How to optimize data decompression and conversion time? Qt C++

Question

I am working on a project, it reads data(24bits per sample) from binary files, decompress it to 32bits and convert it signed int. I have this function for the decompress and convert tasks, and it causes wasting a lot of time in the project for huge data.

signed int convertedSample(QByteArray samp){
QString sample = QString(samp.toHex() + "00");
bool ok;
signed int decimalSample = sample.toInt(&ok, 16);
return decimalSample;
}

To read data from files and store it, I do:

while(!file.atEnd()){
QByteArray chunkSamples = file.read(chunkSize);// chunkSize here is the size of 1 million of samples
for (int i = 0; i < chunkSamples.size() ; i++){
QByteArray samp = chunkSamples.mid(i * 3, 3); // 24bits
buffer.append(convertedSample(samp)); // buffer is a QVector of int
}

Any help to make it faster ?

Answer 1

Thanks to the guys in the comments, I've made some improvements over time. I avoided using strings for the conversion because it seems that using strings is always slow and replaced it with using bitwise shift and OR to combine the individual bytes into int.

I replaced this code:

QString sample = QString(samp.toHex() + "00");
bool ok;
int decimalSample = sample.toInt(&ok, 16);

with :

int32_t decimalSample = ((samp[0] << 24) | ((samp[1] << 16) & 0xFF0000) | ((samp[2] << 8) & 0xFF00));
     if(samp[0] & 0x80){
          decimalSample = - ((~decimalSample + 1) & 0xFFFFFFFF);
     }