fix(csv): avoid rounding drift by allocating shares in minor units with last-share remainder\n\nPreviously, participant shares were computed in major units and rounded per-participant before saldo calculation. For amounts like 1.01 split across 2 people, this caused mismatches (e.g. -0.51 and +0.50) where per‑row saldos did not add up due to early rounding.\n\nThis change mirrors app behaviour: compute shares in minor units (cents), distribute floor(amount*share/total) incrementally and assign the remainder to the last relevant participant. Saldos are derived from these integer shares and only formatted at the end. This fixes the erroneous rounding assignment and keeps totals consistent.

Author: Uli-Z

src/app/groups/[groupId]/expenses/export/csv/route.ts

@@ -126,8 +126,22 @@ export async function GET(
         shares,
       ]),
     ) as Record<string, number>
-    const totalShares = expense.paidFor.reduce(
-      (sum, { shares }) => sum + shares,
+    // Normalize shares based on split mode to mirror app logic
+    const isEvenly = expense.splitMode === 'EVENLY'
+    const normalizedSharesByParticipant: Record<string, number> = {}
+    for (const p of group.participants) {
+      if (isEvenly) {
+        normalizedSharesByParticipant[p.id] = expense.paidFor.some(
+          (pf) => pf.participantId === p.id,
+        )
+          ? 1
+          : 0
+      } else {
+        normalizedSharesByParticipant[p.id] = shareByParticipant[p.id] ?? 0
+      }
+    }
+    const totalShares = Object.values(normalizedSharesByParticipant).reduce(
+      (sum, v) => sum + v,
       0,
     )
 
@@ -153,36 +167,40 @@ export async function GET(
         : null,
       paidBy: participantIdNameMap[expense.paidById],
       splitMode: splitModeLabel[expense.splitMode],
-      // For every participant we export the *saldo* (net effect) of this single expense:
-      // - For the paying participant (paidBy):
-      //     saldo = totalAmount - participantShareAmount
-      //   -> how much they effectively advance for others.
-      // - For all other participants:
-      //     saldo = -participantShareAmount
-      //   -> how much they owe to the payer for this expense.
-      //
-      // The sum of all participant saldos for a given expense is always 0,
-      // which makes the CSV easy to aggregate in tools like Excel.
-      ...Object.fromEntries(
-        group.participants.map((participant) => {
-          const participantShare = shareByParticipant[participant.id] ?? 0
-
-          const participantShareAmount =
-            totalShares === 0
-              ? 0
-              : +formatAmountAsDecimal(
-                  (normalizedAmount / totalShares) * participantShare,
-                  currency,
-                )
-
+      // For every participant we export the saldo (net effect) of this single expense.
+      // Compute participant shares in minor units first to avoid rounding drift.
+      ...(() => {
+        const entries: [string, number][] = []
+        // Determine ordered list of participants that actually have shares
+        const participantsWithShares = group.participants
+          .map((p, idx) => ({ p, idx }))
+          .filter(({ p }) => (normalizedSharesByParticipant[p.id] ?? 0) > 0)
+          .map(({ p }) => p.id)
+
+        let remaining = normalizedAmount // minor units remaining to allocate
+        group.participants.forEach((participant) => {
+          const shares = normalizedSharesByParticipant[participant.id] ?? 0
+          let shareMinor = 0
+          if (totalShares > 0 && shares > 0) {
+            const isLast =
+              participant.id ===
+              participantsWithShares[participantsWithShares.length - 1]
+            if (isLast) {
+              shareMinor = remaining
+            } else {
+              shareMinor = Math.floor((normalizedAmount * shares) / totalShares)
+              remaining -= shareMinor
+            }
+          }
+          const shareMajor = +formatAmountAsDecimal(shareMinor, currency)
           const isPaidByParticipant = expense.paidById === participant.id
           const saldo = isPaidByParticipant
-            ? totalAmount - participantShareAmount
-            : -participantShareAmount
-
-          return [participant.name, saldo]
-        }),
-      ),
+            ? totalAmount - shareMajor
+            : -shareMajor
+          entries.push([participant.name, saldo])
+        })
+        return Object.fromEntries(entries)
+      })(),
     }
   })