From: Jeff King <peff@peff.net>
To: Junio C Hamano <gitster@pobox.com>
Cc: Michael Haggerty <mhagger@alum.mit.edu>,
Kim Gybels <kgybels@infogroep.be>,
Johannes Schindelin <johannes.schindelin@gmx.de>,
git@vger.kernel.org
Subject: Re: [PATCH 3/6] find_reference_location(): make function safe for empty snapshots
Date: Wed, 24 Jan 2018 16:34:11 -0500 [thread overview]
Message-ID: <20180124213410.GA8952@sigill.intra.peff.net> (raw)
In-Reply-To: <xmqq8tcnc68r.fsf@gitster.mtv.corp.google.com>
On Wed, Jan 24, 2018 at 01:11:00PM -0800, Junio C Hamano wrote:
> > This tightens the binary search termination condition. If we ever did
> > see "hi > lo", we'd want to terminate the loop. Is that ever possible?
>
> I think you meant "lo > hi", but I shared the same "Huh?" moment.
Er, yeah. Sorry about that.
> Because "While lo is strictly lower than hi" is a so well
> established binary search pattern, even though we know that it is
> equivalent to "While lo and hi is different" due to your analysis
> below, the new code looks somewhat strange at the first glance.
I thought at first that this was due to the way the record-finding
happens, but I think even in our normal binary searches, it is an
invariant that "lo <= hi".
> > I think the answer is "no". Our "hi" here is an exclusive bound, so we
> > should never go past it via find_end_of_record() when assigning "lo".
> > And "hi" is always assigned from the start of the current record. That
> > can never cross "lo", because find_start_of_record() ensures it.
> >
> > So I think it's fine, but I wanted to double check.
>
> It would be much simpler to reason about if we instead do
>
> #define is_empty_snapshot(s) ((s)->start == NULL)
>
> if (is_empty_snapshot(snapshot))
> return NULL;
>
> or something like that upfront.
Yes, I agree that would also work.
-Peff
next prev parent reply other threads:[~2018-01-24 21:34 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-13 16:11 [PATCH] packed_ref_cache: don't use mmap() for small files Kim Gybels
2018-01-13 18:56 ` Johannes Schindelin
2018-01-14 19:14 ` [PATCH v2] " Kim Gybels
2018-01-15 12:17 ` [PATCH 0/3] Supplements to "packed_ref_cache: don't use mmap() for small files" Michael Haggerty
2018-01-15 12:17 ` [PATCH 1/3] SQUASH? Mention that `snapshot::buf` can be NULL for empty files Michael Haggerty
2018-01-15 12:17 ` [PATCH 2/3] create_snapshot(): exit early if the file was empty Michael Haggerty
2018-01-15 12:17 ` [PATCH 3/3] find_reference_location(): don't invoke if `snapshot->buf` is NULL Michael Haggerty
2018-01-17 20:23 ` [PATCH 0/3] Supplements to "packed_ref_cache: don't use mmap() for small files" Johannes Schindelin
2018-01-17 21:52 ` Junio C Hamano
2018-01-15 21:15 ` [PATCH] packed_ref_cache: don't use mmap() for small files Jeff King
2018-01-15 23:37 ` Kim Gybels
2018-01-15 23:52 ` Jeff King
2018-01-16 19:38 ` [PATCH v3] " Kim Gybels
2018-01-17 22:09 ` Jeff King
2018-01-21 4:41 ` Michael Haggerty
2018-01-22 19:31 ` Junio C Hamano
2018-01-24 11:05 ` Michael Haggerty
2018-01-24 11:14 ` [PATCH 0/6] Yet another approach to handling empty snapshots Michael Haggerty
2018-01-24 11:14 ` [PATCH 1/6] struct snapshot: store `start` rather than `header_len` Michael Haggerty
2018-01-24 20:36 ` Jeff King
2018-01-24 11:14 ` [PATCH 2/6] create_snapshot(): use `xmemdupz()` rather than a strbuf Michael Haggerty
2018-01-24 11:14 ` [PATCH 3/6] find_reference_location(): make function safe for empty snapshots Michael Haggerty
2018-01-24 20:27 ` Jeff King
2018-01-24 21:11 ` Junio C Hamano
2018-01-24 21:34 ` Jeff King [this message]
2018-01-24 11:14 ` [PATCH 4/6] packed_ref_iterator_begin(): make optimization more general Michael Haggerty
2018-01-24 20:32 ` Jeff King
2018-01-24 11:14 ` [PATCH 5/6] load_contents(): don't try to mmap an empty file Michael Haggerty
2018-01-24 11:14 ` [PATCH 6/6] packed_ref_cache: don't use mmap() for small files Michael Haggerty
2018-01-24 20:38 ` [PATCH 0/6] Yet another approach to handling empty snapshots Jeff King
2018-01-24 20:54 ` Junio C Hamano
2018-02-15 16:54 ` Johannes Schindelin
2018-01-24 18:05 ` [PATCH v3] packed_ref_cache: don't use mmap() for small files Junio C Hamano
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