If instead we stopped on the round cap, it'sRoundLimit.The
MaxWeightExceededattribution is sound for the crate's metrics: a cap-blind metric can only exhaust-empty when the value itself is unreachable (otherwise it would have returned an over-cap selection, not failed), so you never falsely land there.Breaking
NoBnbSolutionbecomes an enum; themax_rounds/roundsfields move under theRoundLimitvariant.Display/Errorstill hold.Tests
Deterministic tests for each variant:
run_bnb_reports_{insufficient_funds,max_weight_exceeded,round_limit}.🤖 Generated with Claude Code