util/slicesx: add package for generic slice functions, use

Now that we're using rand.Shuffle in a few locations, create a generic
shuffle function and use it instead. While we're at it, move the
interleaveSlices function to the same package for use.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I0b00920e5b3eea846b6cedc30bd34d978a049fd3

util/slicesx/slicesx.go

@@ -0,0 +1,44 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package slicesx contains some helpful generic slice functions.
+package slicesx
+
+import "math/rand"
+
+// Interleave combines two slices of the form [a, b, c] and [x, y, z] into a
+// slice with elements interleaved; i.e. [a, x, b, y, c, z].
+func Interleave[S ~[]T, T any](a, b S) S {
+	// Avoid allocating an empty slice.
+	if a == nil && b == nil {
+		return nil
+	}
+
+	var (
+		i   int
+		ret = make([]T, 0, len(a)+len(b))
+	)
+	for i = 0; i < len(a) && i < len(b); i++ {
+		ret = append(ret, a[i], b[i])
+	}
+	ret = append(ret, a[i:]...)
+	ret = append(ret, b[i:]...)
+	return ret
+}
+
+// Shuffle randomly shuffles a slice in-place, similar to rand.Shuffle.
+func Shuffle[S ~[]T, T any](s S) {
+	// TODO(andrew): use a pooled Rand?
+
+	// This is the same Fisher-Yates shuffle implementation as rand.Shuffle
+	n := len(s)
+	i := n - 1
+	for ; i > 1<<31-1-1; i-- {
+		j := int(rand.Int63n(int64(i + 1)))
+		s[i], s[j] = s[j], s[i]
+	}
+	for ; i > 0; i-- {
+		j := int(rand.Int31n(int32(i + 1)))
+		s[i], s[j] = s[j], s[i]
+	}
+}