QBitArray: fix fromBits() and actually test it

When I initially added it, it was ony for QCborValue, but I never added
the tests. Turns out there were two bugs:

[ChangeLog][QtCore][QBitArray] Fixed two bugs that caused QBitArrays
created using fromBits() not to compare equal to the equivalent
QBitArray created using other methods if the size was zero or not a
multiple of 4. If the size modulus 8 was 5, 6, or 7, the data was
actually incorrect.

Fixes: QTBUG-77285
Change-Id: Ife213d861bb14c1787e1fffd15b70573d162042c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qbitarray.cpp

@@ -1,7 +1,7 @@
 /****************************************************************************
 **
 ** Copyright (C) 2016 The Qt Company Ltd.
-** Copyright (C) 2016 Intel Corporation.
+** Copyright (C) 2019 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -132,12 +132,12 @@ QT_BEGIN_NAMESPACE
  * We overallocate the byte array by 1 byte. The first user bit is at
  * d.data()[1]. On the extra first byte, we store the difference between the
  * number of bits in the byte array (including this byte) and the number of
- * bits in the bit array. Therefore, it's always a number between 8 and 15.
+ * bits in the bit array. Therefore, for a non-empty QBitArray, it's always a
+ * number between 8 and 15. For the empty one, d is the an empty QByteArray and
+ * *d.constData() is the QByteArray's terminating NUL (0) byte.
  *
  * This allows for fast calculation of the bit array size:
  *    inline int size() const { return (d.size() << 3) - *d.constData(); }
- *
- * Note: for an array of zero size, *d.constData() is the QByteArray implicit NUL.
  */
 
 /*!
@@ -326,6 +326,8 @@ void QBitArray::fill(bool value, int begin, int end)
 QBitArray QBitArray::fromBits(const char *data, qsizetype size)
 {
     QBitArray result;
+    if (size == 0)
+        return result;
     qsizetype nbytes = (size + 7) / 8;
 
     result.d = QByteArray(nbytes + 1, Qt::Uninitialized);
@@ -334,7 +336,7 @@ QBitArray QBitArray::fromBits(const char *data, qsizetype size)
 
     // clear any unused bits from the last byte
     if (size & 7)
-        bits[nbytes] &= 0xffU >> (size & 7);
+        bits[nbytes] &= 0xffU >> (8 - (size & 7));
 
     *bits = result.d.size() * 8 - size;
     return result;

tests/auto/corelib/tools/qbitarray/tst_qbitarray.cpp

@@ -84,6 +84,8 @@ private slots:
     void operator_noteq();
 
     void resize();
+    void fromBits_data();
+    void fromBits();
 };
 
 void tst_QBitArray::size_data()
@@ -610,5 +612,60 @@ void tst_QBitArray::resize()
 
 }
 
+void tst_QBitArray::fromBits_data()
+{
+    QTest::addColumn<QByteArray>("data");
+    QTest::addColumn<int>("size");
+    QTest::addColumn<QBitArray>("expected");
+
+    QTest::newRow("empty") << QByteArray() << 0 << QBitArray();
+
+    auto add = [](const QByteArray &tag, const char *data) {
+        QTest::newRow(tag) << QByteArray(data, (tag.size() + 7) / 8) << tag.size()
+                           << QStringToQBitArray(tag);
+    };
+
+    // "0" to "0000000000000000"
+    for (int i = 1; i < 16; ++i) {
+        char zero[2] = { 0, 0 };
+        QByteArray pattern(i, '0');
+        add(pattern, zero);
+    }
+
+    // "1" to "1111111111111111"
+    for (int i = 1; i < 16; ++i) {
+        char one[2] = { '\xff', '\xff' };
+        QByteArray pattern(i, '1');
+        add(pattern, one);
+    }
+
+    // trailing 0 and 1
+    char zero = 1;
+    char one = 0;
+    QByteArray pzero = "1";
+    QByteArray pone = "0";
+    for (int i = 2; i < 8; ++i) {
+        zero <<= 1;
+        pzero.prepend('0');
+        add(pzero, &zero);
+
+        one = (one << 1) | 1;
+        pone.prepend('1');
+        add(pone, &one);
+    }
+}
+
+void tst_QBitArray::fromBits()
+{
+    QFETCH(QByteArray, data);
+    QFETCH(int, size);
+    QFETCH(QBitArray, expected);
+
+    QBitArray fromBits = QBitArray::fromBits(data, size);
+    QCOMPARE(fromBits, expected);
+
+    QCOMPARE(QBitArray::fromBits(fromBits.bits(), fromBits.size()), expected);
+}
+
 QTEST_APPLESS_MAIN(tst_QBitArray)
 #include "tst_qbitarray.moc"