refactor(budget): the cost preview splits through the shared cent split the settlement uses

client/src/components/Budget/CostsPanel.helpers.test.ts

@@ -104,11 +104,9 @@ describe('rebalancePayers', () => {
 
 // ── Client/server share parity (#2176) ───────────────────────────────────────
 //
-// splitEqualShares exists twice: here (previewing the split in euros) and on the
-// server (netting the settlement in cents — BudgetService.splitEqualShares in
-// server/src/nest/budget/budget.service.ts). The fixture below is duplicated
-// verbatim in server/tests/unit/nest/budget.service.calc.test.ts; if either
-// implementation drifts — sign handling included — its copy of this table fails.
+// The preview's splitEqualShares returns euros on top of the shared cent split
+// the server settles with (shared/src/money/money.ts, whose money.spec.ts holds
+// the same table). This copy checks that the euro conversion keeps every cent.
 const SHARE_PARITY_FIXTURE: { totalCents: number; users: number[]; itemId: number; expected: Record<number, number> }[] = [
   { totalCents: 10000, users: [1, 2, 3], itemId: 0, expected: { 1: 3334, 2: 3333, 3: 3333 } },
   { totalCents: 10000, users: [1, 2, 3], itemId: 1, expected: { 1: 3333, 2: 3334, 3: 3333 } },

client/src/components/Budget/CostsPanel.helpers.ts

@@ -11,6 +11,7 @@
  * Amounts are the raw input strings, parsed on use (same as customAmounts).
  */
 
+import { splitEqualShares as splitEqualMinorShares, toMinor } from '@trek/shared'
 import type { BudgetParticipantFinal, ReceiptRead } from '@trek/shared'
 import { amountToInputString, currencyDecimals } from '../../utils/formatters'
 import type { ExpensePrefill } from './CostsPanel'
@@ -131,38 +132,15 @@ export function rebalancePayers(
 }
 
 /**
- * Split `total` equally across `members`, in whole cents.
- *
- * The remainder cent rotates with the item id rather than always landing on the
- * first member, so across several expenses the rounding evens out instead of
- * always favouring the same person.
- *
- * Must stay share-for-share identical to the server's
- * BudgetService.splitEqualShares (budget.service.ts) — the settlement is netted
- * there, this copy only previews it. The remainder is `totalCents - baseCents*n`
- * rather than `%` so a negative total (a refund, #2176) still yields a remainder
- * in [0, n) and the shares sum back to the total exactly, like the server's do.
- * The parity fixture in CostsPanel.helpers.test.ts pins both sides.
+ * Split `total` equally across `members`, in whole cents, as euros for the
+ * preview. The cents come from the shared `splitEqualShares` the server nets
+ * the settlement with, so the preview and the settlement cannot disagree: the
+ * remainder cent rotates with the item id and a negative total (a refund,
+ * #2176) still sums back exactly.
  */
 export function splitEqualShares(total: number, members: { user_id: number }[], itemId: number): Record<number, number> {
-  const n = members.length
-  if (n === 0) return {}
-
-  const totalCents = Math.round(total * 100)
-  const baseCents = Math.floor(totalCents / n)
-  const remainder = totalCents - baseCents * n
-
-  const shares: Record<number, number> = {}
-  const sortedMembers = [...members].sort((a, b) => a.user_id - b.user_id)
-  const startIndex = itemId % n
-
-  for (let i = 0; i < n; i++) {
-    const member = sortedMembers[i]
-    const hasExtraCent = ((i - startIndex + n) % n) < remainder
-    shares[member.user_id] = (baseCents + (hasExtraCent ? 1 : 0)) / 100
-  }
-
-  return shares
+  const cents = splitEqualMinorShares(toMinor(total), members, itemId)
+  return Object.fromEntries(Object.entries(cents).map(([id, c]) => [id, c / 100]))
 }
 
 /** One line of a receipt: what it cost and who is in on it. */

shared/CLAUDE.md

@@ -65,7 +65,7 @@ The parity rule governs **existing** routes; it is not a license to mint new deb
 
 ## Money (`src/money/money.ts`)
 
-`toMinor`, `sumMinor`, `splitEqualShares` and `currencyDecimals` are the budget arithmetic both sides must agree on to the cent: amounts are netted in whole hundredths, equal splits hand the leftover hundredths out by rotation from `itemId % n`, and the shares always sum back to the total. The server's settlement uses these directly; `money.spec.ts` holds the share table that used to be duplicated between the server and client tests. The client still carries its own `splitEqualShares` (in euros, `client/src/components/Budget/CostsPanel.helpers.ts`) and `currencyDecimals` (`client/src/utils/formatters.ts`); folding them onto these is client work, not a second implementation to copy.
+`toMinor`, `sumMinor`, `splitEqualShares` and `currencyDecimals` are the budget arithmetic both sides must agree on to the cent: amounts are netted in whole hundredths, equal splits hand the leftover hundredths out by rotation from `itemId % n`, and the shares always sum back to the total. The server's settlement uses these directly; `money.spec.ts` holds the share table that used to be duplicated between the server and client tests. The client's euro preview (`splitEqualShares` in `client/src/components/Budget/CostsPanel.helpers.ts`) converts the shared cent split; only its `currencyDecimals` (`client/src/utils/formatters.ts`) is still a copy of its own, and folding it onto this one is client work, not a second implementation to copy.
 
 ## Permissions (`src/permissions/permissions.ts`)