gitformat-chunk.html

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   8 <title>gitformat-chunk(5)</title>
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 422  *
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 452
 453 /*
 454  * html5 specific
 455  *
 456  * */
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 538
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 540 <script type="text/javascript">
 541 /*<![CDATA[*/
 542 var asciidoc = {  // Namespace.
 543
 544 /////////////////////////////////////////////////////////////////////
 545 // Table Of Contents generator
 546 /////////////////////////////////////////////////////////////////////
 547
 548 /* Author: Mihai Bazon, September 2002
 549  * http://students.infoiasi.ro/~mishoo
 550  *
 551  * Table Of Content generator
 552  * Version: 0.4
 553  *
 554  * Feel free to use this script under the terms of the GNU General Public
 555  * License, as long as you do not remove or alter this notice.
 556  */
 557
 558  /* modified by Troy D. Hanson, September 2006. License: GPL */
 559  /* modified by Stuart Rackham, 2006, 2009. License: GPL */
 560
 561 // toclevels = 1..4.
 562 toc: function (toclevels) {
 563
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 598     iterate(el);
 599     return result;
 600   }
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 602   var toc = document.getElementById("toc");
 603   if (!toc) {
 604     return;
 605   }
 606
 607   // Delete existing TOC entries in case we're reloading the TOC.
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 610   for (i = 0; i < toc.childNodes.length; i++) {
 611     var entry = toc.childNodes[i];
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 613      && entry.getAttribute("class")
 614      && entry.getAttribute("class").match(/^toclevel/))
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 616   }
 617   for (i = 0; i < tocEntriesToRemove.length; i++) {
 618     toc.removeChild(tocEntriesToRemove[i]);
 619   }
 620
 621   // Rebuild TOC entries.
 622   var entries = tocEntries(document.getElementById("content"), toclevels);
 623   for (var i = 0; i < entries.length; ++i) {
 624     var entry = entries[i];
 625     if (entry.element.id == "")
 626       entry.element.id = "_toc_" + i;
 627     var a = document.createElement("a");
 628     a.href = "#" + entry.element.id;
 629     a.appendChild(document.createTextNode(entry.text));
 630     var div = document.createElement("div");
 631     div.appendChild(a);
 632     div.className = "toclevel" + entry.toclevel;
 633     toc.appendChild(div);
 634   }
 635   if (entries.length == 0)
 636     toc.parentNode.removeChild(toc);
 637 },
 638
 639
 640 /////////////////////////////////////////////////////////////////////
 641 // Footnotes generator
 642 /////////////////////////////////////////////////////////////////////
 643
 644 /* Based on footnote generation code from:
 645  * http://www.brandspankingnew.net/archive/2005/07/format_footnote.html
 646  */
 647
 648 footnotes: function () {
 649   // Delete existing footnote entries in case we're reloading the footnodes.
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 660   }
 661   for (i = 0; i < entriesToRemove.length; i++) {
 662     noteholder.removeChild(entriesToRemove[i]);
 663   }
 664
 665   // Rebuild footnote entries.
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 667   var spans = cont.getElementsByTagName("span");
 668   var refs = {};
 669   var n = 0;
 670   for (i=0; i<spans.length; i++) {
 671     if (spans[i].className == "footnote") {
 672       n++;
 673       var note = spans[i].getAttribute("data-note");
 674       if (!note) {
 675         // Use [\s\S] in place of . so multi-line matches work.
 676         // Because JavaScript has no s (dotall) regex flag.
 677         note = spans[i].innerHTML.match(/\s*\[([\s\S]*)]\s*/)[1];
 678         spans[i].innerHTML =
 679           "[<a id='_footnoteref_" + n + "' href='#_footnote_" + n +
 680           "' title='View footnote' class='footnote'>" + n + "</a>]";
 681         spans[i].setAttribute("data-note", note);
 682       }
 683       noteholder.innerHTML +=
 684         "<div class='footnote' id='_footnote_" + n + "'>" +
 685         "<a href='#_footnoteref_" + n + "' title='Return to text'>" +
 686         n + "</a>. " + note + "</div>";
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 688       if (id != null) refs["#"+id] = n;
 689     }
 690   }
 691   if (n == 0)
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 693   else {
 694     // Process footnoterefs.
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 696       if (spans[i].className == "footnoteref") {
 697         var href = spans[i].getElementsByTagName("a")[0].getAttribute("href");
 698         href = href.match(/#.*/)[0];  // Because IE return full URL.
 699         n = refs[href];
 700         spans[i].innerHTML =
 701           "[<a href='#_footnote_" + n +
 702           "' title='View footnote' class='footnote'>" + n + "</a>]";
 703       }
 704     }
 705   }
 706 },
 707
 708 install: function(toclevels) {
 709   var timerId;
 710
 711   function reinstall() {
 712     asciidoc.footnotes();
 713     if (toclevels) {
 714       asciidoc.toc(toclevels);
 715     }
 716   }
 717
 718   function reinstallAndRemoveTimer() {
 719     clearInterval(timerId);
 720     reinstall();
 721   }
 722
 723   timerId = setInterval(reinstall, 500);
 724   if (document.addEventListener)
 725     document.addEventListener("DOMContentLoaded", reinstallAndRemoveTimer, false);
 726   else
 727     window.onload = reinstallAndRemoveTimer;
 728 }
 729
 730 }
 731 asciidoc.install();
 732 /*]]>*/
 733 </script>
 734 </head>
 735 <body class="manpage">
 736 <div id="header">
 737 <h1>
 738 gitformat-chunk(5) Manual Page
 739 </h1>
 740 <h2>NAME</h2>
 741 <div class="sectionbody">
 742 <p>gitformat-chunk -
 743    Chunk-based file formats
 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="paragraph"><p>Used by <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a> and the "MIDX" format (see
 752 the pack format documentation in <a href="gitformat-pack.html">gitformat-pack(5)</a>).</p></div>
 753 </div>
 754 </div>
 755 <div class="sect1">
 756 <h2 id="_description">DESCRIPTION</h2>
 757 <div class="sectionbody">
 758 <div class="paragraph"><p>Some file formats in Git use a common concept of "chunks" to describe
 759 sections of the file. This allows structured access to a large file by
 760 scanning a small "table of contents" for the remaining data. This common
 761 format is used by the <code>commit-graph</code> and <code>multi-pack-index</code> files. See
 762 the <code>multi-pack-index</code> format in <a href="gitformat-pack.html">gitformat-pack(5)</a> and
 763 the <code>commit-graph</code> format in <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a> for
 764 how they use the chunks to describe structured data.</p></div>
 765 <div class="paragraph"><p>A chunk-based file format begins with some header information custom to
 766 that format. That header should include enough information to identify
 767 the file type, format version, and number of chunks in the file. From this
 768 information, that file can determine the start of the chunk-based region.</p></div>
 769 <div class="paragraph"><p>The chunk-based region starts with a table of contents describing where
 770 each chunk starts and ends. This consists of (C+1) rows of 12 bytes each,
 771 where C is the number of chunks. Consider the following table:</p></div>
 772 <div class="literalblock">
 773 <div class="content">
 774 <pre><code>| Chunk ID (4 bytes) | Chunk Offset (8 bytes) |
 775 |--------------------|------------------------|
 776 | ID[0]              | OFFSET[0]              |
 777 | ...                | ...                    |
 778 | ID[C]              | OFFSET[C]              |
 779 | 0x0000             | OFFSET[C+1]            |</code></pre>
 780 </div></div>
 781 <div class="paragraph"><p>Each row consists of a 4-byte chunk identifier (ID) and an 8-byte offset.
 782 Each integer is stored in network-byte order.</p></div>
 783 <div class="paragraph"><p>The chunk identifier <code>ID[i]</code> is a label for the data stored within this
 784 fill from <code>OFFSET[i]</code> (inclusive) to <code>OFFSET[i+1]</code> (exclusive). Thus, the
 785 size of the <code>i`th chunk is equal to the difference between `OFFSET[i+1]</code>
 786 and <code>OFFSET[i]</code>. This requires that the chunk data appears contiguously
 787 in the same order as the table of contents.</p></div>
 788 <div class="paragraph"><p>The final entry in the table of contents must be four zero bytes. This
 789 confirms that the table of contents is ending and provides the offset for
 790 the end of the chunk-based data.</p></div>
 791 <div class="paragraph"><p>Note: The chunk-based format expects that the file contains <em>at least</em> a
 792 trailing hash after <code>OFFSET[C+1]</code>.</p></div>
 793 <div class="paragraph"><p>Functions for working with chunk-based file formats are declared in
 794 <code>chunk-format.h</code>. Using these methods provide extra checks that assist
 795 developers when creating new file formats.</p></div>
 796 </div>
 797 </div>
 798 <div class="sect1">
 799 <h2 id="_writing_chunk_based_file_formats">Writing chunk-based file formats</h2>
 800 <div class="sectionbody">
 801 <div class="paragraph"><p>To write a chunk-based file format, create a <code>struct chunkfile</code> by
 802 calling <code>init_chunkfile()</code> and pass a <code>struct hashfile</code> pointer. The
 803 caller is responsible for opening the <code>hashfile</code> and writing header
 804 information so the file format is identifiable before the chunk-based
 805 format begins.</p></div>
 806 <div class="paragraph"><p>Then, call <code>add_chunk()</code> for each chunk that is intended for write. This
 807 populates the <code>chunkfile</code> with information about the order and size of
 808 each chunk to write. Provide a <code>chunk_write_fn</code> function pointer to
 809 perform the write of the chunk data upon request.</p></div>
 810 <div class="paragraph"><p>Call <code>write_chunkfile()</code> to write the table of contents to the <code>hashfile</code>
 811 followed by each of the chunks. This will verify that each chunk wrote
 812 the expected amount of data so the table of contents is correct.</p></div>
 813 <div class="paragraph"><p>Finally, call <code>free_chunkfile()</code> to clear the <code>struct chunkfile</code> data. The
 814 caller is responsible for finalizing the <code>hashfile</code> by writing the trailing
 815 hash and closing the file.</p></div>
 816 </div>
 817 </div>
 818 <div class="sect1">
 819 <h2 id="_reading_chunk_based_file_formats">Reading chunk-based file formats</h2>
 820 <div class="sectionbody">
 821 <div class="paragraph"><p>To read a chunk-based file format, the file must be opened as a
 822 memory-mapped region. The chunk-format API expects that the entire file
 823 is mapped as a contiguous memory region.</p></div>
 824 <div class="paragraph"><p>Initialize a <code>struct chunkfile</code> pointer with <code>init_chunkfile(NULL)</code>.</p></div>
 825 <div class="paragraph"><p>After reading the header information from the beginning of the file,
 826 including the chunk count, call <code>read_table_of_contents()</code> to populate
 827 the <code>struct chunkfile</code> with the list of chunks, their offsets, and their
 828 sizes.</p></div>
 829 <div class="paragraph"><p>Extract the data information for each chunk using <code>pair_chunk()</code> or
 830 <code>read_chunk()</code>:</p></div>
 831 <div class="ulist"><ul>
 832 <li>
 833 <p>
 834 <code>pair_chunk()</code> assigns a given pointer with the location inside the
 835   memory-mapped file corresponding to that chunk&#8217;s offset. If the chunk
 836   does not exist, then the pointer is not modified.
 837 </p>
 838 </li>
 839 <li>
 840 <p>
 841 <code>read_chunk()</code> takes a <code>chunk_read_fn</code> function pointer and calls it
 842   with the appropriate initial pointer and size information. The function
 843   is not called if the chunk does not exist. Use this method to read chunks
 844   if you need to perform immediate parsing or if you need to execute logic
 845   based on the size of the chunk.
 846 </p>
 847 </li>
 848 </ul></div>
 849 <div class="paragraph"><p>After calling these methods, call <code>free_chunkfile()</code> to clear the
 850 <code>struct chunkfile</code> data. This will not close the memory-mapped region.
 851 Callers are expected to own that data for the timeframe the pointers into
 852 the region are needed.</p></div>
 853 </div>
 854 </div>
 855 <div class="sect1">
 856 <h2 id="_examples">Examples</h2>
 857 <div class="sectionbody">
 858 <div class="paragraph"><p>These file formats use the chunk-format API, and can be used as examples
 859 for future formats:</p></div>
 860 <div class="ulist"><ul>
 861 <li>
 862 <p>
 863 <strong>commit-graph:</strong> see <code>write_commit_graph_file()</code> and <code>parse_commit_graph()</code>
 864   in <code>commit-graph.c</code> for how the chunk-format API is used to write and
 865   parse the commit-graph file format documented in
 866   the commit-graph file format in <a href="gitformat-commit-graph.html">gitformat-commit-graph(5)</a>.
 867 </p>
 868 </li>
 869 <li>
 870 <p>
 871 <strong>multi-pack-index:</strong> see <code>write_midx_internal()</code> and <code>load_multi_pack_index()</code>
 872   in <code>midx.c</code> for how the chunk-format API is used to write and
 873   parse the multi-pack-index file format documented in
 874   the multi-pack-index file format section of <a href="gitformat-pack.html">gitformat-pack(5)</a>.
 875 </p>
 876 </li>
 877 </ul></div>
 878 </div>
 879 </div>
 880 <div class="sect1">
 881 <h2 id="_git">GIT</h2>
 882 <div class="sectionbody">
 883 <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
 884 </div>
 885 </div>
 886 </div>
 887 <div id="footnotes"><hr /></div>
 888 <div id="footer">
 889 <div id="footer-text">
 890 Last updated
 891  2022-08-18 14:11:07 PDT
 892 </div>
 893 </div>
 894 </body>
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