JS: Math.random(): fix range to not include 1.0.

[15.8.2.14] specifies that the Math.random() returns a number greator
or equal to 0, but less than 1. Libc however defines it to be less than
or equal to 1, so we have to divide the returned value by RAND_MAX+1.
Of course, in order to do this, we need to widen them to 64bits ints.

Task-number: QTBUG-48753
Change-Id: Ia4d808014dbf2a5575f4226779214bf0d5981f49
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/qml/jsruntime/qv4mathobject.cpp b/src/qml/jsruntime/qv4mathobject.cpp
index f1face007c..3d3ac84576 100644
--- a/src/qml/jsruntime/qv4mathobject.cpp
+++ b/src/qml/jsruntime/qv4mathobject.cpp
@@ -277,10 +277,15 @@ Q_GLOBAL_STATIC(QThreadStorage<bool *>, seedCreatedStorage);
 ReturnedValue MathObject::method_random(CallContext *context)
 {
     if (!seedCreatedStorage()->hasLocalData()) {
-        qsrand(QTime(0,0,0).msecsTo(QTime::currentTime()) ^ reinterpret_cast<quintptr>(context));
+        int msecs = QTime(0,0,0).msecsTo(QTime::currentTime());
+        Q_ASSERT(msecs >= 0);
+        qsrand(uint(uint(msecs) ^ reinterpret_cast<quintptr>(context)));
         seedCreatedStorage()->setLocalData(new bool(true));
     }
-    return Encode(qrand() / (double) RAND_MAX);
+    // rand()/qrand() return a value where the upperbound is RAND_MAX inclusive. So, instead of
+    // dividing by RAND_MAX (which would return 0..RAND_MAX inclusive), we divide by RAND_MAX + 1.
+    qint64 upperLimit = qint64(RAND_MAX) + 1;
+    return Encode(qrand() / double(upperLimit));
 }
 
 ReturnedValue MathObject::method_round(CallContext *context)