add missing file

mixing/expiry.go

@@ -0,0 +1,18 @@
+// Copyright (c) 2023-2024 The Decred developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package mixing
+
+import (
+	"time"
+
+	"github.com/decred/dcrd/chaincfg/v3"
+)
+
+// MaxExpiry returns the maximum allowed expiry for a new pair request message
+// created with a blockchain tip at tipHeight.
+func MaxExpiry(tipHeight uint32, params *chaincfg.Params) uint32 {
+	target := params.TargetTimePerBlock
+	return tipHeight + uint32(2*time.Hour/target) + 1
+}