From f27ade02150fbbbfe7f493ad141de516e72cdcab Mon Sep 17 00:00:00 2001
From: Miguel Otero <miguel.otero@example.invalid>
Date: Wed, 4 Mar 2026 11:02:19 +0000
Subject: [PATCH] feat(ledger): add banker's rounding helper

Long runs of half-up rounding drift the balance upward by a cent
every few thousand postings.
---
 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