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<div id="header">
<h1>GIT bitmap v1 format</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="_pack_and_multi_pack_bitmaps">Pack and multi-pack bitmaps</h2>
<div class="sectionbody">
<div class="paragraph"><p>Bitmaps store reachability information about the set of objects in a packfile,
or a multi-pack index (MIDX). The former is defined obviously, and the latter is
defined as the union of objects in packs contained in the MIDX.</p></div>
<div class="paragraph"><p>A bitmap may belong to either one pack, or the repository&#8217;s multi-pack index (if
it exists). A repository may have at most one bitmap.</p></div>
<div class="paragraph"><p>An object is uniquely described by its bit position within a bitmap:</p></div>
<div class="ulist"><ul>
<li>
<p>
If the bitmap belongs to a packfile, the <em>n</em>th bit corresponds to
the <em>n</em>th object in pack order. For a function <code>offset</code> which maps
objects to their byte offset within a pack, pack order is defined as
follows:
</p>
<div class="literalblock">
<div class="content">
<pre><code>o1 &lt;= o2 &lt;==&gt; offset(o1) &lt;= offset(o2)</code></pre>
</div></div>
</li>
<li>
<p>
If the bitmap belongs to a MIDX, the <em>n</em>th bit corresponds to the
<em>n</em>th object in MIDX order. With an additional function <code>pack</code> which
maps objects to the pack they were selected from by the MIDX, MIDX order
is defined as follows:
</p>
<div class="literalblock">
<div class="content">
<pre><code>o1 &lt;= o2 &lt;==&gt; pack(o1) &lt;= pack(o2) /\ offset(o1) &lt;= offset(o2)</code></pre>
</div></div>
<div class="paragraph"><p>The ordering between packs is done according to the MIDX&#8217;s .rev file.
Notably, the preferred pack sorts ahead of all other packs.</p></div>
</li>
</ul></div>
<div class="paragraph"><p>The on-disk representation (described below) of a bitmap is the same regardless
of whether or not that bitmap belongs to a packfile or a MIDX. The only
difference is the interpretation of the bits, which is described above.</p></div>
<div class="paragraph"><p>Certain bitmap extensions are supported (see: Appendix B). No extensions are
required for bitmaps corresponding to packfiles. For bitmaps that correspond to
MIDXs, both the bit-cache and rev-cache extensions are required.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_on_disk_format">On-disk format</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
A header appears at the beginning:
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
4-byte signature:
</dt>
<dd>
<p>
{<em>B</em>, <em>I</em>, <em>T</em>, <em>M</em>}
</p>
</dd>
<dt class="hdlist1">
2-byte version number (network byte order):
</dt>
<dd>
<p>
The current implementation only supports version 1
of the bitmap index (the same one as JGit).
</p>
</dd>
<dt class="hdlist1">
2-byte flags (network byte order):
</dt>
<dd>
<p>
The following flags are supported:
</p>
<div class="ulist"><ul>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
BITMAP_OPT_FULL_DAG (0x1) REQUIRED:
</dt>
<dd>
<p>
This flag must always be present. It implies that the
bitmap index has been generated for a packfile or
multi-pack index (MIDX) with full closure (i.e. where
every single object in the packfile/MIDX can find its
parent links inside the same packfile/MIDX). This is a
requirement for the bitmap index format, also present in
JGit, that greatly reduces the complexity of the
implementation.
</p>
</dd>
</dl></div>
</li>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
BITMAP_OPT_HASH_CACHE (0x4):
</dt>
<dd>
<p>
If present, the end of the bitmap file contains
<code>N</code> 32-bit name-hash values, one per object in the
pack/MIDX. The format and meaning of the name-hash is
described below.
</p>
</dd>
</dl></div>
</li>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
BITMAP_OPT_LOOKUP_TABLE (0x10):
</dt>
<dd>
<p>
If present, the end of the bitmap file contains a table
containing a list of <code>N</code> &lt;commit_pos, offset, xor_row&gt;
triplets. The format and meaning of the table is described
below.
</p>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">Unlike the xor_offset used to compress an individual bitmap,
<code>xor_row</code> stores an <strong>absolute</strong> index into the lookup table, not a location
relative to the current entry.</td>
</tr></table>
</div>
</dd>
</dl></div>
</li>
</ul></div>
</dd>
<dt class="hdlist1">
4-byte entry count (network byte order):
</dt>
<dd>
<p>
The total count of entries (bitmapped commits) in this bitmap index.
</p>
</dd>
<dt class="hdlist1">
20-byte checksum:
</dt>
<dd>
<p>
The SHA1 checksum of the pack/MIDX this bitmap index
belongs to.
</p>
</dd>
</dl></div>
</li>
<li>
<p>
4 EWAH bitmaps that act as type indexes
</p>
<div class="paragraph"><p>Type indexes are serialized after the hash cache in the shape
of four EWAH bitmaps stored consecutively (see Appendix A for
the serialization format of an EWAH bitmap).</p></div>
<div class="paragraph"><p>There is a bitmap for each Git object type, stored in the following
order:</p></div>
<div class="ulist"><ul>
<li>
<p>
Commits
</p>
</li>
<li>
<p>
Trees
</p>
</li>
<li>
<p>
Blobs
</p>
</li>
<li>
<p>
Tags
</p>
</li>
</ul></div>
<div class="paragraph"><p>In each bitmap, the `n`th bit is set to true if the `n`th object
in the packfile or multi-pack index is of that type.</p></div>
<div class="paragraph"><p>The obvious consequence is that the OR of all 4 bitmaps will result
in a full set (all bits set), and the AND of all 4 bitmaps will
result in an empty bitmap (no bits set).</p></div>
</li>
<li>
<p>
N entries with compressed bitmaps, one for each indexed commit
</p>
<div class="paragraph"><p>Where <code>N</code> is the total amount of entries in this bitmap index.
Each entry contains the following:</p></div>
<div class="ulist"><ul>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
4-byte object position (network byte order):
</dt>
<dd>
<p>
The position <strong>in the index for the packfile or
multi-pack index</strong> where the bitmap for this commit is
found.
</p>
</dd>
</dl></div>
</li>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
1-byte XOR-offset:
</dt>
<dd>
<p>
The xor offset used to compress this bitmap. For an entry
in position <code>x</code>, a XOR offset of <code>y</code> means that the actual
bitmap representing this commit is composed by XORing the
bitmap for this entry with the bitmap in entry <code>x-y</code> (i.e.
the bitmap <code>y</code> entries before this one).
</p>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">This compression can be recursive. In order to
XOR this entry with a previous one, the previous entry needs
to be decompressed first, and so on.</td>
</tr></table>
</div>
<div class="paragraph"><p>The hard-limit for this offset is 160 (an entry can only be
xor&#8217;ed against one of the 160 entries preceding it). This
number is always positive, and hence entries are always xor&#8217;ed
with <strong>previous</strong> bitmaps, not bitmaps that will come afterwards
in the index.</p></div>
</dd>
</dl></div>
</li>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
1-byte flags for this bitmap:
</dt>
<dd>
<p>
At the moment the only available flag is <code>0x1</code>, which hints
that this bitmap can be re-used when rebuilding bitmap indexes
for the repository.
</p>
</dd>
</dl></div>
</li>
<li>
<p>
The compressed bitmap itself, see Appendix A.
</p>
</li>
</ul></div>
</li>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
TRAILER:
</dt>
<dd>
<p>
Trailing checksum of the preceding contents.
</p>
</dd>
</dl></div>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_appendix_a_serialization_format_for_an_ewah_bitmap">Appendix A: Serialization format for an EWAH bitmap</h2>
<div class="sectionbody">
<div class="paragraph"><p>Ewah bitmaps are serialized in the same protocol as the JAVAEWAH
library, making them backwards compatible with the JGit
implementation:</p></div>
<div class="ulist"><ul>
<li>
<p>
4-byte number of bits of the resulting UNCOMPRESSED bitmap
</p>
</li>
<li>
<p>
4-byte number of words of the COMPRESSED bitmap, when stored
</p>
</li>
<li>
<p>
N x 8-byte words, as specified by the previous field
</p>
<div class="paragraph"><p>This is the actual content of the compressed bitmap.</p></div>
</li>
<li>
<p>
4-byte position of the current RLW for the compressed
bitmap
</p>
</li>
</ul></div>
<div class="paragraph"><p>All words are stored in network byte order for their corresponding
sizes.</p></div>
<div class="paragraph"><p>The compressed bitmap is stored in a form of run-length encoding, as
follows. It consists of a concatenation of an arbitrary number of
chunks. Each chunk consists of one or more 64-bit words</p></div>
<div class="literalblock">
<div class="content">
<pre><code>H L_1 L_2 L_3 .... L_M</code></pre>
</div></div>
<div class="paragraph"><p>H is called RLW (run length word). It consists of (from lower to higher
order bits):</p></div>
<div class="ulist"><ul>
<li>
<p>
1 bit: the repeated bit B
</p>
</li>
<li>
<p>
32 bits: repetition count K (unsigned)
</p>
</li>
<li>
<p>
31 bits: literal word count M (unsigned)
</p>
</li>
</ul></div>
<div class="paragraph"><p>The bitstream represented by the above chunk is then:</p></div>
<div class="ulist"><ul>
<li>
<p>
K repetitions of B
</p>
</li>
<li>
<p>
The bits stored in <code>L_1</code> through <code>L_M</code>. Within a word, bits at
lower order come earlier in the stream than those at higher
order.
</p>
</li>
</ul></div>
<div class="paragraph"><p>The next word after <code>L_M</code> (if any) must again be a RLW, for the next
chunk. For efficient appending to the bitstream, the EWAH stores a
pointer to the last RLW in the stream.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_appendix_b_optional_bitmap_sections">Appendix B: Optional Bitmap Sections</h2>
<div class="sectionbody">
<div class="paragraph"><p>These sections may or may not be present in the <code>.bitmap</code> file; their
presence is indicated by the header flags section described above.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_name_hash_cache">Name-hash cache</h2>
<div class="sectionbody">
<div class="paragraph"><p>If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains
a cache of 32-bit values, one per object in the pack/MIDX. The value at
position <code>i</code> is the hash of the pathname at which the `i`th object
(counting in index or multi-pack index order) in the pack/MIDX can be found.
This can be fed into the delta heuristics to compare objects with similar
pathnames.</p></div>
<div class="paragraph"><p>The hash algorithm used is:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>hash = 0;
while ((c = *name++))
if (!isspace(c))
hash = (hash &gt;&gt; 2) + (c &lt;&lt; 24);</code></pre>
</div></div>
<div class="paragraph"><p>Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.
If implementations want to choose a different hashing scheme, they are
free to do so, but MUST allocate a new header flag (because comparing
hashes made under two different schemes would be pointless).</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_commit_lookup_table">Commit lookup table</h2>
<div class="sectionbody">
<div class="paragraph"><p>If the BITMAP_OPT_LOOKUP_TABLE flag is set, the last <code>N * (4 + 8 + 4)</code>
bytes (preceding the name-hash cache and trailing hash) of the <code>.bitmap</code>
file contains a lookup table specifying the information needed to get
the desired bitmap from the entries without parsing previous unnecessary
bitmaps.</p></div>
<div class="paragraph"><p>For a <code>.bitmap</code> containing <code>nr_entries</code> reachability bitmaps, the table
contains a list of <code>nr_entries</code> &lt;commit_pos, offset, xor_row&gt; triplets
(sorted in the ascending order of <code>commit_pos</code>). The content of i&#8217;th
triplet is -</p></div>
<div class="ulist"><ul>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
commit_pos (4 byte integer, network byte order):
</dt>
<dd>
<p>
It stores the object position of a commit (in the midx or pack
index).
</p>
</dd>
</dl></div>
</li>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
offset (8 byte integer, network byte order):
</dt>
<dd>
<p>
The offset from which that commit&#8217;s bitmap can be read.
</p>
</dd>
</dl></div>
</li>
<li>
<p>
</p>
<div class="dlist"><dl>
<dt class="hdlist1">
xor_row (4 byte integer, network byte order):
</dt>
<dd>
<p>
The position of the triplet whose bitmap is used to compress
this one, or <code>0xffffffff</code> if no such bitmap exists.
</p>
</dd>
</dl></div>
</li>
</ul></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated
2023-04-17 17:57:53 PDT
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