Twice Shell

[ EDITING: bitmap-format.html ]

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<title>GIT bitmap v1 format</title>
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</head>
<body class="article">
<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>o1 &lt;= o2 &lt;==&gt; offset(o1) &lt;= offset(o2)</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>o1 &lt;= o2 &lt;==&gt; pack(o1) &lt;= pack(o2) /\ offset(o1) &lt;= offset(o2)</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 note">
<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 number 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>, an 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 note">
<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>H  L_1  L_2  L_3 .... L_M</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>hash = 0;
while ((c = *name++))
 if (!isspace(c))
  hash = (hash &gt;&gt; 2) + (c &lt;&lt; 24);</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</code> <code>*</code> (<code>4</code> <code>+</code> <code>8</code> <code>+</code> <code>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 the 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 class="sect1">
<h2 id="_pseudo_merge_bitmaps">Pseudo-merge bitmaps</h2>
<div class="sectionbody">
<div class="paragraph">
<p>If the <code>BITMAP_OPT_PSEUDO_MERGES</code> flag is set, a variable number of
bytes (preceding the name-hash cache, commit lookup table, and trailing
checksum) of the .<code>bitmap</code> file is used to store pseudo-merge bitmaps.</p>
</div>
<div class="paragraph">
<p>For more information on what pseudo-merges are, why they are useful, and
how to configure them, see the information in <a href="../gitpacking.html">gitpacking(7)</a>.</p>
</div>
<div class="sect2">
<h3 id="_file_format">File format</h3>
<div class="paragraph">
<p>If enabled, pseudo-merge bitmaps are stored in an optional section at
the end of a .<code>bitmap</code> file. The format is as follows:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>+-------------------------------------------+
|               .bitmap File                |
+-------------------------------------------+
|                                           |
|  Pseudo-merge bitmaps (Variable Length)   |
|  +---------------------------+            |
|  | commits_bitmap (EWAH)     |            |
|  +---------------------------+            |
|  | merge_bitmap (EWAH)       |            |
|  +---------------------------+            |
|                                           |
+-------------------------------------------+
|                                           |
|  Lookup Table                             |
|  +---------------------------+            |
|  | commit_pos (4 bytes)      |            |
|  +---------------------------+            |
|  | offset (8 bytes)          |            |
|  +------------+--------------+            |
|                                           |
|  Offset Cases:                            |
|  -------------                            |
|                                           |
|  1. MSB Unset: single pseudo-merge bitmap |
|     + offset to pseudo-merge bitmap       |
|                                           |
|  2. MSB Set: multiple pseudo-merges       |
|     + offset to extended lookup table     |
|                                           |
+-------------------------------------------+
|                                           |
|  Extended Lookup Table (Optional)         |
|  +----+----------+----------+----------+  |
|  | N  | Offset 1 |   ....   | Offset N |  |
|  +----+----------+----------+----------+  |
|  |    |  8 bytes |   ....   |  8 bytes |  |
|  +----+----------+----------+----------+  |
|                                           |
+-------------------------------------------+
|                                           |
|  Pseudo-merge position table              |
|  +----+----------+----------+----------+  |
|  | N  | Offset 1 |   ....   | Offset N |  |
|  +----+----------+----------+----------+  |
|  |    |  8 bytes |   ....   |  8 bytes |  |
|  +----+----------+----------+----------+  |
|                                           |
+-------------------------------------------+
|                                           |
|  Pseudo-merge Metadata                    |
|  +-----------------------------------+    |
|  | # pseudo-merges (4 bytes)         |    |
|  +-----------------------------------+    |
|  | # commits (4 bytes)               |    |
|  +-----------------------------------+    |
|  | Lookup offset (8 bytes)           |    |
|  +-----------------------------------+    |
|  | Extension size (8 bytes)          |    |
|  +-----------------------------------+    |
|                                           |
+-------------------------------------------+</pre>
</div>
</div>
<div class="ulist">
<ul>
<li>
<p>One or more pseudo-merge bitmaps, each containing:</p>
<div class="ulist">
<ul>
<li>
<p><code>commits_bitmap</code>, an EWAH-compressed bitmap describing the set of
commits included in the this psuedo-merge.</p>
</li>
<li>
<p><code>merge_bitmap</code>, an EWAH-compressed bitmap describing the union of
the set of objects reachable from all commits listed in the
<code>commits_bitmap</code>.</p>
</li>
</ul>
</div>
</li>
<li>
<p>A lookup table, mapping pseudo-merged commits to the pseudo-merges
they belong to. Entries appear in increasing order of each commit&#8217;s
bit position. Each entry is 12 bytes wide, and is comprised of the
following:</p>
<div class="ulist">
<ul>
<li>
<p><code>commit_pos</code>, a 4-byte unsigned value (in network byte-order)
containing the bit position for this commit.</p>
</li>
<li>
<p><code>offset</code>, an 8-byte unsigned value (also in network byte-order)
containing either one of two possible offsets, depending on whether or
not the most-significant bit is set.</p>
<div class="ulist">
<ul>
<li>
<p>If unset (i.e. <code>offset</code> &amp; ((<code>uint64_t</code>)<code>1</code>&lt;&lt;63) <code>==</code> <code>0</code>), the offset
(relative to the beginning of the .<code>bitmap</code> file) at which the
pseudo-merge bitmap for this commit can be read. This indicates
only a single pseudo-merge bitmap contains this commit.</p>
</li>
<li>
<p>If set (i.e. <code>offset</code> &amp; ((<code>uint64_t</code>)<code>1</code>&lt;&lt;63) != <code>0</code>), the offset
(again relative to the beginning of the .<code>bitmap</code> file) at which
the extended offset table can be located describing the set of
pseudo-merge bitmaps which contain this commit. This indicates
that multiple pseudo-merge bitmaps contain this commit.</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</li>
<li>
<p>An (optional) extended lookup table (written if and only if there is
at least one commit which appears in more than one pseudo-merge).
There are as many entries as commits which appear in multiple
pseudo-merges. Each entry contains the following:</p>
<div class="ulist">
<ul>
<li>
<p><code>N</code>, a 4-byte unsigned value equal to the number of pseudo-merges
which contain a given commit.</p>
</li>
<li>
<p>An array of <code>N</code> 8-byte unsigned values, each of which is
interpreted as an offset (relative to the beginning of the
.<code>bitmap</code> file) at which a pseudo-merge bitmap for this commit can
be read. These values occur in no particular order.</p>
</li>
</ul>
</div>
</li>
<li>
<p>Positions for all pseudo-merges, each stored as an 8-byte unsigned
value (in network byte-order) containing the offset (relative to the
beginning of the .<code>bitmap</code> file) of each consecutive pseudo-merge.</p>
</li>
<li>
<p>A 4-byte unsigned value (in network byte-order) equal to the number of
pseudo-merges.</p>
</li>
<li>
<p>A 4-byte unsigned value (in network byte-order) equal to the number of
unique commits which appear in any pseudo-merge.</p>
</li>
<li>
<p>An 8-byte unsigned value (in network byte-order) equal to the number
of bytes between the start of the pseudo-merge section and the
beginning of the lookup table.</p>
</li>
<li>
<p>An 8-byte unsigned value (in network byte-order) equal to the number
of bytes in the pseudo-merge section (including this field).</p>
</li>
</ul>
</div>
</div>
</div>
</div>
</div>
<div id="footer">
<div id="footer-text">
Last updated 2026-07-10 14:59:56 UTC
</div>
</div>
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