Implement CheckedNumeric::UnsignedAbs()

base/numerics/safe_math_impl.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_NUMERICS_SAFE_MATH_IMPL_H_
 #define BASE_NUMERICS_SAFE_MATH_IMPL_H_
 
 #include <stdint.h>
 
 #include <cmath>
@@ skipping 72 matching lines @@
           sizeof(Integer) * 2,
           std::numeric_limits<Integer>::is_signed>::type>::type type;
 };
 
 template <typename Integer>
 struct PositionOfSignBit {
   static const typename enable_if<std::numeric_limits<Integer>::is_integer,
                                   size_t>::type value = 8 * sizeof(Integer) - 1;
 };
 
+// This is used for UnsignedAbs, where we need to support floating-point
+// template instantiations even though we don't actually support the operations.
+// However, there is no corresponding implementation of e.g. CheckedUnsignedAbs,
+// so the float versions will not compile.
+template <typename Numeric,
+          bool IsInteger = std::numeric_limits<Numeric>::is_integer,
+          bool IsFloat = std::numeric_limits<Numeric>::is_iec559>
+struct UnsignedOrFloatForSize;
+
+template <typename Numeric>
+struct UnsignedOrFloatForSize<Numeric, true, false> {
+  typedef typename UnsignedIntegerForSize<Numeric>::type type;
+};
+
+template <typename Numeric>
+struct UnsignedOrFloatForSize<Numeric, false, true> {
+  typedef typename Numeric type;
+};
+
 // Helper templates for integer manipulations.
 
 template <typename T>
 bool HasSignBit(T x) {
   // Cast to unsigned since right shift on signed is undefined.
   return !!(static_cast<typename UnsignedIntegerForSize<T>::type>(x) >>
             PositionOfSignBit<T>::value);
 }
 
 // This wrapper undoes the standard integer promotions.
@@ skipping 179 matching lines @@
 template <typename T>
 typename enable_if<
     std::numeric_limits<T>::is_integer && !std::numeric_limits<T>::is_signed,
     T>::type
 CheckedAbs(T value, RangeConstraint* validity) {
   // Absolute value of a positive is just its identiy.
   *validity = RANGE_VALID;
   return value;
 }
 
+template <typename T>
+typename enable_if<std::numeric_limits<T>::is_integer &&
+                       std::numeric_limits<T>::is_signed,
+                   typename UnsignedIntegerForSize<T>::type>::type
+CheckedUnsignedAbs(T value) {
+  typedef typename UnsignedIntegerForSize<T>::type UnsignedT;
+  return value == std::numeric_limits<T>::min()
+             ? static_cast<UnsignedT>(std::numeric_limits<T>::max()) + 1
+             : static_cast<UnsignedT>(std::abs(value));
+}
+
+template <typename T>
+typename enable_if<std::numeric_limits<T>::is_integer &&
+                       !std::numeric_limits<T>::is_signed,
+                   T>::type
+CheckedUnsignedAbs(T value) {
+  // Absolute value of a positive is just its identiy.

[Jeffrey Yasskin, 2015/09/15 23:58:25]

"T is unsigned, so |value| must already be positive."?

+  return value;
+}
+
 // These are the floating point stubs that the compiler needs to see. Only the
 // negation operation is ever called.
 #define BASE_FLOAT_ARITHMETIC_STUBS(NAME)                        \
   template <typename T>                                          \
   typename enable_if<std::numeric_limits<T>::is_iec559, T>::type \
   Checked##NAME(T, T, RangeConstraint*) {                        \
     NOTREACHED();                                                \
     return 0;                                                    \
   }
 
@@ skipping 190 matching lines @@
                             sizeof(T) >= (2 * sizeof(Lhs)) &&
                             StaticDstRangeRelationToSrcRange<T, Rhs>::value !=
                                 NUMERIC_RANGE_CONTAINED &&
                             sizeof(T) >= (2 * sizeof(Rhs));
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_NUMERICS_SAFE_MATH_IMPL_H_