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From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: "Derrick Stolee" <stolee@gmail.com>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Stefan Beller" <sbeller@google.com>, git <git@vger.kernel.org>,
	"Duy Nguyen" <pclouds@gmail.com>
Subject: Re: [PoC -- do not apply 3/3] test-tree-bitmap: replace ewah with custom rle encoding
Date: Wed, 10 Oct 2018 09:58:51 +0900
Message-ID: <xmqqr2gyobw4.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <20181009231441.GC23730@sigill.intra.peff.net> (Jeff King's message of "Tue, 9 Oct 2018 19:14:41 -0400")

Jeff King <peff@peff.net> writes:

> +static void strbuf_add_varint(struct strbuf *out, uintmax_t val)
> +{
> +	size_t len;
> +	strbuf_grow(out, 16); /* enough for any varint */
> +	len = encode_varint(val, (unsigned char *)out->buf + out->len);
> +	strbuf_setlen(out, out->len + len);
> +}
> +
> +static void bitmap_to_rle(struct strbuf *out, struct bitmap *bitmap)
> +{
> +	int curval = 0; /* count zeroes, then ones, then zeroes, etc */
> +	size_t run = 0;
> +	size_t word;
> +	size_t orig_len = out->len;
> +
> +	for (word = 0; word < bitmap->word_alloc; word++) {
> +		int bit;
> +
> +		for (bit = 0; bit < BITS_IN_EWORD; bit++) {
> +			int val = !!(bitmap->words[word] & (((eword_t)1) << bit));
> +			if (val == curval)
> +				run++;
> +			else {
> +				strbuf_add_varint(out, run);
> +				curval = 1 - curval; /* flip 0/1 */
> +				run = 1;
> +			}
> +		}

OK.  I find it a bit disturbing to see that the loop knows a bit too
much about how "struct bitmap" is implemented, but that is a complaint
against the bitmap API, not this new user of the API.

We do not try to handle the case where bitmap has bits that is not
multiple of BITS_IN_EWORD and instead pretend that size of such a
bitmap can be rounded up, because we ignore trailing 0-bit anyway,
and we know the "struct bitmap" would pad with 0-bit at the tail?

> +	}
> +
> +	/*
> +	 * complete the run, but do not bother with trailing zeroes, unless we
> +	 * failed to write even an initial run of 0's.
> +	 */
> +	if (curval && run)
> +		strbuf_add_varint(out, run);
> +	else if (orig_len == out->len)
> +		strbuf_add_varint(out, 0);
> +
> +	/* signal end-of-input with an empty run */
> +	strbuf_add_varint(out, 0);
> +}

OK.

> +static size_t rle_each_bit(const unsigned char *in, size_t len,
> +			   void (*fn)(size_t, void *), void *data)
> +{
> +	int curval = 0; /* look for zeroes first, then ones, etc */
> +	const unsigned char *cur = in;
> +	const unsigned char *end = in + len;
> +	size_t pos;
> +
> +	/* we always have a first run, even if it's 0 zeroes */
> +	pos = decode_varint(&cur);
> +
> +	/*
> +	 * ugh, varint does not seem to have a way to prevent reading past
> +	 * the end of the buffer. We'll do a length check after each one,
> +	 * so the worst case is bounded.
> +	 */

Sorry about that :-).

> +	if (cur > end) {
> +		error("input underflow in rle");
> +		return len;
> +	}
> +
> +	while (1) {
> +		size_t run = decode_varint(&cur);
> +
> +		if (cur > end) {
> +			error("input underflow in rle");
> +			return len;
> +		}
> +
> +		if (!run)
> +			break; /* empty run signals end */
> +
> +		curval = 1 - curval; /* flip 0/1 */
> +		if (curval) {
> +			/* we have a run of 1's; deliver them */
> +			size_t i;
> +			for (i = 0; i < run; i++)
> +				fn(pos + i, data);
> +		}
> +		pos += run;
> +	}
> +
> +	return cur - in;
> +}

Makes sense.


  reply index

Thread overview: 76+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-03 13:23 We should add a "git gc --auto" after "git clone" due to commit graph Ævar Arnfjörð Bjarmason
2018-10-03 13:36 ` SZEDER Gábor
2018-10-03 13:42   ` Derrick Stolee
2018-10-03 14:18     ` Ævar Arnfjörð Bjarmason
2018-10-03 14:01   ` Ævar Arnfjörð Bjarmason
2018-10-03 14:17     ` SZEDER Gábor
2018-10-03 14:22       ` Ævar Arnfjörð Bjarmason
2018-10-03 14:53         ` SZEDER Gábor
2018-10-03 15:19           ` Ævar Arnfjörð Bjarmason
2018-10-03 16:59             ` SZEDER Gábor
2018-10-05  6:09               ` Junio C Hamano
2018-10-10 22:07                 ` SZEDER Gábor
2018-10-10 23:01                   ` Ævar Arnfjörð Bjarmason
2018-10-03 19:08           ` Stefan Beller
2018-10-03 19:21             ` Jeff King
2018-10-03 20:35               ` Ævar Arnfjörð Bjarmason
2018-10-03 17:47         ` Stefan Beller
2018-10-03 18:47           ` Ævar Arnfjörð Bjarmason
2018-10-03 18:51             ` Jeff King
2018-10-03 18:59               ` Derrick Stolee
2018-10-03 19:18                 ` Jeff King
2018-10-08 16:41                   ` SZEDER Gábor
2018-10-08 16:57                     ` Derrick Stolee
2018-10-08 18:10                       ` SZEDER Gábor
2018-10-08 18:29                         ` Derrick Stolee
2018-10-09  3:08                           ` Jeff King
2018-10-09 13:48                             ` Bloom Filters (was Re: We should add a "git gc --auto" after "git clone" due to commit graph) Derrick Stolee
2018-10-09 18:45                               ` Ævar Arnfjörð Bjarmason
2018-10-09 18:46                               ` Jeff King
2018-10-09 19:03                                 ` Derrick Stolee
2018-10-09 21:14                                   ` Jeff King
2018-10-09 23:12                                     ` Bloom Filters Jeff King
2018-10-09 23:13                                       ` [PoC -- do not apply 1/3] initial tree-bitmap proof of concept Jeff King
2018-10-09 23:14                                       ` [PoC -- do not apply 2/3] test-tree-bitmap: add "dump" mode Jeff King
2018-10-10  0:48                                         ` Junio C Hamano
2018-10-11  3:13                                           ` Jeff King
2018-10-09 23:14                                       ` [PoC -- do not apply 3/3] test-tree-bitmap: replace ewah with custom rle encoding Jeff King
2018-10-10  0:58                                         ` Junio C Hamano [this message]
2018-10-11  3:20                                           ` Jeff King
2018-10-11 12:33                                       ` Bloom Filters Derrick Stolee
2018-10-11 13:43                                         ` Jeff King
2018-10-09 21:30                             ` We should add a "git gc --auto" after "git clone" due to commit graph SZEDER Gábor
2018-10-09 19:34                       ` [PATCH 0/4] Bloom filter experiment SZEDER Gábor
2018-10-09 19:34                         ` [PATCH 1/4] Add a (very) barebones Bloom filter implementation SZEDER Gábor
2018-10-09 19:34                         ` [PATCH 2/4] commit-graph: write a Bloom filter containing changed paths for each commit SZEDER Gábor
2018-10-09 21:06                           ` Jeff King
2018-10-09 21:37                             ` SZEDER Gábor
2018-10-09 19:34                         ` [PATCH 3/4] revision.c: use the Bloom filter to speed up path-limited revision walks SZEDER Gábor
2018-10-09 19:34                         ` [PATCH 4/4] revision.c: add GIT_TRACE_BLOOM_FILTER for a bit of statistics SZEDER Gábor
2018-10-09 19:47                         ` [PATCH 0/4] Bloom filter experiment Derrick Stolee
2018-10-11  1:21                         ` [PATCH 0/2] Per-commit filter proof of concept Jonathan Tan
2018-10-11  1:21                           ` [PATCH 1/2] One filter per commit Jonathan Tan
2018-10-11  1:21                           ` [PATCH 2/2] Only make bloom filter for first parent Jonathan Tan
2018-10-11  7:37                           ` [PATCH 0/2] Per-commit filter proof of concept Ævar Arnfjörð Bjarmason
2018-10-15 14:39                         ` [PATCH 0/4] Bloom filter experiment Derrick Stolee
2018-10-16  4:45                           ` Junio C Hamano
2018-10-16 11:13                             ` Derrick Stolee
2018-10-16 12:57                               ` Ævar Arnfjörð Bjarmason
2018-10-16 13:03                                 ` Derrick Stolee
2018-10-18  2:00                                 ` Junio C Hamano
2018-10-16 23:41                           ` Jonathan Tan
2018-10-08 23:02                     ` We should add a "git gc --auto" after "git clone" due to commit graph Junio C Hamano
2018-10-03 14:32     ` Duy Nguyen
2018-10-03 16:45 ` Duy Nguyen
2018-10-04 21:42 ` [RFC PATCH] " Ævar Arnfjörð Bjarmason
2018-10-05 12:05   ` Derrick Stolee
2018-10-05 13:05     ` Ævar Arnfjörð Bjarmason
2018-10-05 13:45       ` Derrick Stolee
2018-10-05 14:04         ` Ævar Arnfjörð Bjarmason
2018-10-05 19:21         ` Jeff King
2018-10-05 19:41           ` Derrick Stolee
2018-10-05 19:47             ` Jeff King
2018-10-05 20:00               ` Derrick Stolee
2018-10-05 20:02                 ` Jeff King
2018-10-05 20:01               ` Ævar Arnfjörð Bjarmason
2018-10-05 20:09                 ` Jeff King
2018-10-11 12:49 [PATCH 1/2] One filter per commit Derrick Stolee
2018-10-11 19:11 ` [PATCH] Per-commit and per-parent filters for 2 parents Jonathan Tan

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