types/views: optimize SliceEqualAnyOrderFunc for small slices

If the total number of differences is less than a small amount, just do
the dumb quadratic thing and compare every single object instead of
allocating a map.

Updates tailscale/corp#25479

Signed-off-by: Andrew Dunham <[email protected]>
Change-Id: I8931b4355a2da4ec0f19739927311cf88711a840

Author: andrew-d

types/views/views.go

@@ -381,6 +381,29 @@ func SliceEqualAnyOrderFunc[T any, V comparable](a, b Slice[T], cmp func(T) V) b
 		return true
 	}
 
+	// For a small number of items, avoid the allocation of a map and just
+	// do the quadratic thing. We can also only check the items between
+	// diffStart and the end.
+	nRemain := a.Len() - diffStart
+	if nRemain <= 5 {
+		maxLen := a.Len() // same as b.Len()
+		for i := diffStart; i < maxLen; i++ {
+			av := cmp(a.At(i))
+			found := false
+			for j := diffStart; j < maxLen; j++ {
+				bv := cmp(b.At(j))
+				if av == bv {
+					found = true
+					break
+				}
+			}
+			if !found {
+				return false
+			}
+		}
+		return true
+	}
+
 	// count the occurrences of remaining values and compare
 	valueCount := make(map[V]int)
 	for i, n := diffStart, a.Len(); i < n; i++ {

types/views/views_test.go

@@ -188,6 +188,15 @@ func TestSliceEqualAnyOrderFunc(t *testing.T) {
 
 	// Nothing shared
 	c.Check(SliceEqualAnyOrderFunc(v, ncFrom("baz", "qux"), cmp), qt.Equals, false)
+
+	// Long slice that matches
+	longSlice := ncFrom("a", "b", "c", "d", "e", "f", "g", "h", "i", "j")
+	longSame := ncFrom("b", "a", "c", "d", "e", "f", "g", "h", "i", "j") // first 2 elems swapped
+	c.Check(SliceEqualAnyOrderFunc(longSlice, longSame, cmp), qt.Equals, true)
+
+	// Long difference; past the quadratic limit
+	longDiff := ncFrom("b", "a", "c", "d", "e", "f", "g", "h", "i", "k") // differs at end
+	c.Check(SliceEqualAnyOrderFunc(longSlice, longDiff, cmp), qt.Equals, false)
 }
 
 func TestSliceEqual(t *testing.T) {