add checked_sub

Change-Id: I440cd18c249f38194cfd3dfd4a1fc4b7f80858d6
Reviewed-on: https://gerrit.libreoffice.org/43810
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/include/o3tl/safeint.hxx b/include/o3tl/safeint.hxx
index 1c8bf280171e..8b735fe2edfa 100644
--- a/include/o3tl/safeint.hxx
+++ b/include/o3tl/safeint.hxx
@@ -68,6 +68,11 @@ template<typename T> inline bool checked_add(T a, T b, T& result)
     return !msl::utilities::SafeAdd(a, b, result);
 }
 
+template<typename T> inline bool checked_sub(T a, T b, T& result)
+{
+    return !msl::utilities::SafeSubtract(a, b, result);
+}
+
 #elif (defined __GNUC__ && __GNUC__ >= 5) || (__has_builtin(__builtin_mul_overflow) && !(defined ANDROID && defined __clang__))
 
 template<typename T> inline bool checked_multiply(T a, T b, T& result)
@@ -80,6 +85,11 @@ template<typename T> inline bool checked_add(T a, T b, T& result)
     return __builtin_add_overflow(a, b, &result);
 }
 
+template<typename T> inline bool checked_sub(T a, T b, T& result)
+{
+    return __builtin_sub_overflow(a, b, &result);
+}
+
 #else
 
 //https://www.securecoding.cert.org/confluence/display/c/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow
@@ -149,6 +159,31 @@ template<typename T> inline typename std::enable_if<std::is_unsigned<T>::value,
     return false;
 }
 
+//https://www.securecoding.cert.org/confluence/display/c/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow
+template<typename T> inline typename std::enable_if<std::is_signed<T>::value, bool>::type checked_sub(T a, T b, T& result)
+{
+    if ((b > 0 && a < std::numeric_limits<T>::min() + b) ||
+        (b < 0 && a > std::numeric_limits<T>::max() + b)) {
+        return true;
+    }
+
+    result = a - b;
+
+    return false;
+}
+
+//https://www.securecoding.cert.org/confluence/display/c/INT30-C.+Ensure+that+unsigned+integer+operations+do+not+wrap
+template<typename T> inline typename std::enable_if<std::is_unsigned<T>::value, bool>::type checked_sub(T a, T b, T& result)
+{
+    if (a < b) {
+        return true;
+    }
+
+    result = a - b;
+
+    return false;
+}
+
 #endif
 
 }