to_ordering to convert from int to ordering

Summary: [Folly] `to_ordering` to convert from `int` to `ordering`.

Reviewed By: stevegury

Differential Revision: D7596709

fbshipit-source-id: cf1316a7560b9398b94d1c834258f09a797a98f8

folly/lang/Ordering.h

@@ -20,4 +20,9 @@ namespace folly {
 
 enum class ordering : int { lt = -1, eq = 0, gt = 1 };
 
+template <typename T>
+constexpr ordering to_ordering(T c) {
+  return c < T(0) ? ordering::lt : c > T(0) ? ordering::gt : ordering::eq;
+}
+
 } // namespace folly

folly/lang/test/OrderingTest.cpp

@@ -27,3 +27,13 @@ TEST_F(OrderingTest, ordering) {
   EXPECT_EQ(0, int(ordering::eq));
   EXPECT_EQ(+1, int(ordering::gt));
 }
+
+TEST_F(OrderingTest, to_ordering) {
+  EXPECT_EQ(ordering::lt, to_ordering(int(ordering::lt)));
+  EXPECT_EQ(ordering::eq, to_ordering(int(ordering::eq)));
+  EXPECT_EQ(ordering::gt, to_ordering(int(ordering::gt)));
+
+  EXPECT_EQ(ordering::lt, to_ordering(-22));
+  EXPECT_EQ(ordering::eq, to_ordering(0));
+  EXPECT_EQ(ordering::gt, to_ordering(+44));
+}