types/views: add SliceContains, View.ContainsFunc, View.IndexFunc

We were starting to write these elsewhere as little unexported copies
in misc places.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/views/views.go

@@ -132,6 +132,43 @@ func (v Slice[T]) AsSlice() []T {
 	return v.AppendTo(v.ж[:0:0])
 }
 
+// IndexFunc returns the first index of an element in v satisfying f(e),
+// or -1 if none do.
+//
+// As it runs in O(n) time, use with care.
+func (v Slice[T]) IndexFunc(f func(T) bool) int {
+	for i := 0; i < v.Len(); i++ {
+		if f(v.At(i)) {
+			return i
+		}
+	}
+	return -1
+}
+
+// ContainsFunc reports whether any element in v satisfies f(e).
+//
+// As it runs in O(n) time, use with care.
+func (v Slice[T]) ContainsFunc(f func(T) bool) bool {
+	for i := 0; i < v.Len(); i++ {
+		if f(v.At(i)) {
+			return true
+		}
+	}
+	return false
+}
+
+// SliceContains reports whether v contains element e.
+//
+// As it runs in O(n) time, use with care.
+func SliceContains[T comparable](v Slice[T], e T) bool {
+	for i := 0; i < v.Len(); i++ {
+		if v.At(i) == e {
+			return true
+		}
+	}
+	return false
+}
+
 // IPPrefixSlice is a read-only accessor for a slice of netaddr.IPPrefix.
 type IPPrefixSlice struct {
 	ж Slice[netaddr.IPPrefix]

types/views/views_test.go

@@ -11,6 +11,7 @@
 	"strings"
 	"testing"
 
+	qt "github.com/frankban/quicktest"
 	"inet.af/netaddr"
 )
 
@@ -73,3 +74,15 @@ type viewStruct struct {
 		}
 	}
 }
+
+func TestViewUtils(t *testing.T) {
+	v := SliceOf([]string{"foo", "bar"})
+	c := qt.New(t)
+
+	c.Check(v.ContainsFunc(func(s string) bool { return strings.HasPrefix(s, "f") }), qt.Equals, true)
+	c.Check(v.ContainsFunc(func(s string) bool { return strings.HasPrefix(s, "g") }), qt.Equals, false)
+	c.Check(v.IndexFunc(func(s string) bool { return strings.HasPrefix(s, "b") }), qt.Equals, 1)
+	c.Check(v.IndexFunc(func(s string) bool { return strings.HasPrefix(s, "z") }), qt.Equals, -1)
+	c.Check(SliceContains(v, "bar"), qt.Equals, true)
+	c.Check(SliceContains(v, "baz"), qt.Equals, false)
+}