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]
+	}
+}

util/slicesx/slicesx_test.go

@@ -0,0 +1,66 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package slicesx
+
+import (
+	"reflect"
+	"testing"
+
+	"golang.org/x/exp/slices"
+)
+
+func TestInterleave(t *testing.T) {
+	testCases := []struct {
+		name string
+		a, b []int
+		want []int
+	}{
+		{name: "equal", a: []int{1, 3, 5}, b: []int{2, 4, 6}, want: []int{1, 2, 3, 4, 5, 6}},
+		{name: "short_b", a: []int{1, 3, 5}, b: []int{2, 4}, want: []int{1, 2, 3, 4, 5}},
+		{name: "short_a", a: []int{1, 3}, b: []int{2, 4, 6}, want: []int{1, 2, 3, 4, 6}},
+		{name: "len_1", a: []int{1}, b: []int{2, 4, 6}, want: []int{1, 2, 4, 6}},
+		{name: "nil_a", a: nil, b: []int{2, 4, 6}, want: []int{2, 4, 6}},
+		{name: "nil_all", a: nil, b: nil, want: nil},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			merged := Interleave(tc.a, tc.b)
+			if !reflect.DeepEqual(merged, tc.want) {
+				t.Errorf("got %v; want %v", merged, tc.want)
+			}
+		})
+	}
+}
+
+func BenchmarkInterleave(b *testing.B) {
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Interleave(
+			[]int{1, 2, 3},
+			[]int{9, 8, 7},
+		)
+	}
+}
+func TestShuffle(t *testing.T) {
+	var sl []int
+	for i := 0; i < 100; i++ {
+		sl = append(sl, i)
+	}
+
+	var wasShuffled bool
+	for try := 0; try < 10; try++ {
+		shuffled := slices.Clone(sl)
+		Shuffle(shuffled)
+		if !reflect.DeepEqual(shuffled, sl) {
+			wasShuffled = true
+			break
+		}
+	}
+
+	if !wasShuffled {
+		t.Errorf("expected shuffle after 10 tries")
+	}
+}