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28 lines
1.2 KiB
Markdown
28 lines
1.2 KiB
Markdown
# Incident 2026-09-24: order totals off by one cent
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## Root cause
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**C-2** — the totals refactor in `src/totals.js`.
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The refactor replaced integer-cent arithmetic with a decimal discount factor
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(`priceCents * quantity * (1 - discountPercent / 100)`). Decimal factors such
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as 0.7 or 0.93 have no exact binary floating-point representation, so for
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line amounts whose exact discounted value lands precisely on a half-cent
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boundary (e.g. 165 cents at 30% off = 115.5), the float result lands just
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below the boundary and `Math.round` rounds down instead of half-up. Every
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affected order is undercharged by exactly one cent, matching the finance
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findings in `evidence/incident.txt`.
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## Evidence
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- `evidence/incident.txt`: every flagged order is off by exactly one cent in the
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store's favor, and all of them appeared after the 2026-09-23 deploy.
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- C-1 (logging) and C-3 (inventory timeout) cannot change totals; C-2 touched
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the totals computation itself.
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## Fix
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`src/totals.js` now computes line discounts with exact integer arithmetic:
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`floor((priceCents * quantity * (100 - discountPercent) + 50) / 100)`, which
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rounds half-up on exact cent boundaries with no floating-point error.
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