QRandomGenerator: assert that bounded() calls have correct arguments

Otherwise, the math will fail badly. Documentation improved to reflect
reality.

Change-Id: I9e3d261ad9bf41cfb2b6fffd159085cd38e3c388
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 18 insertions, 8 deletions

diff --git a/src/corelib/global/qrandom.cpp b/src/corelib/global/qrandom.cpp
index 90df8653a7..bf01b7ae2a 100644
--- a/src/corelib/global/qrandom.cpp
+++ b/src/corelib/global/qrandom.cpp
@@ -903,6 +903,10 @@ inline QRandomGenerator::SystemGenerator &QRandomGenerator::SystemGenerator::sel
 
     \snippet code/src_corelib_global_qrandom.cpp 12
 
+    If the \a highest parameter is negative, the result will be negative too;
+    if it is infinite or NaN, the result will be infinite or NaN too (that is,
+    not random).
+
     \sa generateDouble(), bounded()
  */
 
@@ -934,7 +938,7 @@ inline QRandomGenerator::SystemGenerator &QRandomGenerator::SystemGenerator::sel
     \overload
 
     Generates one random 32-bit quantity in the range between 0 (inclusive) and
-    \a highest (exclusive). \a highest must not be negative.
+    \a highest (exclusive). \a highest must be positive.
 
     Note that this function cannot be used to obtain values in the full 32-bit
     range of int. Instead, use generate() and cast to int.
@@ -946,8 +950,11 @@ inline QRandomGenerator::SystemGenerator &QRandomGenerator::SystemGenerator::sel
     \fn quint32 QRandomGenerator::bounded(quint32 lowest, quint32 highest)
     \overload
 
-    Generates one random 32-bit quantity in the range between \a lowest (inclusive)
-    and \a highest (exclusive). The same result may also be obtained by using
+    Generates one random 32-bit quantity in the range between \a lowest
+    (inclusive) and \a highest (exclusive). The \a highest parameter must be
+    greater than \a lowest.
+
+    The same result may also be obtained by using
     \l{http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution}{\c std::uniform_int_distribution}
     with parameters \a lowest and \c{\a highest - 1}. That class can also be used to
     obtain quantities larger than 32 bits.
@@ -968,7 +975,8 @@ inline QRandomGenerator::SystemGenerator &QRandomGenerator::SystemGenerator::sel
     \overload
 
     Generates one random 32-bit quantity in the range between \a lowest
-    (inclusive) and \a highest (exclusive), both of which may be negative.
+    (inclusive) and \a highest (exclusive), both of which may be negative, but
+    \a highest must be greater than \a lowest.
 
     Note that this function cannot be used to obtain values in the full 32-bit
     range of int. Instead, use generate() and cast to int.
diff --git a/src/corelib/global/qrandom.h b/src/corelib/global/qrandom.h
index 46d3e0e152..260fc1501a 100644
--- a/src/corelib/global/qrandom.h
+++ b/src/corelib/global/qrandom.h
@@ -122,14 +122,16 @@ public:
         return quint32(value);
     }
 
-    int bounded(int highest)
+    quint32 bounded(quint32 lowest, quint32 highest)
     {
-        return int(bounded(quint32(highest)));
+        Q_ASSERT(highest > lowest);
+        return bounded(highest - lowest) + lowest;
     }
 
-    quint32 bounded(quint32 lowest, quint32 highest)
+    int bounded(int highest)
     {
-        return bounded(highest - lowest) + lowest;
+        Q_ASSERT(highest > 0);
+        return int(bounded(0U, quint32(highest)));
     }
 
     int bounded(int lowest, int highest)