QCryptographicHash: Add a static method to retrieve hash length

It's useful when you need to check how long a hash will be without first
generating one.

[ChangeLog][QtCore][QCryptographicHash] Add a static method, hashLength,
which returns the length of the output of a hash function in bytes.

Change-Id: Id6a454016523de83d157fd95c50105c6db4bb1d9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qcryptographichash.cpp

@@ -544,6 +544,46 @@ QByteArray QCryptographicHash::hash(const QByteArray &data, Algorithm method)
     return hash.result();
 }
 
+/*!
+  Returns the size of the output of the selected hash \a method in bytes.
+
+  \since 5.12
+*/
+int QCryptographicHash::hashLength(QCryptographicHash::Algorithm method)
+{
+    switch (method) {
+    case QCryptographicHash::Sha1:
+        return 20;
+#ifndef QT_CRYPTOGRAPHICHASH_ONLY_SHA1
+    case QCryptographicHash::Md4:
+        return 16;
+    case QCryptographicHash::Md5:
+        return 16;
+    case QCryptographicHash::Sha224:
+        return SHA224HashSize;
+    case QCryptographicHash::Sha256:
+        return SHA256HashSize;
+    case QCryptographicHash::Sha384:
+        return SHA384HashSize;
+    case QCryptographicHash::Sha512:
+        return SHA512HashSize;
+    case QCryptographicHash::RealSha3_224:
+    case QCryptographicHash::Keccak_224:
+        return 224 / 8;
+    case QCryptographicHash::RealSha3_256:
+    case QCryptographicHash::Keccak_256:
+        return 256 / 8;
+    case QCryptographicHash::RealSha3_384:
+    case QCryptographicHash::Keccak_384:
+        return 384 / 8;
+    case QCryptographicHash::RealSha3_512:
+    case QCryptographicHash::Keccak_512:
+        return 512 / 8;
+#endif
+    }
+    return 0;
+}
+
 QT_END_NAMESPACE
 
 #ifndef QT_NO_QOBJECT

src/corelib/tools/qcryptographichash.h

@@ -101,6 +101,7 @@ public:
     QByteArray result() const;
 
     static QByteArray hash(const QByteArray &data, Algorithm method);
+    static int hashLength(Algorithm method);
 private:
     Q_DISABLE_COPY(QCryptographicHash)
     QCryptographicHashPrivate *d;

tests/auto/corelib/tools/qcryptographichash/tst_qcryptographichash.cpp

@@ -29,6 +29,7 @@
 
 #include <QtCore/QCoreApplication>
 #include <QtTest/QtTest>
+#include <QtCore/QMetaEnum>
 
 Q_DECLARE_METATYPE(QCryptographicHash::Algorithm)
 
@@ -45,6 +46,7 @@ private slots:
     void sha3();
     void files_data();
     void files();
+    void hashLength();
 };
 
 void tst_QCryptographicHash::repeated_result_data()
@@ -291,6 +293,15 @@ void tst_QCryptographicHash::files()
     }
 }
 
+void tst_QCryptographicHash::hashLength()
+{
+    auto metaEnum = QMetaEnum::fromType<QCryptographicHash::Algorithm>();
+    for (int i = 0, value = metaEnum.value(i); value != -1; value = metaEnum.value(++i)) {
+        auto algorithm = QCryptographicHash::Algorithm(value);
+        QByteArray output = QCryptographicHash::hash(QByteArrayLiteral("test"), algorithm);
+        QCOMPARE(QCryptographicHash::hashLength(algorithm), output.length());
+    }
+}
 
 QTEST_MAIN(tst_QCryptographicHash)
 #include "tst_qcryptographichash.moc"