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p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3} h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid} #header,#content,#footnotes,#footer{max-width:none} #toc,.sidebarblock,.exampleblock>.content{background:none!important} #toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important} body.book #header{text-align:center} body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em} body.book #header .details{border:0!important;display:block;padding:0!important} body.book #header .details span:first-child{margin-left:0!important} body.book #header .details br{display:block} body.book #header .details br+span::before{content:none!important} body.book #toc{border:0!important;text-align:left!important;padding:0!important;margin:0!important} body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-break-before:always} .listingblock code[data-lang]::before{display:block} #footer{padding:0 .9375em} .hide-on-print{display:none!important} .print-only{display:block!important} .hide-for-print{display:none!important} .show-for-print{display:inherit!important}} @media print,amzn-kf8{#header>h1:first-child{margin-top:1.25rem} .sect1{padding:0!important} .sect1+.sect1{border:0} #footer{background:none} #footer-text{color:rgba(0,0,0,.6);font-size:.9em}} @media amzn-kf8{#header,#content,#footnotes,#footer{padding:0}} </style> </head> <body class="article"> <div id="header"> <h1>GIT bitmap v1 format</h1> <div class="details"> <span id="revdate"></span> </div> </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’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 <= o2 <==> offset(o1) <= 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 <= o2 <==> pack(o1) <= pack(o2) /\ offset(o1) <= offset(o2)</pre> </div> </div> <div class="paragraph"> <p>The ordering between packs is done according to the MIDX’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> <commit_pos, offset, xor_row> 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’ed against one of the 160 entries preceding it). This number is always positive, and hence entries are always xor’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 >> 2) + (c << 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 * (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> <commit_pos, offset, xor_row> triplets (sorted in the ascending order of <code>commit_pos</code>). The content of the i’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’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’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 & ((uint64_t)1<<63) == 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 & ((uint64_t)1<<63) != 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 2025-06-13 22:20:03 UTC </div> </div> </body> </html>
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