On Tue, 11 Dec 2007, Davide Libenzi wrote:
>
> > That's not the problem. The problem with xdl_trim_ends() is that it
> > happens *after* you have done all the hashing, so as an optimization it's
> > fairly useless, because it still leaves the real cost (the per-line
> > hashing) on the table.
>
> Careful. The real cost of diffing, is not the O(1) pass of the prepare
> phase. It's the potentially O(N*M) worst case of the cross-record compare.
> So that optimization is far from useless. That optimization is indeed
> mainly targeted to avoid such worst case.
I'm not saying it's useless. I'm saying it's ineffective.
My simple patch that you saw, speeded up a real-life case by A FACTOR OF
THREE. We're not talking small potatoes here.
> Also, you'd need some code at the end that hands you back at least the N
> lines you want for context.
Sure. The special case I added it to specifically wanted a context of zero
in the caller, so I could just ignore that.
But doing this in general and handing back the context is a simple matter
of
while (size < orig && context_lines) {
if (src->buffer[size++] == '\n')
context_lines--;
}
which will usually hit in a really short time (ie three lines by default,
just a few tens of bytes).
Linus
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