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 735 <body class="manpage">
 736 <div id="header">
 737 <h1>
 738 gitformat-commit-graph(5) Manual Page
 739 </h1>
 740 <h2>NAME</h2>
 741 <div class="sectionbody">
 742 <p>gitformat-commit-graph -
 743    Git commit graph format
 744 </p>
 745 </div>
 746 </div>
 747 <div id="content">
 748 <div class="sect1">
 749 <h2 id="_synopsis">SYNOPSIS</h2>
 750 <div class="sectionbody">
 751 <div class="verseblock">
 752 <pre class="content">$GIT_DIR/objects/info/commit-graph
 753 $GIT_DIR/objects/info/commit-graphs/*</pre>
 754 <div class="attribution">
 755 </div></div>
 756 </div>
 757 </div>
 758 <div class="sect1">
 759 <h2 id="_description">DESCRIPTION</h2>
 760 <div class="sectionbody">
 761 <div class="paragraph"><p>The Git commit graph stores a list of commit OIDs and some associated
 762 metadata, including:</p></div>
 763 <div class="ulist"><ul>
 764 <li>
 765 <p>
 766 The generation number of the commit.
 767 </p>
 768 </li>
 769 <li>
 770 <p>
 771 The root tree OID.
 772 </p>
 773 </li>
 774 <li>
 775 <p>
 776 The commit date.
 777 </p>
 778 </li>
 779 <li>
 780 <p>
 781 The parents of the commit, stored using positional references within
 782   the graph file.
 783 </p>
 784 </li>
 785 <li>
 786 <p>
 787 The Bloom filter of the commit carrying the paths that were changed between
 788   the commit and its first parent, if requested.
 789 </p>
 790 </li>
 791 </ul></div>
 792 <div class="paragraph"><p>These positional references are stored as unsigned 32-bit integers
 793 corresponding to the array position within the list of commit OIDs. Due
 794 to some special constants we use to track parents, we can store at most
 795 (1 &lt;&lt; 30) + (1 &lt;&lt; 29) + (1 &lt;&lt; 28) - 1 (around 1.8 billion) commits.</p></div>
 796 </div>
 797 </div>
 798 <div class="sect1">
 799 <h2 id="_commit_graph_files_have_the_following_format">Commit graph files have the following format:</h2>
 800 <div class="sectionbody">
 801 <div class="paragraph"><p>In order to allow extensions that add extra data to the graph, we organize
 802 the body into "chunks" and provide a binary lookup table at the beginning
 803 of the body. The header includes certain values, such as number of chunks
 804 and hash type.</p></div>
 805 <div class="paragraph"><p>All multi-byte numbers are in network byte order.</p></div>
 806 <div class="sect2">
 807 <h3 id="_header">HEADER:</h3>
 808 <div class="literalblock">
 809 <div class="content">
 810 <pre><code>4-byte signature:
 811     The signature is: {'C', 'G', 'P', 'H'}</code></pre>
 812 </div></div>
 813 <div class="literalblock">
 814 <div class="content">
 815 <pre><code>1-byte version number:
 816     Currently, the only valid version is 1.</code></pre>
 817 </div></div>
 818 <div class="literalblock">
 819 <div class="content">
 820 <pre><code>1-byte Hash Version
 821     We infer the hash length (H) from this value:
 822       1 =&gt; SHA-1
 823       2 =&gt; SHA-256
 824     If the hash type does not match the repository's hash algorithm, the
 825     commit-graph file should be ignored with a warning presented to the
 826     user.</code></pre>
 827 </div></div>
 828 <div class="literalblock">
 829 <div class="content">
 830 <pre><code>1-byte number (C) of "chunks"</code></pre>
 831 </div></div>
 832 <div class="literalblock">
 833 <div class="content">
 834 <pre><code>1-byte number (B) of base commit-graphs
 835     We infer the length (H*B) of the Base Graphs chunk
 836     from this value.</code></pre>
 837 </div></div>
 838 </div>
 839 <div class="sect2">
 840 <h3 id="_chunk_lookup">CHUNK LOOKUP:</h3>
 841 <div class="literalblock">
 842 <div class="content">
 843 <pre><code>(C + 1) * 12 bytes listing the table of contents for the chunks:
 844     First 4 bytes describe the chunk id. Value 0 is a terminating label.
 845     Other 8 bytes provide the byte-offset in current file for chunk to
 846     start. (Chunks are ordered contiguously in the file, so you can infer
 847     the length using the next chunk position if necessary.) Each chunk
 848     ID appears at most once.</code></pre>
 849 </div></div>
 850 <div class="literalblock">
 851 <div class="content">
 852 <pre><code>The CHUNK LOOKUP matches the table of contents from
 853 the chunk-based file format, see linkgit:gitformat-chunk[5]</code></pre>
 854 </div></div>
 855 <div class="literalblock">
 856 <div class="content">
 857 <pre><code>The remaining data in the body is described one chunk at a time, and
 858 these chunks may be given in any order. Chunks are required unless
 859 otherwise specified.</code></pre>
 860 </div></div>
 861 </div>
 862 <div class="sect2">
 863 <h3 id="_chunk_data">CHUNK DATA:</h3>
 864 <div class="sect3">
 865 <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>
 866 <div class="literalblock">
 867 <div class="content">
 868 <pre><code>The ith entry, F[i], stores the number of OIDs with first
 869 byte at most i. Thus F[255] stores the total
 870 number of commits (N).</code></pre>
 871 </div></div>
 872 </div>
 873 <div class="sect3">
 874 <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>
 875 <div class="literalblock">
 876 <div class="content">
 877 <pre><code>The OIDs for all commits in the graph, sorted in ascending order.</code></pre>
 878 </div></div>
 879 </div>
 880 <div class="sect3">
 881 <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>
 882 <div class="ulist"><ul>
 883 <li>
 884 <p>
 885 The first H bytes are for the OID of the root tree.
 886 </p>
 887 </li>
 888 <li>
 889 <p>
 890 The next 8 bytes are for the positions of the first two parents
 891       of the ith commit. Stores value 0x70000000 if no parent in that
 892       position. If there are more than two parents, the second value
 893       has its most-significant bit on and the other bits store an array
 894       position into the Extra Edge List chunk.
 895 </p>
 896 </li>
 897 <li>
 898 <p>
 899 The next 8 bytes store the topological level (generation number v1)
 900       of the commit and
 901       the commit time in seconds since EPOCH. The generation number
 902       uses the higher 30 bits of the first 4 bytes, while the commit
 903       time uses the 32 bits of the second 4 bytes, along with the lowest
 904       2 bits of the lowest byte, storing the 33rd and 34th bit of the
 905       commit time.
 906 </p>
 907 </li>
 908 </ul></div>
 909 </div>
 910 <div class="sect3">
 911 <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>
 912 <div class="ulist"><ul>
 913 <li>
 914 <p>
 915 This list of 4-byte values store corrected commit date offsets for the
 916       commits, arranged in the same order as commit data chunk.
 917 </p>
 918 </li>
 919 <li>
 920 <p>
 921 If the corrected commit date offset cannot be stored within 31 bits,
 922       the value has its most-significant bit on and the other bits store
 923       the position of corrected commit date into the Generation Data Overflow
 924       chunk.
 925 </p>
 926 </li>
 927 <li>
 928 <p>
 929 Generation Data chunk is present only when commit-graph file is written
 930       by compatible versions of Git and in case of split commit-graph chains,
 931       the topmost layer also has Generation Data chunk.
 932 </p>
 933 </li>
 934 </ul></div>
 935 </div>
 936 <div class="sect3">
 937 <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>
 938 <div class="ulist"><ul>
 939 <li>
 940 <p>
 941 This list of 8-byte values stores the corrected commit date offsets
 942       for commits with corrected commit date offsets that cannot be
 943       stored within 31 bits.
 944 </p>
 945 </li>
 946 <li>
 947 <p>
 948 Generation Data Overflow chunk is present only when Generation Data
 949       chunk is present and atleast one corrected commit date offset cannot
 950       be stored within 31 bits.
 951 </p>
 952 </li>
 953 </ul></div>
 954 </div>
 955 <div class="sect3">
 956 <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>
 957 <div class="literalblock">
 958 <div class="content">
 959 <pre><code>This list of 4-byte values store the second through nth parents for
 960 all octopus merges. The second parent value in the commit data stores
 961 an array position within this list along with the most-significant bit
 962 on. Starting at that array position, iterate through this list of commit
 963 positions for the parents until reaching a value with the most-significant
 964 bit on. The other bits correspond to the position of the last parent.</code></pre>
 965 </div></div>
 966 </div>
 967 <div class="sect3">
 968 <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>
 969 <div class="ulist"><ul>
 970 <li>
 971 <p>
 972 The ith entry, BIDX[i], stores the number of bytes in all Bloom filters
 973       from commit 0 to commit i (inclusive) in lexicographic order. The Bloom
 974       filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header
 975       length), where BIDX[-1] is 0.
 976 </p>
 977 </li>
 978 <li>
 979 <p>
 980 The BIDX chunk is ignored if the BDAT chunk is not present.
 981 </p>
 982 </li>
 983 </ul></div>
 984 </div>
 985 <div class="sect3">
 986 <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>
 987 <div class="ulist"><ul>
 988 <li>
 989 <p>
 990 It starts with header consisting of three unsigned 32-bit integers:
 991 </p>
 992 <div class="ulist"><ul>
 993 <li>
 994 <p>
 995 Version of the hash algorithm being used. We currently only support
 996         value 1 which corresponds to the 32-bit version of the murmur3 hash
 997         implemented exactly as described in
 998         <a href="https://en.wikipedia.org/wiki/MurmurHash#Algorithm">https://en.wikipedia.org/wiki/MurmurHash#Algorithm</a> and the double
 999         hashing technique using seed values 0x293ae76f and 0x7e646e2 as
1000         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
1001         in Probabilistic Verification"
1002 </p>
1003 </li>
1004 <li>
1005 <p>
1006 The number of times a path is hashed and hence the number of bit positions
1007               that cumulatively determine whether a file is present in the commit.
1008 </p>
1009 </li>
1010 <li>
1011 <p>
1012 The minimum number of bits <em>b</em> per entry in the Bloom filter. If the filter
1013               contains <em>n</em> entries, then the filter size is the minimum number of 64-bit
1014               words that contain n*b bits.
1015 </p>
1016 </li>
1017 </ul></div>
1018 </li>
1019 <li>
1020 <p>
1021 The rest of the chunk is the concatenation of all the computed Bloom
1022       filters for the commits in lexicographic order.
1023 </p>
1024 </li>
1025 <li>
1026 <p>
1027 Note: Commits with no changes or more than 512 changes have Bloom filters
1028       of length one, with either all bits set to zero or one respectively.
1029 </p>
1030 </li>
1031 <li>
1032 <p>
1033 The BDAT chunk is present if and only if BIDX is present.
1034 </p>
1035 </li>
1036 </ul></div>
1037 </div>
1038 <div class="sect3">
1039 <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>
1040 <div class="literalblock">
1041 <div class="content">
1042 <pre><code>This list of H-byte hashes describe a set of B commit-graph files that
1043 form a commit-graph chain. The graph position for the ith commit in this
1044 file's OID Lookup chunk is equal to i plus the number of commits in all
1045 base graphs.  If B is non-zero, this chunk must exist.</code></pre>
1046 </div></div>
1047 </div>
1048 </div>
1049 <div class="sect2">
1050 <h3 id="_trailer">TRAILER:</h3>
1051 <div class="literalblock">
1052 <div class="content">
1053 <pre><code>H-byte HASH-checksum of all of the above.</code></pre>
1054 </div></div>
1055 </div>
1056 </div>
1057 </div>
1058 <div class="sect1">
1059 <h2 id="_historical_notes">Historical Notes:</h2>
1060 <div class="sectionbody">
1061 <div class="paragraph"><p>The Generation Data (GDA2) and Generation Data Overflow (GDO2) chunks have
1062 the number <em>2</em> in their chunk IDs because a previous version of Git wrote
1063 possibly erroneous data in these chunks with the IDs "GDAT" and "GDOV". By
1064 changing the IDs, newer versions of Git will silently ignore those older
1065 chunks and write the new information without trusting the incorrect data.</p></div>
1066 </div>
1067 </div>
1068 <div class="sect1">
1069 <h2 id="_git">GIT</h2>
1070 <div class="sectionbody">
1071 <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
1072 </div>
1073 </div>
1074 </div>
1075 <div id="footnotes"><hr /></div>
1076 <div id="footer">
1077 <div id="footer-text">
1078 Last updated
1079  2022-08-18 14:11:07 PDT
1080 </div>
1081 </div>
1082 </body>
1083 </html>