[core] Use Mersenne Twister engine with C++11

instead of the std::random_device.
Fixed max value probability for the default (C++03) rand().

srtcore/sync.cpp

@@ -283,10 +283,11 @@ bool srt::sync::CGlobEvent::waitForEvent()
 namespace srt
 {
 #if HAVE_CXX11
-static std::random_device& randomDevice()
+static std::mt19937& randomGen()
 {
     static std::random_device s_RandomDevice;
-    return s_RandomDevice;
+    static std::mt19937 s_GenMT19937(s_RandomDevice());
+    return s_GenMT19937;
 }
 #elif defined(_WIN32) && defined(__MINGW32__)
 static void initRandSeed()
@@ -300,7 +301,7 @@ static pthread_once_t s_InitRandSeedOnce = PTHREAD_ONCE_INIT;
 static unsigned int genRandSeed()
 {
     // Duration::count() does not depend on any global objects,
-    // therefore it is preferred over count)microseconds(..).
+    // therefore it is preferred over count_microseconds(..).
     const int64_t seed = sync::steady_clock::now().time_since_epoch().count();
     return (unsigned int) seed;
 }
@@ -325,7 +326,7 @@ int srt::sync::genRandomInt(int minVal, int maxVal)
     sync::ScopedLock lck(s_mtxRandomDevice);
 #if HAVE_CXX11
     uniform_int_distribution<> dis(minVal, maxVal); 
-    return dis(randomDevice());
+    return dis(randomGen());
 #else
 #if defined(__MINGW32__)
     // No rand_r(..) for MinGW.
@@ -342,9 +343,13 @@ int srt::sync::genRandomInt(int minVal, int maxVal)
 #endif
 
     // Map onto [minVal, maxVal].
-    // Note. The probablity to get maxVal as the result is minuscule.
-    const int res = minVal + static_cast<int>((maxVal - minVal) * rand_0_1);
-    return res;
+    // Note. There is a minuscule probablity to get maxVal+1 as the result.
+    // So we have to use long long to handle cases when maxVal = INT32_MAX.
+    // Also we must check 'res' does not exceed maxVal,
+    // which may happen if rand_0_1 = 1, even though the chances are low.
+    const long long llMaxVal = maxVal;
+    const int res = minVal + static_cast<int>((llMaxVal + 1 - minVal) * rand_0_1);
+    return min(res, maxVal);
 #endif // HAVE_CXX11
 }