util/rands: add Shuffle and Perm functions with on-stack RNG state

The new math/rand/v2 package includes an m-local global random number
generator that can not be reseeded by the user, which is suitable for
most uses without the RNG pools we have in a number of areas of the code
base.

The new API still does not have an allocation-free way of performing a
seeded operations, due to the long term compiler bug around interface
parameter escapes, and the Source interface.

This change introduces the two APIs that math/rand/v2 can not yet
replace efficiently: seeded Perm() and Shuffle() operations. This
implementation chooses to use the PCG random source from math/rand/v2,
as with sufficient compiler optimization, this source should boil down
to only two on-stack registers for random state under ideal conditions.

Updates #17243

Signed-off-by: James Tucker <james@tailscale.com>

util/rands/cheap_test.go

@@ -0,0 +1,96 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package rands
+
+import (
+	"slices"
+	"testing"
+
+	randv2 "math/rand/v2"
+)
+
+func TestShuffleNoAllocs(t *testing.T) {
+	seed := randv2.Uint64()
+	data := make([]int, 100)
+	for i := range data {
+		data[i] = i
+	}
+	if n := testing.AllocsPerRun(1000, func() {
+		Shuffle(seed, data)
+	}); n > 0 {
+		t.Errorf("Rand got %v allocs per run", n)
+	}
+}
+
+func BenchmarkStdRandV2Shuffle(b *testing.B) {
+	seed := randv2.Uint64()
+	data := make([]int, 100)
+	for i := range data {
+		data[i] = i
+	}
+	b.ReportAllocs()
+	for range b.N {
+		// PCG is the lightest source, taking just two uint64s, the chacha8
+		// source has much larger state.
+		rng := randv2.New(randv2.NewPCG(seed, seed))
+		rng.Shuffle(len(data), func(i, j int) { data[i], data[j] = data[j], data[i] })
+	}
+}
+
+func BenchmarkLocalShuffle(b *testing.B) {
+	seed := randv2.Uint64()
+	data := make([]int, 100)
+	for i := range data {
+		data[i] = i
+	}
+	b.ReportAllocs()
+	for range b.N {
+		Shuffle(seed, data)
+	}
+}
+
+func TestPerm(t *testing.T) {
+	seed := uint64(12345)
+	p := Perm(seed, 100)
+	if len(p) != 100 {
+		t.Errorf("got %v; want 100", len(p))
+	}
+	expect := [][]int{
+		{5, 7, 1, 4, 0, 9, 2, 3, 6, 8},
+		{0, 5, 9, 8, 1, 6, 2, 4, 3, 7},
+		{5, 2, 3, 1, 9, 7, 6, 8, 4, 0},
+		{4, 5, 7, 1, 6, 3, 8, 2, 0, 9},
+		{5, 7, 0, 9, 2, 1, 8, 4, 6, 3},
+	}
+	for i := range 5 {
+		got := Perm(seed+uint64(i), 10)
+		want := expect[i]
+		if !slices.Equal(got, want) {
+			t.Errorf("got %v; want %v", got, want)
+		}
+	}
+}
+
+func TestShuffle(t *testing.T) {
+	seed := uint64(12345)
+	p := Perm(seed, 10)
+	if len(p) != 10 {
+		t.Errorf("got %v; want 10", len(p))
+	}
+
+	expect := [][]int{
+		{9, 3, 7, 0, 5, 8, 1, 4, 2, 6},
+		{9, 8, 6, 2, 3, 1, 7, 5, 0, 4},
+		{1, 6, 2, 8, 4, 5, 7, 0, 3, 9},
+		{4, 5, 0, 6, 7, 8, 3, 2, 1, 9},
+		{8, 2, 4, 9, 0, 5, 1, 7, 3, 6},
+	}
+	for i := range 5 {
+		Shuffle(seed+uint64(i), p)
+		want := expect[i]
+		if !slices.Equal(p, want) {
+			t.Errorf("got %v; want %v", p, want)
+		}
+	}
+}