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| author | Junio C Hamano <gitster@pobox.com> | 2022-08-18 14:13:08 -0700 |
|---|---|---|
| committer | Junio C Hamano <gitster@pobox.com> | 2022-08-18 14:13:08 -0700 |
| commit | 04495a1941c77d95cb6ad521f45451da4f714af2 (patch) | |
| tree | ad4d9204d5b4c2ac489f99a1051faa1bf48d6ed7 /gitformat-commit-graph.html | |
| parent | 472c81a0f3cf8038d39479da1e3840ac33fe6ef8 (diff) | |
| download | git-htmldocs-04495a1941c77d95cb6ad521f45451da4f714af2.tar.gz | |
Autogenerated HTML docs for v2.37.2-382-g795ea
Diffstat (limited to 'gitformat-commit-graph.html')
| -rw-r--r-- | gitformat-commit-graph.html | 1083 |
1 files changed, 1083 insertions, 0 deletions
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+<body class="manpage">
+<div id="header">
+<h1>
+gitformat-commit-graph(5) Manual Page
+</h1>
+<h2>NAME</h2>
+<div class="sectionbody">
+<p>gitformat-commit-graph -
+ Git commit graph format
+</p>
+</div>
+</div>
+<div id="content">
+<div class="sect1">
+<h2 id="_synopsis">SYNOPSIS</h2>
+<div class="sectionbody">
+<div class="verseblock">
+<pre class="content">$GIT_DIR/objects/info/commit-graph
+$GIT_DIR/objects/info/commit-graphs/*</pre>
+<div class="attribution">
+</div></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_description">DESCRIPTION</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>The Git commit graph stores a list of commit OIDs and some associated
+metadata, including:</p></div>
+<div class="ulist"><ul>
+<li>
+<p>
+The generation number of the commit.
+</p>
+</li>
+<li>
+<p>
+The root tree OID.
+</p>
+</li>
+<li>
+<p>
+The commit date.
+</p>
+</li>
+<li>
+<p>
+The parents of the commit, stored using positional references within
+ the graph file.
+</p>
+</li>
+<li>
+<p>
+The Bloom filter of the commit carrying the paths that were changed between
+ the commit and its first parent, if requested.
+</p>
+</li>
+</ul></div>
+<div class="paragraph"><p>These positional references are stored as unsigned 32-bit integers
+corresponding to the array position within the list of commit OIDs. Due
+to some special constants we use to track parents, we can store at most
+(1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 billion) commits.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_commit_graph_files_have_the_following_format">Commit graph files have the following format:</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>In order to allow extensions that add extra data to the graph, we organize
+the body into "chunks" and provide a binary lookup table at the beginning
+of the body. The header includes certain values, such as number of chunks
+and hash type.</p></div>
+<div class="paragraph"><p>All multi-byte numbers are in network byte order.</p></div>
+<div class="sect2">
+<h3 id="_header">HEADER:</h3>
+<div class="literalblock">
+<div class="content">
+<pre><code>4-byte signature:
+ The signature is: {'C', 'G', 'P', 'H'}</code></pre>
+</div></div>
+<div class="literalblock">
+<div class="content">
+<pre><code>1-byte version number:
+ Currently, the only valid version is 1.</code></pre>
+</div></div>
+<div class="literalblock">
+<div class="content">
+<pre><code>1-byte Hash Version
+ We infer the hash length (H) from this value:
+ 1 => SHA-1
+ 2 => SHA-256
+ If the hash type does not match the repository's hash algorithm, the
+ commit-graph file should be ignored with a warning presented to the
+ user.</code></pre>
+</div></div>
+<div class="literalblock">
+<div class="content">
+<pre><code>1-byte number (C) of "chunks"</code></pre>
+</div></div>
+<div class="literalblock">
+<div class="content">
+<pre><code>1-byte number (B) of base commit-graphs
+ We infer the length (H*B) of the Base Graphs chunk
+ from this value.</code></pre>
+</div></div>
+</div>
+<div class="sect2">
+<h3 id="_chunk_lookup">CHUNK LOOKUP:</h3>
+<div class="literalblock">
+<div class="content">
+<pre><code>(C + 1) * 12 bytes listing the table of contents for the chunks:
+ First 4 bytes describe the chunk id. Value 0 is a terminating label.
+ Other 8 bytes provide the byte-offset in current file for chunk to
+ start. (Chunks are ordered contiguously in the file, so you can infer
+ the length using the next chunk position if necessary.) Each chunk
+ ID appears at most once.</code></pre>
+</div></div>
+<div class="literalblock">
+<div class="content">
+<pre><code>The CHUNK LOOKUP matches the table of contents from
+the chunk-based file format, see linkgit:gitformat-chunk[5]</code></pre>
+</div></div>
+<div class="literalblock">
+<div class="content">
+<pre><code>The remaining data in the body is described one chunk at a time, and
+these chunks may be given in any order. Chunks are required unless
+otherwise specified.</code></pre>
+</div></div>
+</div>
+<div class="sect2">
+<h3 id="_chunk_data">CHUNK DATA:</h3>
+<div class="sect3">
+<h4 id="_oid_fanout_id_em_o_em_em_i_em_em_d_em_em_f_em_256_4_bytes">OID Fanout (ID: {<em>O</em>, <em>I</em>, <em>D</em>, <em>F</em>}) (256 * 4 bytes)</h4>
+<div class="literalblock">
+<div class="content">
+<pre><code>The ith entry, F[i], stores the number of OIDs with first
+byte at most i. Thus F[255] stores the total
+number of commits (N).</code></pre>
+</div></div>
+</div>
+<div class="sect3">
+<h4 id="_oid_lookup_id_em_o_em_em_i_em_em_d_em_em_l_em_n_h_bytes">OID Lookup (ID: {<em>O</em>, <em>I</em>, <em>D</em>, <em>L</em>}) (N * H bytes)</h4>
+<div class="literalblock">
+<div class="content">
+<pre><code>The OIDs for all commits in the graph, sorted in ascending order.</code></pre>
+</div></div>
+</div>
+<div class="sect3">
+<h4 id="_commit_data_id_em_c_em_em_d_em_em_a_em_em_t_em_n_h_16_bytes">Commit Data (ID: {<em>C</em>, <em>D</em>, <em>A</em>, <em>T</em> }) (N * (H + 16) bytes)</h4>
+<div class="ulist"><ul>
+<li>
+<p>
+The first H bytes are for the OID of the root tree.
+</p>
+</li>
+<li>
+<p>
+The next 8 bytes are for the positions of the first two parents
+ of the ith commit. Stores value 0x70000000 if no parent in that
+ position. If there are more than two parents, the second value
+ has its most-significant bit on and the other bits store an array
+ position into the Extra Edge List chunk.
+</p>
+</li>
+<li>
+<p>
+The next 8 bytes store the topological level (generation number v1)
+ of the commit and
+ the commit time in seconds since EPOCH. The generation number
+ uses the higher 30 bits of the first 4 bytes, while the commit
+ time uses the 32 bits of the second 4 bytes, along with the lowest
+ 2 bits of the lowest byte, storing the 33rd and 34th bit of the
+ commit time.
+</p>
+</li>
+</ul></div>
+</div>
+<div class="sect3">
+<h4 id="_generation_data_id_em_g_em_em_d_em_em_a_em_em_2_em_n_4_bytes_optional">Generation Data (ID: {<em>G</em>, <em>D</em>, <em>A</em>, <em>2</em> }) (N * 4 bytes) [Optional]</h4>
+<div class="ulist"><ul>
+<li>
+<p>
+This list of 4-byte values store corrected commit date offsets for the
+ commits, arranged in the same order as commit data chunk.
+</p>
+</li>
+<li>
+<p>
+If the corrected commit date offset cannot be stored within 31 bits,
+ the value has its most-significant bit on and the other bits store
+ the position of corrected commit date into the Generation Data Overflow
+ chunk.
+</p>
+</li>
+<li>
+<p>
+Generation Data chunk is present only when commit-graph file is written
+ by compatible versions of Git and in case of split commit-graph chains,
+ the topmost layer also has Generation Data chunk.
+</p>
+</li>
+</ul></div>
+</div>
+<div class="sect3">
+<h4 id="_generation_data_overflow_id_em_g_em_em_d_em_em_o_em_em_2_em_optional">Generation Data Overflow (ID: {<em>G</em>, <em>D</em>, <em>O</em>, <em>2</em> }) [Optional]</h4>
+<div class="ulist"><ul>
+<li>
+<p>
+This list of 8-byte values stores the corrected commit date offsets
+ for commits with corrected commit date offsets that cannot be
+ stored within 31 bits.
+</p>
+</li>
+<li>
+<p>
+Generation Data Overflow chunk is present only when Generation Data
+ chunk is present and atleast one corrected commit date offset cannot
+ be stored within 31 bits.
+</p>
+</li>
+</ul></div>
+</div>
+<div class="sect3">
+<h4 id="_extra_edge_list_id_em_e_em_em_d_em_em_g_em_em_e_em_optional">Extra Edge List (ID: {<em>E</em>, <em>D</em>, <em>G</em>, <em>E</em>}) [Optional]</h4>
+<div class="literalblock">
+<div class="content">
+<pre><code>This list of 4-byte values store the second through nth parents for
+all octopus merges. The second parent value in the commit data stores
+an array position within this list along with the most-significant bit
+on. Starting at that array position, iterate through this list of commit
+positions for the parents until reaching a value with the most-significant
+bit on. The other bits correspond to the position of the last parent.</code></pre>
+</div></div>
+</div>
+<div class="sect3">
+<h4 id="_bloom_filter_index_id_em_b_em_em_i_em_em_d_em_em_x_em_n_4_bytes_optional">Bloom Filter Index (ID: {<em>B</em>, <em>I</em>, <em>D</em>, <em>X</em>}) (N * 4 bytes) [Optional]</h4>
+<div class="ulist"><ul>
+<li>
+<p>
+The ith entry, BIDX[i], stores the number of bytes in all Bloom filters
+ from commit 0 to commit i (inclusive) in lexicographic order. The Bloom
+ filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header
+ length), where BIDX[-1] is 0.
+</p>
+</li>
+<li>
+<p>
+The BIDX chunk is ignored if the BDAT chunk is not present.
+</p>
+</li>
+</ul></div>
+</div>
+<div class="sect3">
+<h4 id="_bloom_filter_data_id_em_b_em_em_d_em_em_a_em_em_t_em_optional">Bloom Filter Data (ID: {<em>B</em>, <em>D</em>, <em>A</em>, <em>T</em>}) [Optional]</h4>
+<div class="ulist"><ul>
+<li>
+<p>
+It starts with header consisting of three unsigned 32-bit integers:
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+Version of the hash algorithm being used. We currently only support
+ value 1 which corresponds to the 32-bit version of the murmur3 hash
+ implemented exactly as described in
+ <a href="https://en.wikipedia.org/wiki/MurmurHash#Algorithm">https://en.wikipedia.org/wiki/MurmurHash#Algorithm</a> and the double
+ hashing technique using seed values 0x293ae76f and 0x7e646e2 as
+ described in <a href="https://doi.org/10.1007/978-3-540-30494-4_26">https://doi.org/10.1007/978-3-540-30494-4_26</a> "Bloom Filters
+ in Probabilistic Verification"
+</p>
+</li>
+<li>
+<p>
+The number of times a path is hashed and hence the number of bit positions
+ that cumulatively determine whether a file is present in the commit.
+</p>
+</li>
+<li>
+<p>
+The minimum number of bits <em>b</em> per entry in the Bloom filter. If the filter
+ contains <em>n</em> entries, then the filter size is the minimum number of 64-bit
+ words that contain n*b bits.
+</p>
+</li>
+</ul></div>
+</li>
+<li>
+<p>
+The rest of the chunk is the concatenation of all the computed Bloom
+ filters for the commits in lexicographic order.
+</p>
+</li>
+<li>
+<p>
+Note: Commits with no changes or more than 512 changes have Bloom filters
+ of length one, with either all bits set to zero or one respectively.
+</p>
+</li>
+<li>
+<p>
+The BDAT chunk is present if and only if BIDX is present.
+</p>
+</li>
+</ul></div>
+</div>
+<div class="sect3">
+<h4 id="_base_graphs_list_id_em_b_em_em_a_em_em_s_em_em_e_em_optional">Base Graphs List (ID: {<em>B</em>, <em>A</em>, <em>S</em>, <em>E</em>}) [Optional]</h4>
+<div class="literalblock">
+<div class="content">
+<pre><code>This list of H-byte hashes describe a set of B commit-graph files that
+form a commit-graph chain. The graph position for the ith commit in this
+file's OID Lookup chunk is equal to i plus the number of commits in all
+base graphs. If B is non-zero, this chunk must exist.</code></pre>
+</div></div>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_trailer">TRAILER:</h3>
+<div class="literalblock">
+<div class="content">
+<pre><code>H-byte HASH-checksum of all of the above.</code></pre>
+</div></div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_historical_notes">Historical Notes:</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>The Generation Data (GDA2) and Generation Data Overflow (GDO2) chunks have
+the number <em>2</em> in their chunk IDs because a previous version of Git wrote
+possibly erroneous data in these chunks with the IDs "GDAT" and "GDOV". By
+changing the IDs, newer versions of Git will silently ignore those older
+chunks and write the new information without trusting the incorrect data.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_git">GIT</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
+</div>
+</div>
+</div>
+<div id="footnotes"><hr /></div>
+<div id="footer">
+<div id="footer-text">
+Last updated
+ 2022-08-18 14:11:07 PDT
+</div>
+</div>
+</body>
+</html>
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