constexpr_ceil

Summary: [Folly] `constexpr_ceil`, an integral rounding-up util.

Reviewed By: Orvid

Differential Revision: D6558042

fbshipit-source-id: 6b42add9bf2e3605baf71391130c2a2c88cc4385

folly/ConstexprMath.h

@@ -115,4 +115,11 @@ constexpr T constexpr_log2(T t) {
   return detail::constexpr_log2(T(0), t);
 }
 
+template <typename T>
+constexpr T constexpr_ceil(T t, T round) {
+  return round == T(0)
+      ? t
+      : ((t + (t < T(0) ? T(0) : round - T(1))) / round) * round;
+}
+
 } // namespace folly

folly/test/ConstexprMathTest.cpp

@@ -122,3 +122,24 @@ TEST_F(ConstexprMathTest, constexpr_log2_64) {
   EXPECT_EQ(6ull, a);
   EXPECT_TRUE((std::is_same<decltype(v), decltype(a)>::value));
 }
+
+TEST_F(ConstexprMathTest, constexpr_ceil) {
+  {
+    constexpr auto roundable = 20ull;
+    constexpr auto round = 6ull;
+    constexpr auto rounded = folly::constexpr_ceil(roundable, round);
+    EXPECT_EQ(24ull, rounded);
+  }
+  {
+    constexpr auto roundable = -20ll;
+    constexpr auto round = 6ll;
+    constexpr auto rounded = folly::constexpr_ceil(roundable, round);
+    EXPECT_EQ(-18ll, rounded);
+  }
+  {
+    constexpr auto roundable = -20ll;
+    constexpr auto round = 0ll;
+    constexpr auto rounded = folly::constexpr_ceil(roundable, round);
+    EXPECT_EQ(-20ll, rounded);
+  }
+}