[Interest] Qt 5.5.0 Compile Time Hash Example

[mark diener]

Hello List:

Have you ever wanted an efficient switch statement on QString values?

There are many references to the subject with various algorithms, but they
all have some major flaw in the implementation.

For those looking for a way to have compile and run time hash on their
strings, I spent a little while doing some research on the subject and
wanted to share back to Qt community:

The tests ran successfully with the C++11 example for OSX/IOS Xcode 6.3.2
and Android NDK 10 r10e,

I went ahead and tested the function on MSVC 2013 with CTP November and it
failed.  MSVC 2015 worked, but I could not build the QT 5.5.0 source code
successfully, so I just built a console app in MSVC2015 and the C++11
function worked.

The C++14 example did not work on OSX or Android, but it did work on IOS.
With 2 out of 3 failures, I did not feel motivated to test for MSVC 2015 on
Windows 10.

So for those wanting to verify and make use of it, here are the required
code snippets:

First, in your .PRO file:

#CONFIG += c++14

CONFIG += c++11


Now for a source sample:


//main.cpp ######################//VALID C++14 Constant Expression
Function (Compiles for IOS on Qt5.5.0 and XCode 6.3.2, but does not
Compile for OSX Desktop with Clang)/*constexpr unsigned int
fnhash(const char *gpstr){    unsigned int hash = 0;    size_t gidx =
0 ;    while (gpstr[gidx] > 0)    {        hash = 65599 * hash +
gpstr[gidx];        if (gidx > 15) break ;        gidx++ ;    }
return hash ^ (hash >> 16);}*/// ######################//C++11
Constant Expression Function
//Forward declarationconstexpr unsigned int fnhashrec(unsigned int
hash, const char* str) ;
constexpr unsigned int fnhash(const char* str){   return ( !str ? 0 :
fnhashrec(5381, str) );}
constexpr unsigned int fnhashrec(unsigned int hash, const char* str){
 return ( !*str ? hash : fnhashrec(((hash << 5) + hash) + *str, str +
1));}// ######################
int main(int argc, char *argv[]){    QGuiApplication gapp(argc, argv);
   QQmlApplicationEngine geng;
    QString gval = "JOKER" ;

    //NOW look at the run time eval from QString -> char*

    switch (fnhash(gval.toUtf8().data()))    {    case fnhash("PETE"):
 //Compile time optimized        qDebug() << "POOR PETE IS NOT A
JOKER" << endl ;        break ;    case fnhash("JOKER"): //Compile
time optimized        qDebug() << "BLOODY JOKER WORKED -> SUCCESS" <<
endl ;        break ;    }
    geng.load(QUrl(QStringLiteral("qrc:/main.qml")));    return gapp.exec();}

Cheers,

md
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