How to optimize this Qt code (QByteArray conversion)? - qt

Question

I need to perform some regexp operations on binary data. I wrote a function to convert QByteArray data in a hexa string representation. Each byte is prepended by 'x' for parsing purpose.
How could this code be optimized?
QByteArray data;
QByteArray newData;
for (int i = 0; i < data.size(); i++) {
QString hex;
hex.setNum(data[i], 16);
if (data[i] < 10) {
hex.prepend("x0");
} else {
hex.prepend("x");
}
newData.append(hex.toLatin1());
}

Answer 1

The code you posted has two bugs in it that I corrected.
1) Assuming you always want two hex digits you want to check if the value is less than 16, not 10.
2) QString::setNum has no overload for char, so the value is promoted to a larger type. For a value like 128, which is negative in a signed char, you would get x0ffffffffffffff80 due to sign extension.
The function foo1 is your original code with the bugs fixed, and foo2 is a more optimal version that avoids creating a temporary QString since the conversion to unicode and back isn't free, and prepending values to a string requires additional copying.
I used QElapsedTimer because on Windows where I am testing it uses the high resolution PerformanceCounter clock. If you are on another platform it might be less accurate. You can see the different types of clocks it may use in the documentation.
Set display_converted_string to true if you want the converted string printed to verify they are identical.
#include <QString>
#include <QByteArray>
#include <QElapsedTimer>
#include <iostream>
QByteArray foo1(QByteArray data)
{
QByteArray newData;
for (int i = 0; i < data.size(); i++) {
unsigned char c = data[i];
QString hex;
hex.setNum(c, 16);
if (c < 16) {
hex.prepend("x0");
} else {
hex.prepend("x");
}
newData.append(hex.toLatin1());
}
return newData;
}
QByteArray foo2(QByteArray data)
{
static const char digits[] = {'0','1','2','3','4','5','6','7',
'8','9','a','b','c','d','e','f'};
QByteArray newData;
newData.reserve(data.size() * 3);
for (int i = 0; i < data.size(); i++)
{
unsigned char c = data[i];
newData.append('x');
newData.append(digits[(c >> 4) & 0x0f]);
newData.append(digits[c & 0x0f]);
}
return newData;
}
int main()
{
const int iterations = 10000;
const bool display_converted_string = false;
QElapsedTimer t;
std::cout << "Using clock type " << t.clockType() << ".\n";
QByteArray data(256, 0);
QByteArray newData;
qint64 elapsed1 = 0, elapsed2 = 0;
//Set the values in data to 0-255 to make sure all values are converted properly.
for(int i = 0; i < data.size(); ++i)
{
data[i] = i;
}
t.start();
for(int i = 0; i < iterations; ++i)
{
newData = foo1(data);
}
elapsed1 = t.nsecsElapsed();
std::cout << "foo1 elapsed time = " << elapsed1 << "\n";
if(display_converted_string)
{
std::cout << "newData = " << newData.data() << "\n";
}
t.restart();
for(int i = 0; i < iterations; ++i)
{
newData = foo2(data);
}
elapsed2 = t.nsecsElapsed();
std::cout << "foo2 elapsed time = " << elapsed2 << "\n";
if(display_converted_string)
{
std::cout << "newData = " << newData.data() << "\n";
}
return 0;
}