Re: [Qemu-devel] [PATCH 01/11] Add append method to qstring and empty constructor

[Jamie Lokier <jamie@shareable.org>]

Luiz Capitulino wrote:
> >      qstring = qemu_malloc(sizeof(*qstring));
> > -    qstring->string = qemu_strdup(str);
> > +
> > +    qstring->length = strlen(str);
> > +    qstring->capacity = qstring->length;
> > +
> > +    qstring->string = qemu_malloc(qstring->capacity + 1);
> > +    memcpy(qstring->string, str, qstring->length);
> > +    qstring->string[qstring->length] = 0;
> 
>  Couldn't this be:
> 
> qstring->string = qemu_strdup(str);
> qstring->length = qstring->capacity = strlen(str);

Probably to have one call to strlen() instead of two (one inside
qemu_strdup()).

> > +void qstring_append(QString *qstring, const char *str)
> > +{
> > +    size_t len = strlen(str);
> > +
> > +    if (qstring->capacity < (qstring->length + len)) {
> > +        qstring->capacity += len;
> > +        qstring->capacity *= 2; /* use exponential growth */
> > +
> > +        qstring->string = qemu_realloc(qstring->string, qstring->capacity + 1);
> > +    }
> 
>  Why do we need to double it? Wouldn't be enough to only keep track
> of the current string length and add 'len' to it? We could drop
> 'capacity' then.

You need exponential growth if large stringes are to be grown in O(n)
time where n is the number of characters, appended in small pieces.

Think about the time spent copying bytes every time qemu_realloc() is called.

If you just add 'len' each time, think about appending 1 byte 10^4
times.  It will copy approximately 10^8/2 bytes, which is a lot just to
make a string 10^4 bytes long.

But += len; *= 2 is not necessary.  *= 2 is enough, provided the
result is large enough.

> > +    memcpy(qstring->string + qstring->length, str, len);
> > +    qstring->length += len;
> > +    qstring->string[qstring->length] = 0;
> 
>  I would use strcat().

Again, that's an extra call to strlen(), traversing the string twice instead of once.
Doesn't make much different for small strings, only large ones.

-- Jamie