Git Format-Patch Series - 0002 of 3

From ee74572dbf48ad511e3367f43bd88bf25c87615b Mon Sep 17 00:00:00 2001
From: Ada Fernsby <ada.fernsby@example.invalid>
Date: Mon, 2 Mar 2026 09:31:50 +0000
Subject: [PATCH 2/3] feat(ledger): add banker's rounding helper
In-Reply-To: <024f75262757878b20f7a3e64c1472d87a75d6b8.1772442000.git.ada.fernsby@example.invalid>
References: <024f75262757878b20f7a3e64c1472d87a75d6b8.1772442000.git.ada.fernsby@example.invalid>

Half-up rounding drifts the balance upward over long runs.

Signed-off-by: Ada Fernsby <ada.fernsby@example.invalid>
---
 internal/ledger/rounding.go | 17 +++++++++++++++++
 1 file changed, 17 insertions(+), 0 deletions(-)
 create mode 100644 internal/ledger/rounding.go

diff --git a/internal/ledger/rounding.go b/internal/ledger/rounding.go
new file mode 100644
index 0000000..d6f0c19
--- /dev/null
+++ b/internal/ledger/rounding.go
@@ -0,0 +1,17 @@
+package ledger
+
+// HalfEven rounds a minor-unit amount to the nearest multiple of step using
+// banker's rounding, which keeps long runs of postings from drifting upward.
+func HalfEven(minor, step int64) int64 {
+	if step <= 1 {
+		return minor
+	}
+	q, r := minor/step, minor%step
+	switch {
+	case 2*r > step:
+		q++
+	case 2*r == step && q%2 != 0:
+		q++
+	}
+	return q * step
+}
-- 
2.43.0