FA-86219 / Game economy crafting balance / Member archive

Currency exchange desk: Exchange fee truncates · case 04

Small conversions avoid most of the fee.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

rates map currency -> copper value. Same currency returns [amount, 0] without fee. Otherwise value = amount*rate[from]; fee = ceil(value*fee_bp/10000), at least 1 copper when value > 0 and fee_bp > 0 and 0 otherwise; net = value - fee; return [net // rate[to], net % rate[to]] (change in copper).

Why this case matters

Game economies leak or destroy currency when one crafting or pricing rule is off by one boundary, rounding stage or state update; the defect is observable in exact integer outcomes.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
fee rounds up #1	`[7, 49]`	`[7, 48]`	Failed

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