Hello all,
It looks like the standard QString serialization writes a 32-bit size
(uses 0xFFFFFFFF for Null String, and uses 0xFFFFFFFE if 32-bit isn't big
enough and then writes a 64-bit size), followed by 16 bits for each
character.
To save space, I want to use a scheme that writes an 8-bit size, and
reserves the last four values (255, 254, 254, 252) for a null QString, or
another 16-bit, 32-bit, or 64-bit size depending on how large the QString
is, followed by utf8 data.
This will make a null QString 1 byte instead of 4, a single Latin
character like 'a' 2 bytes instead of 6, and most long Latin1 strings
around half the size...
I wrote a quick implementation, and it seems to work well.
My primary question is: is there a way to avoid doing a copy for the utf8
data (the char *buffer in operator>> and the QByteArray utf8 in
operator<<)?
Any other obvious ways to speed it up, or other suggestions?
Josh
QDataStream &operator<<(QDataStream &out, const QString &str)
{
if(str.isNull())
out << (quint8)255; //null marker
else
{
QByteArray utf8=str.toUtf8();
qsizetype size=utf8.size();
if(size<252)
{
out << (quint8)size;
out.writeRawData(utf8.data(), size);
}
else if(size<65536)
{
out << (quint8)254 << (quint16)size;
out.writeRawData(utf8.data(), size);
}
else if(size<4294967296)
{
out << (quint8)253 << (quint32)size;
out.writeRawData(utf8.data(), size);
}
else
{
out << (quint8)252 << (qsizetype)size;
out.writeRawData(utf8.data(), size);
}
}
return(out);
}
QDataStream &operator>>(QDataStream &in, QString &str)
{
quint8 firstSize;
in >> firstSize;
if(firstSize==255) //null marker
str = QString();
else if(firstSize<252)
{
char* buffer = new char[firstSize];
in.readRawData(buffer, firstSize);
str = QString::fromUtf8(buffer, firstSize);
delete[] buffer;
}
else if(firstSize==254)
{
quint16 secondSize;
in >> secondSize;
char* buffer = new char[secondSize];
in.readRawData(buffer, secondSize);
str = QString::fromUtf8(buffer, secondSize);
delete[] buffer;
}
else if(firstSize==253)
{
quint32 secondSize;
in >> secondSize;
char* buffer = new char[secondSize];
in.readRawData(buffer, secondSize);
str = QString::fromUtf8(buffer, secondSize);
delete[] buffer;
}
else if(firstSize==252)
{
qsizetype secondSize;
in >> secondSize;
char* buffer = new char[secondSize];
in.readRawData(buffer, secondSize);
str = QString::fromUtf8(buffer, secondSize);
delete[] buffer;
}
return(in);
}
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