util/set: move Slice type from corp to oss

This is an exact copy of the files misc/set/set{,_test}.go from
tailscale/corp@a5415daa9ca80ce9b798cd80148e4bc8818f8e15, plus the
license headers.

For use in #7877

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I712d09c6d1a180c6633abe3acf8feb59b27e2866

util/set/slice.go

@@ -0,0 +1,69 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package set
+
+import (
+	"golang.org/x/exp/slices"
+	"tailscale.com/types/views"
+)
+
+// Slice is a set of elements tracked in a slice of unique elements.
+type Slice[T comparable] struct {
+	slice []T
+	set   map[T]bool // nil until/unless slice is large enough
+}
+
+// Slice returns the a view of the underlying slice.
+// The elements are in order of insertion.
+// The returned value is only valid until ss is modified again.
+func (ss *Slice[T]) Slice() views.Slice[T] { return views.SliceOf(ss.slice) }
+
+// Contains reports whether v is in the set.
+// The amortized cost is O(1).
+func (ss *Slice[T]) Contains(v T) bool {
+	if ss.set != nil {
+		return ss.set[v]
+	}
+	return slices.Index(ss.slice, v) != -1
+}
+
+// Remove removes v from the set.
+// The cost is O(n).
+func (ss *Slice[T]) Remove(v T) {
+	if ss.set != nil {
+		if !ss.set[v] {
+			return
+		}
+		delete(ss.set, v)
+	}
+	if ix := slices.Index(ss.slice, v); ix != -1 {
+		ss.slice = append(ss.slice[:ix], ss.slice[ix+1:]...)
+	}
+}
+
+// Add adds each element in vs to the set.
+// The amortized cost is O(1) per element.
+func (ss *Slice[T]) Add(vs ...T) {
+	for _, v := range vs {
+		if ss.Contains(v) {
+			continue
+		}
+		ss.slice = append(ss.slice, v)
+		if ss.set != nil {
+			ss.set[v] = true
+		} else if len(ss.slice) > 8 {
+			ss.set = make(map[T]bool, len(ss.slice))
+			for _, v := range ss.slice {
+				ss.set[v] = true
+			}
+		}
+	}
+}
+
+// AddSlice adds all elements in vs to the set.
+func (ss *Slice[T]) AddSlice(vs views.Slice[T]) {
+	for i := 0; i < vs.Len(); i++ {
+		ss.Add(vs.At(i))
+	}
+}

util/set/slice_test.go

@@ -0,0 +1,56 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package set
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestSliceSet(t *testing.T) {
+	c := qt.New(t)
+
+	var ss Slice[int]
+	c.Check(len(ss.slice), qt.Equals, 0)
+	ss.Add(1)
+	c.Check(len(ss.slice), qt.Equals, 1)
+	c.Check(len(ss.set), qt.Equals, 0)
+	c.Check(ss.Contains(1), qt.Equals, true)
+	c.Check(ss.Contains(2), qt.Equals, false)
+
+	ss.Add(1)
+	c.Check(len(ss.slice), qt.Equals, 1)
+	c.Check(len(ss.set), qt.Equals, 0)
+
+	ss.Add(2)
+	ss.Add(3)
+	ss.Add(4)
+	ss.Add(5)
+	ss.Add(6)
+	ss.Add(7)
+	ss.Add(8)
+	c.Check(len(ss.slice), qt.Equals, 8)
+	c.Check(len(ss.set), qt.Equals, 0)
+
+	ss.Add(9)
+	c.Check(len(ss.slice), qt.Equals, 9)
+	c.Check(len(ss.set), qt.Equals, 9)
+
+	ss.Remove(4)
+	c.Check(len(ss.slice), qt.Equals, 8)
+	c.Check(len(ss.set), qt.Equals, 8)
+	c.Assert(ss.Contains(4), qt.IsFalse)
+
+	// Ensure that the order of insertion is maintained
+	c.Assert(ss.Slice().AsSlice(), qt.DeepEquals, []int{1, 2, 3, 5, 6, 7, 8, 9})
+	ss.Add(4)
+	c.Check(len(ss.slice), qt.Equals, 9)
+	c.Check(len(ss.set), qt.Equals, 9)
+	c.Assert(ss.Contains(4), qt.IsTrue)
+	c.Assert(ss.Slice().AsSlice(), qt.DeepEquals, []int{1, 2, 3, 5, 6, 7, 8, 9, 4})
+
+	ss.Add(1, 234, 556)
+	c.Assert(ss.Slice().AsSlice(), qt.DeepEquals, []int{1, 2, 3, 5, 6, 7, 8, 9, 4, 234, 556})
+}