QRect: fix UB (int overflow) in center()
QRect::center() should be defined for any QRect(x1,y1,x2,x2), INT_MIN <= x1, x2, y1, y2 <= INT_MAX because the average of two signed integers is always representable as a signed integer. But not when it's calculated as (x1+x2)/2, since that expression overflows when x1 > INT_MAX - x2. Instead of playing games with Hacker's Delight-style expressions, or use Google's patented algorithm, which requires two divisions, take advantage of the fact that int is not intmax_t and perform the calculation in the qint64 domain. The cast back to int is always well- defined since, as mentioned, the result is always representable in an int. Fix a test-case that expected a nonsensical result due to overflow. [ChangeLog][QtCore][QRect] Fixed integer overflow in center(). This fixes the result for some corner-cases like a 1x1 rectangle at (INT_MIN, INT_MIN), for which the previous implementation could return anything (due to invoking undefined behavior), but commonly returned (0, 0). Change-Id: I1a885ca6dff770327dd31655c3eb473fcfeb8878 Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
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@ -245,7 +245,7 @@ Q_DECL_CONSTEXPR inline QPoint QRect::bottomLeft() const Q_DECL_NOTHROW
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{ return QPoint(x1, y2); }
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Q_DECL_CONSTEXPR inline QPoint QRect::center() const Q_DECL_NOTHROW
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{ return QPoint((x1+x2)/2, (y1+y2)/2); }
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{ return QPoint(int((qint64(x1)+x2)/2), int((qint64(y1)+y2)/2)); } // cast avoids overflow on addition
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Q_DECL_CONSTEXPR inline int QRect::width() const Q_DECL_NOTHROW
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{ return x2 - x1 + 1; }
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@ -2349,7 +2349,7 @@ void tst_QRect::center_data()
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QTest::newRow( "SmallestQRect" ) << getQRectCase( SmallestQRect ) << QPoint(1,1);
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QTest::newRow( "MiddleQRect" ) << getQRectCase( MiddleQRect ) << QPoint(0,0);
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QTest::newRow( "LargestQRect" ) << getQRectCase( LargestQRect ) << QPoint(INT_MAX/2,INT_MAX/2);
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QTest::newRow( "SmallestCoordQRect" ) << getQRectCase( SmallestCoordQRect ) << QPoint(0,0);
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QTest::newRow( "SmallestCoordQRect" ) << getQRectCase( SmallestCoordQRect ) << QPoint(INT_MIN, INT_MIN);
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QTest::newRow( "LargestCoordQRect" ) << getQRectCase( LargestCoordQRect ) << QPoint(0,0);
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QTest::newRow( "RandomQRect" ) << getQRectCase( RandomQRect ) << QPoint(105,207);
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QTest::newRow( "NegativeSizeQRect" ) << getQRectCase( NegativeSizeQRect ) << QPoint(-4,-4);
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