logging: fix compiler compatibility for one more constexpr function

[folly.git] / folly / portability / Constexpr.h

diff --git a/folly/portability/Constexpr.h b/folly/portability/Constexpr.h
old mode 100755 (executable)
new mode 100644 (file)
index 810f987..4d99b62
--- a/folly/portability/Constexpr.h
+++ b/folly/portability/Constexpr.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,40 +14,97 @@
  * limitations under the License.
  */
 
-#ifndef FOLLY_CONSTEXPR_H_
-#define FOLLY_CONSTEXPR_H_
+#pragma once
 
 #include <cstdint>
+#include <cstring>
+#include <type_traits>
 
 namespace folly {
 
+// TLDR: Prefer using operator< for ordering. And when
+// a and b are equivalent objects, we return b to make
+// sorting stable.
+// See http://stepanovpapers.com/notes.pdf for details.
 template <typename T>
 constexpr T constexpr_max(T a, T b) {
-  return a > b ? a : b;
+  return b < a ? a : b;
 }
 
+// When a and b are equivalent objects, we return a to
+// make sorting stable.
 template <typename T>
 constexpr T constexpr_min(T a, T b) {
-  return a < b ? a : b;
+  return b < a ? b : a;
 }
 
-#ifdef _MSC_VER
-constexpr size_t constexpr_strlen_internal(const char* s, size_t len) {
-  return *s == '\0' ? len : constexpr_strlen_internal(s + 1, len + 1);
+namespace detail {
+
+template <typename T, typename = void>
+struct constexpr_abs_helper {};
+
+template <typename T>
+struct constexpr_abs_helper<
+    T,
+    typename std::enable_if<std::is_floating_point<T>::value>::type> {
+  static constexpr T go(T t) {
+    return t < static_cast<T>(0) ? -t : t;
+  }
+};
+
+template <typename T>
+struct constexpr_abs_helper<
+    T,
+    typename std::enable_if<
+        std::is_integral<T>::value && !std::is_same<T, bool>::value &&
+        std::is_unsigned<T>::value>::type> {
+  static constexpr T go(T t) {
+    return t;
+  }
+};
+
+template <typename T>
+struct constexpr_abs_helper<
+    T,
+    typename std::enable_if<
+        std::is_integral<T>::value && !std::is_same<T, bool>::value &&
+        std::is_signed<T>::value>::type> {
+  static constexpr typename std::make_unsigned<T>::type go(T t) {
+    return typename std::make_unsigned<T>::type(t < static_cast<T>(0) ? -t : t);
+  }
+};
+} // namespace detail
+
+template <typename T>
+constexpr auto constexpr_abs(T t)
+    -> decltype(detail::constexpr_abs_helper<T>::go(t)) {
+  return detail::constexpr_abs_helper<T>::go(t);
 }
-static_assert(constexpr_strlen_internal("123456789", 0) == 9,
-              "Someone appears to have broken constexpr_strlen...");
-#endif
 
+namespace detail {
+
+template <typename Char>
+constexpr size_t constexpr_strlen_internal(const Char* s, size_t len) {
+  return *s == Char(0) ? len : constexpr_strlen_internal(s + 1, len + 1);
+}
+static_assert(
+    constexpr_strlen_internal("123456789", 0) == 9,
+    "Someone appears to have broken constexpr_strlen...");
+} // namespace detail
+
+template <typename Char>
+constexpr size_t constexpr_strlen(const Char* s) {
+  return detail::constexpr_strlen_internal(s, 0);
+}
+
+template <>
 constexpr size_t constexpr_strlen(const char* s) {
 #if defined(__clang__)
   return __builtin_strlen(s);
 #elif defined(_MSC_VER)
-  return s == nullptr ? 0 : constexpr_strlen_internal(s, 0);
+  return detail::constexpr_strlen_internal(s, 0);
 #else
-  return strlen(s);
+  return std::strlen(s);
 #endif
 }
 }
-
-#endif