hphp/util/overflow.h

   1 /*
   2    +----------------------------------------------------------------------+
   3    | HipHop for PHP                                                       |
   4    +----------------------------------------------------------------------+
   5    | Copyright (c) 2010-present Facebook, Inc. (http://www.facebook.com)  |
   6    +----------------------------------------------------------------------+
   7    | This source file is subject to version 3.01 of the PHP license,      |
   8    | that is bundled with this package in the file LICENSE, and is        |
   9    | available through the world-wide-web at the following url:           |
  10    | http://www.php.net/license/3_01.txt                                  |
  11    | If you did not receive a copy of the PHP license and are unable to   |
  12    | obtain it through the world-wide-web, please send a note to          |
  13    | license@php.net so we can mail you a copy immediately.               |
  14    +----------------------------------------------------------------------+
  15 */
  16
  17 #ifndef incl_HPHP_UTIL_OVERFLOW_H
  18 #define incl_HPHP_UTIL_OVERFLOW_H
  19
  20 namespace HPHP {
  21
  22 template<typename T>
  23 typename std::enable_if<std::is_integral<T>::value, bool>::type
  24 add_overflow(T a, T b) {
  25   // Cast to unsigned so that ua + ub isn't undefined.
  26   auto const ua = static_cast<typename std::make_unsigned<T>::type>(a);
  27   auto const ub = static_cast<typename std::make_unsigned<T>::type>(b);
  28
  29   // Overflow if the inputs have the same sign, and the result of addition has
  30   // the opposite sign.
  31   return (~(ua ^ ub) & (ua ^ (ua + ub))) >> std::numeric_limits<T>::digits;
  32 }
  33
  34 template<typename T>
  35 typename std::enable_if<std::is_integral<T>::value, bool>::type
  36 sub_overflow(T a, T b) {
  37   if (b == std::numeric_limits<T>::min()) {
  38     // a - (INT_MIN)  -->  a - (-huge)  -->  a + huge
  39     return a >= 0;
  40   }
  41   return add_overflow(a, -b);
  42 }
  43
  44 template<typename T>
  45 typename std::enable_if<std::is_integral<T>::value, bool>::type
  46 mul_overflow(T a, T b) {
  47   auto max = std::numeric_limits<T>::max();
  48   auto min = std::numeric_limits<T>::min();
  49
  50   // Handle bad div cases first.
  51   if (a == 0 || b == 0) {
  52     return false;
  53   }
  54   if ((a == min && b == -1) || (b == min && a == -1)) {
  55     return true;
  56   }
  57
  58   if (a > 0) {
  59     if (b > 0) {
  60       return a > max / b;
  61     } else {
  62       return b < min / a;
  63     }
  64   } else {
  65     if (b < 0) {
  66       return a < max / b;
  67     } else {
  68       return a < min / b;
  69     }
  70   }
  71 }
  72
  73 }
  74
  75 #endif