From 311c071ae60b8246abd4faf58c9c23a94063dd95 Mon Sep 17 00:00:00 2001
From: Pat Whittingslow <graded.sp@gmail.com>
Date: Mon, 6 Jul 2026 22:35:41 -0300
Subject: [PATCH] add runtime.fastrandn (#5502)

---
 src/runtime/algorithm.go | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/runtime/algorithm.go b/src/runtime/algorithm.go
index fda226c03..cdc0eceb1 100644
--- a/src/runtime/algorithm.go
+++ b/src/runtime/algorithm.go
@@ -15,6 +15,14 @@ func fastrand() uint32 {
 	return xorshift32State
 }
 
+// fastrandn is the equivalent of fastrand()%n, but faster. Some packages
+// (e.g. wireguard-go's device/timers.go) reach it via
+// //go:linkname ... runtime.fastrandn, so it must exist in the runtime.
+func fastrandn(n uint32) uint32 {
+	// See https://lemire.me/blog/2016/06/30/fast-random-shuffling/
+	return uint32(uint64(fastrand()) * uint64(n) >> 32)
+}
+
 func initRand() {
 	r, _ := hardwareRand()
 	xorshift64State = uint64(r | 1) // protect against 0