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Volume discount tiers on quantity changes: discounted unit rounding · case 03

Discounted prorations drift by cents proportional to the quantity.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Input {unit, old, new quantities, tiers: ascending [[min_qty, pct_off]], left, period}. A quantity's discount is the pct of the last tier with qty >= min_qty and applies to all units: total(q) = q*unit*(100-pct) in cent-percent. Proration of q = total(q)*left/(100*period) half-up. Return [-proration(old), proration(new)].

Why this case matters

Crossing a volume tier on a mid-cycle quantity change re-prices every unit for the rest of the period.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression 0	`[-38652, 28421]`	`[-38646, 28424]`	Failed

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