Reinstate @johnynek's optimization from typelevel#3279

travisbrown · Feb 26, 2020 · 4b643d8 · 4b643d8
1 parent 2e12f4d
commit 4b643d8
Showing 1 changed file with 7 additions and 0 deletions.
diff --git a/kernel/src/main/scala/cats/kernel/CommutativeSemigroup.scala b/kernel/src/main/scala/cats/kernel/CommutativeSemigroup.scala
@@ -13,6 +13,13 @@ trait CommutativeSemigroup[@sp(Int, Long, Float, Double) A] extends Any with Sem
     new CommutativeSemigroup[A] {
       def combine(a: A, b: A): A =
         self.combine(a, self.combine(middle, b))
+
+      override def combineN(a: A, n: Int): A =
+        if (n <= 1) self.combineN(a, n)
+        else {
+          // a + m + a ... = combineN(a, n) + combineN(m, n - 1)
+          self.combine(self.combineN(a, n), self.combineN(middle, n - 1))
+        }
     }
 }