technical/bitmap-format.html

   1 <?xml version="1.0" encoding="UTF-8"?>
   2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
   3     "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
   4 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
   5 <head>
   6 <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />
   7 <meta name="generator" content="AsciiDoc 10.2.0" />
   8 <title>GIT bitmap v1 format</title>
   9 <style type="text/css">
  10 /* Shared CSS for AsciiDoc xhtml11 and html5 backends */
  11
  12 /* Default font. */
  13 body {
  14   font-family: Georgia,serif;
  15 }
  16
  17 /* Title font. */
  18 h1, h2, h3, h4, h5, h6,
  19 div.title, caption.title,
  20 thead, p.table.header,
  21 #toctitle,
  22 #author, #revnumber, #revdate, #revremark,
  23 #footer {
  24   font-family: Arial,Helvetica,sans-serif;
  25 }
  26
  27 body {
  28   margin: 1em 5% 1em 5%;
  29 }
  30
  31 a {
  32   color: blue;
  33   text-decoration: underline;
  34 }
  35 a:visited {
  36   color: fuchsia;
  37 }
  38
  39 em {
  40   font-style: italic;
  41   color: navy;
  42 }
  43
  44 strong {
  45   font-weight: bold;
  46   color: #083194;
  47 }
  48
  49 h1, h2, h3, h4, h5, h6 {
  50   color: #527bbd;
  51   margin-top: 1.2em;
  52   margin-bottom: 0.5em;
  53   line-height: 1.3;
  54 }
  55
  56 h1, h2, h3 {
  57   border-bottom: 2px solid silver;
  58 }
  59 h2 {
  60   padding-top: 0.5em;
  61 }
  62 h3 {
  63   float: left;
  64 }
  65 h3 + * {
  66   clear: left;
  67 }
  68 h5 {
  69   font-size: 1.0em;
  70 }
  71
  72 div.sectionbody {
  73   margin-left: 0;
  74 }
  75
  76 hr {
  77   border: 1px solid silver;
  78 }
  79
  80 p {
  81   margin-top: 0.5em;
  82   margin-bottom: 0.5em;
  83 }
  84
  85 ul, ol, li > p {
  86   margin-top: 0;
  87 }
  88 ul > li     { color: #aaa; }
  89 ul > li > * { color: black; }
  90
  91 .monospaced, code, pre {
  92   font-family: "Courier New", Courier, monospace;
  93   font-size: inherit;
  94   color: navy;
  95   padding: 0;
  96   margin: 0;
  97 }
  98 pre {
  99   white-space: pre-wrap;
 100 }
 101
 102 #author {
 103   color: #527bbd;
 104   font-weight: bold;
 105   font-size: 1.1em;
 106 }
 107 #email {
 108 }
 109 #revnumber, #revdate, #revremark {
 110 }
 111
 112 #footer {
 113   font-size: small;
 114   border-top: 2px solid silver;
 115   padding-top: 0.5em;
 116   margin-top: 4.0em;
 117 }
 118 #footer-text {
 119   float: left;
 120   padding-bottom: 0.5em;
 121 }
 122 #footer-badges {
 123   float: right;
 124   padding-bottom: 0.5em;
 125 }
 126
 127 #preamble {
 128   margin-top: 1.5em;
 129   margin-bottom: 1.5em;
 130 }
 131 div.imageblock, div.exampleblock, div.verseblock,
 132 div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock,
 133 div.admonitionblock {
 134   margin-top: 1.0em;
 135   margin-bottom: 1.5em;
 136 }
 137 div.admonitionblock {
 138   margin-top: 2.0em;
 139   margin-bottom: 2.0em;
 140   margin-right: 10%;
 141   color: #606060;
 142 }
 143
 144 div.content { /* Block element content. */
 145   padding: 0;
 146 }
 147
 148 /* Block element titles. */
 149 div.title, caption.title {
 150   color: #527bbd;
 151   font-weight: bold;
 152   text-align: left;
 153   margin-top: 1.0em;
 154   margin-bottom: 0.5em;
 155 }
 156 div.title + * {
 157   margin-top: 0;
 158 }
 159
 160 td div.title:first-child {
 161   margin-top: 0.0em;
 162 }
 163 div.content div.title:first-child {
 164   margin-top: 0.0em;
 165 }
 166 div.content + div.title {
 167   margin-top: 0.0em;
 168 }
 169
 170 div.sidebarblock > div.content {
 171   background: #ffffee;
 172   border: 1px solid #dddddd;
 173   border-left: 4px solid #f0f0f0;
 174   padding: 0.5em;
 175 }
 176
 177 div.listingblock > div.content {
 178   border: 1px solid #dddddd;
 179   border-left: 5px solid #f0f0f0;
 180   background: #f8f8f8;
 181   padding: 0.5em;
 182 }
 183
 184 div.quoteblock, div.verseblock {
 185   padding-left: 1.0em;
 186   margin-left: 1.0em;
 187   margin-right: 10%;
 188   border-left: 5px solid #f0f0f0;
 189   color: #888;
 190 }
 191
 192 div.quoteblock > div.attribution {
 193   padding-top: 0.5em;
 194   text-align: right;
 195 }
 196
 197 div.verseblock > pre.content {
 198   font-family: inherit;
 199   font-size: inherit;
 200 }
 201 div.verseblock > div.attribution {
 202   padding-top: 0.75em;
 203   text-align: left;
 204 }
 205 /* DEPRECATED: Pre version 8.2.7 verse style literal block. */
 206 div.verseblock + div.attribution {
 207   text-align: left;
 208 }
 209
 210 div.admonitionblock .icon {
 211   vertical-align: top;
 212   font-size: 1.1em;
 213   font-weight: bold;
 214   text-decoration: underline;
 215   color: #527bbd;
 216   padding-right: 0.5em;
 217 }
 218 div.admonitionblock td.content {
 219   padding-left: 0.5em;
 220   border-left: 3px solid #dddddd;
 221 }
 222
 223 div.exampleblock > div.content {
 224   border-left: 3px solid #dddddd;
 225   padding-left: 0.5em;
 226 }
 227
 228 div.imageblock div.content { padding-left: 0; }
 229 span.image img { border-style: none; vertical-align: text-bottom; }
 230 a.image:visited { color: white; }
 231
 232 dl {
 233   margin-top: 0.8em;
 234   margin-bottom: 0.8em;
 235 }
 236 dt {
 237   margin-top: 0.5em;
 238   margin-bottom: 0;
 239   font-style: normal;
 240   color: navy;
 241 }
 242 dd > *:first-child {
 243   margin-top: 0.1em;
 244 }
 245
 246 ul, ol {
 247     list-style-position: outside;
 248 }
 249 ol.arabic {
 250   list-style-type: decimal;
 251 }
 252 ol.loweralpha {
 253   list-style-type: lower-alpha;
 254 }
 255 ol.upperalpha {
 256   list-style-type: upper-alpha;
 257 }
 258 ol.lowerroman {
 259   list-style-type: lower-roman;
 260 }
 261 ol.upperroman {
 262   list-style-type: upper-roman;
 263 }
 264
 265 div.compact ul, div.compact ol,
 266 div.compact p, div.compact p,
 267 div.compact div, div.compact div {
 268   margin-top: 0.1em;
 269   margin-bottom: 0.1em;
 270 }
 271
 272 tfoot {
 273   font-weight: bold;
 274 }
 275 td > div.verse {
 276   white-space: pre;
 277 }
 278
 279 div.hdlist {
 280   margin-top: 0.8em;
 281   margin-bottom: 0.8em;
 282 }
 283 div.hdlist tr {
 284   padding-bottom: 15px;
 285 }
 286 dt.hdlist1.strong, td.hdlist1.strong {
 287   font-weight: bold;
 288 }
 289 td.hdlist1 {
 290   vertical-align: top;
 291   font-style: normal;
 292   padding-right: 0.8em;
 293   color: navy;
 294 }
 295 td.hdlist2 {
 296   vertical-align: top;
 297 }
 298 div.hdlist.compact tr {
 299   margin: 0;
 300   padding-bottom: 0;
 301 }
 302
 303 .comment {
 304   background: yellow;
 305 }
 306
 307 .footnote, .footnoteref {
 308   font-size: 0.8em;
 309 }
 310
 311 span.footnote, span.footnoteref {
 312   vertical-align: super;
 313 }
 314
 315 #footnotes {
 316   margin: 20px 0 20px 0;
 317   padding: 7px 0 0 0;
 318 }
 319
 320 #footnotes div.footnote {
 321   margin: 0 0 5px 0;
 322 }
 323
 324 #footnotes hr {
 325   border: none;
 326   border-top: 1px solid silver;
 327   height: 1px;
 328   text-align: left;
 329   margin-left: 0;
 330   width: 20%;
 331   min-width: 100px;
 332 }
 333
 334 div.colist td {
 335   padding-right: 0.5em;
 336   padding-bottom: 0.3em;
 337   vertical-align: top;
 338 }
 339 div.colist td img {
 340   margin-top: 0.3em;
 341 }
 342
 343 @media print {
 344   #footer-badges { display: none; }
 345 }
 346
 347 #toc {
 348   margin-bottom: 2.5em;
 349 }
 350
 351 #toctitle {
 352   color: #527bbd;
 353   font-size: 1.1em;
 354   font-weight: bold;
 355   margin-top: 1.0em;
 356   margin-bottom: 0.1em;
 357 }
 358
 359 div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 {
 360   margin-top: 0;
 361   margin-bottom: 0;
 362 }
 363 div.toclevel2 {
 364   margin-left: 2em;
 365   font-size: 0.9em;
 366 }
 367 div.toclevel3 {
 368   margin-left: 4em;
 369   font-size: 0.9em;
 370 }
 371 div.toclevel4 {
 372   margin-left: 6em;
 373   font-size: 0.9em;
 374 }
 375
 376 span.aqua { color: aqua; }
 377 span.black { color: black; }
 378 span.blue { color: blue; }
 379 span.fuchsia { color: fuchsia; }
 380 span.gray { color: gray; }
 381 span.green { color: green; }
 382 span.lime { color: lime; }
 383 span.maroon { color: maroon; }
 384 span.navy { color: navy; }
 385 span.olive { color: olive; }
 386 span.purple { color: purple; }
 387 span.red { color: red; }
 388 span.silver { color: silver; }
 389 span.teal { color: teal; }
 390 span.white { color: white; }
 391 span.yellow { color: yellow; }
 392
 393 span.aqua-background { background: aqua; }
 394 span.black-background { background: black; }
 395 span.blue-background { background: blue; }
 396 span.fuchsia-background { background: fuchsia; }
 397 span.gray-background { background: gray; }
 398 span.green-background { background: green; }
 399 span.lime-background { background: lime; }
 400 span.maroon-background { background: maroon; }
 401 span.navy-background { background: navy; }
 402 span.olive-background { background: olive; }
 403 span.purple-background { background: purple; }
 404 span.red-background { background: red; }
 405 span.silver-background { background: silver; }
 406 span.teal-background { background: teal; }
 407 span.white-background { background: white; }
 408 span.yellow-background { background: yellow; }
 409
 410 span.big { font-size: 2em; }
 411 span.small { font-size: 0.6em; }
 412
 413 span.underline { text-decoration: underline; }
 414 span.overline { text-decoration: overline; }
 415 span.line-through { text-decoration: line-through; }
 416
 417 div.unbreakable { page-break-inside: avoid; }
 418
 419
 420 /*
 421  * xhtml11 specific
 422  *
 423  * */
 424
 425 div.tableblock {
 426   margin-top: 1.0em;
 427   margin-bottom: 1.5em;
 428 }
 429 div.tableblock > table {
 430   border: 3px solid #527bbd;
 431 }
 432 thead, p.table.header {
 433   font-weight: bold;
 434   color: #527bbd;
 435 }
 436 p.table {
 437   margin-top: 0;
 438 }
 439 /* Because the table frame attribute is overridden by CSS in most browsers. */
 440 div.tableblock > table[frame="void"] {
 441   border-style: none;
 442 }
 443 div.tableblock > table[frame="hsides"] {
 444   border-left-style: none;
 445   border-right-style: none;
 446 }
 447 div.tableblock > table[frame="vsides"] {
 448   border-top-style: none;
 449   border-bottom-style: none;
 450 }
 451
 452
 453 /*
 454  * html5 specific
 455  *
 456  * */
 457
 458 table.tableblock {
 459   margin-top: 1.0em;
 460   margin-bottom: 1.5em;
 461 }
 462 thead, p.tableblock.header {
 463   font-weight: bold;
 464   color: #527bbd;
 465 }
 466 p.tableblock {
 467   margin-top: 0;
 468 }
 469 table.tableblock {
 470   border-width: 3px;
 471   border-spacing: 0px;
 472   border-style: solid;
 473   border-color: #527bbd;
 474   border-collapse: collapse;
 475 }
 476 th.tableblock, td.tableblock {
 477   border-width: 1px;
 478   padding: 4px;
 479   border-style: solid;
 480   border-color: #527bbd;
 481 }
 482
 483 table.tableblock.frame-topbot {
 484   border-left-style: hidden;
 485   border-right-style: hidden;
 486 }
 487 table.tableblock.frame-sides {
 488   border-top-style: hidden;
 489   border-bottom-style: hidden;
 490 }
 491 table.tableblock.frame-none {
 492   border-style: hidden;
 493 }
 494
 495 th.tableblock.halign-left, td.tableblock.halign-left {
 496   text-align: left;
 497 }
 498 th.tableblock.halign-center, td.tableblock.halign-center {
 499   text-align: center;
 500 }
 501 th.tableblock.halign-right, td.tableblock.halign-right {
 502   text-align: right;
 503 }
 504
 505 th.tableblock.valign-top, td.tableblock.valign-top {
 506   vertical-align: top;
 507 }
 508 th.tableblock.valign-middle, td.tableblock.valign-middle {
 509   vertical-align: middle;
 510 }
 511 th.tableblock.valign-bottom, td.tableblock.valign-bottom {
 512   vertical-align: bottom;
 513 }
 514
 515
 516 /*
 517  * manpage specific
 518  *
 519  * */
 520
 521 body.manpage h1 {
 522   padding-top: 0.5em;
 523   padding-bottom: 0.5em;
 524   border-top: 2px solid silver;
 525   border-bottom: 2px solid silver;
 526 }
 527 body.manpage h2 {
 528   border-style: none;
 529 }
 530 body.manpage div.sectionbody {
 531   margin-left: 3em;
 532 }
 533
 534 @media print {
 535   body.manpage div#toc { display: none; }
 536 }
 537
 538
 539 </style>
 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
 564   function getText(el) {
 565     var text = "";
 566     for (var i = el.firstChild; i != null; i = i.nextSibling) {
 567       if (i.nodeType == 3 /* Node.TEXT_NODE */) // IE doesn't speak constants.
 568         text += i.data;
 569       else if (i.firstChild != null)
 570         text += getText(i);
 571     }
 572     return text;
 573   }
 574
 575   function TocEntry(el, text, toclevel) {
 576     this.element = el;
 577     this.text = text;
 578     this.toclevel = toclevel;
 579   }
 580
 581   function tocEntries(el, toclevels) {
 582     var result = new Array;
 583     var re = new RegExp('[hH]([1-'+(toclevels+1)+'])');
 584     // Function that scans the DOM tree for header elements (the DOM2
 585     // nodeIterator API would be a better technique but not supported by all
 586     // browsers).
 587     var iterate = function (el) {
 588       for (var i = el.firstChild; i != null; i = i.nextSibling) {
 589         if (i.nodeType == 1 /* Node.ELEMENT_NODE */) {
 590           var mo = re.exec(i.tagName);
 591           if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") {
 592             result[result.length] = new TocEntry(i, getText(i), mo[1]-1);
 593           }
 594           iterate(i);
 595         }
 596       }
 597     }
 598     iterate(el);
 599     return result;
 600   }
 601
 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.
 608   var tocEntriesToRemove = [];
 609   var i;
 610   for (i = 0; i < toc.childNodes.length; i++) {
 611     var entry = toc.childNodes[i];
 612     if (entry.nodeName.toLowerCase() == 'div'
 613      && entry.getAttribute("class")
 614      && entry.getAttribute("class").match(/^toclevel/))
 615       tocEntriesToRemove.push(entry);
 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.
 650   var i;
 651   var noteholder = document.getElementById("footnotes");
 652   if (!noteholder) {
 653     return;
 654   }
 655   var entriesToRemove = [];
 656   for (i = 0; i < noteholder.childNodes.length; i++) {
 657     var entry = noteholder.childNodes[i];
 658     if (entry.nodeName.toLowerCase() == 'div' && entry.getAttribute("class") == "footnote")
 659       entriesToRemove.push(entry);
 660   }
 661   for (i = 0; i < entriesToRemove.length; i++) {
 662     noteholder.removeChild(entriesToRemove[i]);
 663   }
 664
 665   // Rebuild footnote entries.
 666   var cont = document.getElementById("content");
 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>";
 687       var id =spans[i].getAttribute("id");
 688       if (id != null) refs["#"+id] = n;
 689     }
 690   }
 691   if (n == 0)
 692     noteholder.parentNode.removeChild(noteholder);
 693   else {
 694     // Process footnoterefs.
 695     for (i=0; i<spans.length; i++) {
 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="article">
 736 <div id="header">
 737 <h1>GIT bitmap v1 format</h1>
 738 <span id="revdate">2023-10-29</span>
 739 </div>
 740 <div id="content">
 741 <div class="sect1">
 742 <h2 id="_pack_and_multi_pack_bitmaps">Pack and multi-pack bitmaps</h2>
 743 <div class="sectionbody">
 744 <div class="paragraph"><p>Bitmaps store reachability information about the set of objects in a packfile,
 745 or a multi-pack index (MIDX). The former is defined obviously, and the latter is
 746 defined as the union of objects in packs contained in the MIDX.</p></div>
 747 <div class="paragraph"><p>A bitmap may belong to either one pack, or the repository&#8217;s multi-pack index (if
 748 it exists). A repository may have at most one bitmap.</p></div>
 749 <div class="paragraph"><p>An object is uniquely described by its bit position within a bitmap:</p></div>
 750 <div class="ulist"><ul>
 751 <li>
 752 <p>
 753 If the bitmap belongs to a packfile, the <em>n</em>th bit corresponds to
 754         the <em>n</em>th object in pack order. For a function <code>offset</code> which maps
 755         objects to their byte offset within a pack, pack order is defined as
 756         follows:
 757 </p>
 758 <div class="literalblock">
 759 <div class="content">
 760 <pre><code>o1 &lt;= o2 &lt;==&gt; offset(o1) &lt;= offset(o2)</code></pre>
 761 </div></div>
 762 </li>
 763 <li>
 764 <p>
 765 If the bitmap belongs to a MIDX, the <em>n</em>th bit corresponds to the
 766         <em>n</em>th object in MIDX order. With an additional function <code>pack</code> which
 767         maps objects to the pack they were selected from by the MIDX, MIDX order
 768         is defined as follows:
 769 </p>
 770 <div class="literalblock">
 771 <div class="content">
 772 <pre><code>o1 &lt;= o2 &lt;==&gt; pack(o1) &lt;= pack(o2) /\ offset(o1) &lt;= offset(o2)</code></pre>
 773 </div></div>
 774 <div class="paragraph"><p>The ordering between packs is done according to the MIDX&#8217;s .rev file.
 775 Notably, the preferred pack sorts ahead of all other packs.</p></div>
 776 </li>
 777 </ul></div>
 778 <div class="paragraph"><p>The on-disk representation (described below) of a bitmap is the same regardless
 779 of whether or not that bitmap belongs to a packfile or a MIDX. The only
 780 difference is the interpretation of the bits, which is described above.</p></div>
 781 <div class="paragraph"><p>Certain bitmap extensions are supported (see: Appendix B). No extensions are
 782 required for bitmaps corresponding to packfiles. For bitmaps that correspond to
 783 MIDXs, both the bit-cache and rev-cache extensions are required.</p></div>
 784 </div>
 785 </div>
 786 <div class="sect1">
 787 <h2 id="_on_disk_format">On-disk format</h2>
 788 <div class="sectionbody">
 789 <div class="ulist"><ul>
 790 <li>
 791 <p>
 792 A header appears at the beginning:
 793 </p>
 794 <div class="dlist"><dl>
 795 <dt class="hdlist1">
 796 4-byte signature:
 797 </dt>
 798 <dd>
 799 <p>
 800 {<em>B</em>, <em>I</em>, <em>T</em>, <em>M</em>}
 801 </p>
 802 </dd>
 803 <dt class="hdlist1">
 804 2-byte version number (network byte order):
 805 </dt>
 806 <dd>
 807 <p>
 808             The current implementation only supports version 1
 809             of the bitmap index (the same one as JGit).
 810 </p>
 811 </dd>
 812 <dt class="hdlist1">
 813 2-byte flags (network byte order):
 814 </dt>
 815 <dd>
 816 <p>
 817             The following flags are supported:
 818 </p>
 819 <div class="ulist"><ul>
 820 <li>
 821 <p>
 822 </p>
 823 <div class="dlist"><dl>
 824 <dt class="hdlist1">
 825 BITMAP_OPT_FULL_DAG (0x1) REQUIRED:
 826 </dt>
 827 <dd>
 828 <p>
 829             This flag must always be present. It implies that the
 830             bitmap index has been generated for a packfile or
 831             multi-pack index (MIDX) with full closure (i.e. where
 832             every single object in the packfile/MIDX can find its
 833             parent links inside the same packfile/MIDX). This is a
 834             requirement for the bitmap index format, also present in
 835             JGit, that greatly reduces the complexity of the
 836             implementation.
 837 </p>
 838 </dd>
 839 </dl></div>
 840 </li>
 841 <li>
 842 <p>
 843 </p>
 844 <div class="dlist"><dl>
 845 <dt class="hdlist1">
 846 BITMAP_OPT_HASH_CACHE (0x4):
 847 </dt>
 848 <dd>
 849 <p>
 850             If present, the end of the bitmap file contains
 851             <code>N</code> 32-bit name-hash values, one per object in the
 852             pack/MIDX. The format and meaning of the name-hash is
 853             described below.
 854 </p>
 855 </dd>
 856 </dl></div>
 857 </li>
 858 <li>
 859 <p>
 860 </p>
 861 <div class="dlist"><dl>
 862 <dt class="hdlist1">
 863 BITMAP_OPT_LOOKUP_TABLE (0x10):
 864 </dt>
 865 <dd>
 866 <p>
 867                 If present, the end of the bitmap file contains a table
 868                 containing a list of <code>N</code> &lt;commit_pos, offset, xor_row&gt;
 869                 triplets. The format and meaning of the table is described
 870                 below.
 871 </p>
 872 <div class="admonitionblock">
 873 <table><tr>
 874 <td class="icon">
 875 <div class="title">Note</div>
 876 </td>
 877 <td class="content">Unlike the xor_offset used to compress an individual bitmap,
 878 <code>xor_row</code> stores an <strong>absolute</strong> index into the lookup table, not a location
 879 relative to the current entry.</td>
 880 </tr></table>
 881 </div>
 882 </dd>
 883 </dl></div>
 884 </li>
 885 </ul></div>
 886 </dd>
 887 <dt class="hdlist1">
 888 4-byte entry count (network byte order):
 889 </dt>
 890 <dd>
 891 <p>
 892             The total count of entries (bitmapped commits) in this bitmap index.
 893 </p>
 894 </dd>
 895 <dt class="hdlist1">
 896 20-byte checksum:
 897 </dt>
 898 <dd>
 899 <p>
 900             The SHA1 checksum of the pack/MIDX this bitmap index
 901             belongs to.
 902 </p>
 903 </dd>
 904 </dl></div>
 905 </li>
 906 <li>
 907 <p>
 908 4 EWAH bitmaps that act as type indexes
 909 </p>
 910 <div class="paragraph"><p>Type indexes are serialized after the hash cache in the shape
 911 of four EWAH bitmaps stored consecutively (see Appendix A for
 912 the serialization format of an EWAH bitmap).</p></div>
 913 <div class="paragraph"><p>There is a bitmap for each Git object type, stored in the following
 914 order:</p></div>
 915 <div class="ulist"><ul>
 916 <li>
 917 <p>
 918 Commits
 919 </p>
 920 </li>
 921 <li>
 922 <p>
 923 Trees
 924 </p>
 925 </li>
 926 <li>
 927 <p>
 928 Blobs
 929 </p>
 930 </li>
 931 <li>
 932 <p>
 933 Tags
 934 </p>
 935 </li>
 936 </ul></div>
 937 <div class="paragraph"><p>In each bitmap, the `n`th bit is set to true if the `n`th object
 938 in the packfile or multi-pack index is of that type.</p></div>
 939 <div class="paragraph"><p>The obvious consequence is that the OR of all 4 bitmaps will result
 940 in a full set (all bits set), and the AND of all 4 bitmaps will
 941 result in an empty bitmap (no bits set).</p></div>
 942 </li>
 943 <li>
 944 <p>
 945 N entries with compressed bitmaps, one for each indexed commit
 946 </p>
 947 <div class="paragraph"><p>Where <code>N</code> is the total number of entries in this bitmap index.
 948 Each entry contains the following:</p></div>
 949 <div class="ulist"><ul>
 950 <li>
 951 <p>
 952 </p>
 953 <div class="dlist"><dl>
 954 <dt class="hdlist1">
 955 4-byte object position (network byte order):
 956 </dt>
 957 <dd>
 958 <p>
 959             The position <strong>in the index for the packfile or
 960             multi-pack index</strong> where the bitmap for this commit is
 961             found.
 962 </p>
 963 </dd>
 964 </dl></div>
 965 </li>
 966 <li>
 967 <p>
 968 </p>
 969 <div class="dlist"><dl>
 970 <dt class="hdlist1">
 971 1-byte XOR-offset:
 972 </dt>
 973 <dd>
 974 <p>
 975             The xor offset used to compress this bitmap. For an entry
 976             in position <code>x</code>, an XOR offset of <code>y</code> means that the actual
 977             bitmap representing this commit is composed by XORing the
 978             bitmap for this entry with the bitmap in entry <code>x-y</code> (i.e.
 979             the bitmap <code>y</code> entries before this one).
 980 </p>
 981 <div class="admonitionblock">
 982 <table><tr>
 983 <td class="icon">
 984 <div class="title">Note</div>
 985 </td>
 986 <td class="content">This compression can be recursive. In order to
 987 XOR this entry with a previous one, the previous entry needs
 988 to be decompressed first, and so on.</td>
 989 </tr></table>
 990 </div>
 991 <div class="paragraph"><p>The hard-limit for this offset is 160 (an entry can only be
 992 xor&#8217;ed against one of the 160 entries preceding it). This
 993 number is always positive, and hence entries are always xor&#8217;ed
 994 with <strong>previous</strong> bitmaps, not bitmaps that will come afterwards
 995 in the index.</p></div>
 996 </dd>
 997 </dl></div>
 998 </li>
 999 <li>
1000 <p>
1001 </p>
1002 <div class="dlist"><dl>
1003 <dt class="hdlist1">
1004 1-byte flags for this bitmap:
1005 </dt>
1006 <dd>
1007 <p>
1008             At the moment the only available flag is <code>0x1</code>, which hints
1009             that this bitmap can be re-used when rebuilding bitmap indexes
1010             for the repository.
1011 </p>
1012 </dd>
1013 </dl></div>
1014 </li>
1015 <li>
1016 <p>
1017 The compressed bitmap itself, see Appendix A.
1018 </p>
1019 </li>
1020 </ul></div>
1021 </li>
1022 <li>
1023 <p>
1024 </p>
1025 <div class="dlist"><dl>
1026 <dt class="hdlist1">
1027 TRAILER:
1028 </dt>
1029 <dd>
1030 <p>
1031                 Trailing checksum of the preceding contents.
1032 </p>
1033 </dd>
1034 </dl></div>
1035 </li>
1036 </ul></div>
1037 </div>
1038 </div>
1039 <div class="sect1">
1040 <h2 id="_appendix_a_serialization_format_for_an_ewah_bitmap">Appendix A: Serialization format for an EWAH bitmap</h2>
1041 <div class="sectionbody">
1042 <div class="paragraph"><p>Ewah bitmaps are serialized in the same protocol as the JAVAEWAH
1043 library, making them backwards compatible with the JGit
1044 implementation:</p></div>
1045 <div class="ulist"><ul>
1046 <li>
1047 <p>
1048 4-byte number of bits of the resulting UNCOMPRESSED bitmap
1049 </p>
1050 </li>
1051 <li>
1052 <p>
1053 4-byte number of words of the COMPRESSED bitmap, when stored
1054 </p>
1055 </li>
1056 <li>
1057 <p>
1058 N x 8-byte words, as specified by the previous field
1059 </p>
1060 <div class="paragraph"><p>This is the actual content of the compressed bitmap.</p></div>
1061 </li>
1062 <li>
1063 <p>
1064 4-byte position of the current RLW for the compressed
1065                 bitmap
1066 </p>
1067 </li>
1068 </ul></div>
1069 <div class="paragraph"><p>All words are stored in network byte order for their corresponding
1070 sizes.</p></div>
1071 <div class="paragraph"><p>The compressed bitmap is stored in a form of run-length encoding, as
1072 follows.  It consists of a concatenation of an arbitrary number of
1073 chunks.  Each chunk consists of one or more 64-bit words</p></div>
1074 <div class="literalblock">
1075 <div class="content">
1076 <pre><code>H  L_1  L_2  L_3 .... L_M</code></pre>
1077 </div></div>
1078 <div class="paragraph"><p>H is called RLW (run length word).  It consists of (from lower to higher
1079 order bits):</p></div>
1080 <div class="ulist"><ul>
1081 <li>
1082 <p>
1083 1 bit: the repeated bit B
1084 </p>
1085 </li>
1086 <li>
1087 <p>
1088 32 bits: repetition count K (unsigned)
1089 </p>
1090 </li>
1091 <li>
1092 <p>
1093 31 bits: literal word count M (unsigned)
1094 </p>
1095 </li>
1096 </ul></div>
1097 <div class="paragraph"><p>The bitstream represented by the above chunk is then:</p></div>
1098 <div class="ulist"><ul>
1099 <li>
1100 <p>
1101 K repetitions of B
1102 </p>
1103 </li>
1104 <li>
1105 <p>
1106 The bits stored in <code>L_1</code> through <code>L_M</code>.  Within a word, bits at
1107        lower order come earlier in the stream than those at higher
1108        order.
1109 </p>
1110 </li>
1111 </ul></div>
1112 <div class="paragraph"><p>The next word after <code>L_M</code> (if any) must again be a RLW, for the next
1113 chunk.  For efficient appending to the bitstream, the EWAH stores a
1114 pointer to the last RLW in the stream.</p></div>
1115 </div>
1116 </div>
1117 <div class="sect1">
1118 <h2 id="_appendix_b_optional_bitmap_sections">Appendix B: Optional Bitmap Sections</h2>
1119 <div class="sectionbody">
1120 <div class="paragraph"><p>These sections may or may not be present in the <code>.bitmap</code> file; their
1121 presence is indicated by the header flags section described above.</p></div>
1122 </div>
1123 </div>
1124 <div class="sect1">
1125 <h2 id="_name_hash_cache">Name-hash cache</h2>
1126 <div class="sectionbody">
1127 <div class="paragraph"><p>If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains
1128 a cache of 32-bit values, one per object in the pack/MIDX. The value at
1129 position <code>i</code> is the hash of the pathname at which the `i`th object
1130 (counting in index or multi-pack index order) in the pack/MIDX can be found.
1131 This can be fed into the delta heuristics to compare objects with similar
1132 pathnames.</p></div>
1133 <div class="paragraph"><p>The hash algorithm used is:</p></div>
1134 <div class="literalblock">
1135 <div class="content">
1136 <pre><code>hash = 0;
1137 while ((c = *name++))
1138         if (!isspace(c))
1139                 hash = (hash &gt;&gt; 2) + (c &lt;&lt; 24);</code></pre>
1140 </div></div>
1141 <div class="paragraph"><p>Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.
1142 If implementations want to choose a different hashing scheme, they are
1143 free to do so, but MUST allocate a new header flag (because comparing
1144 hashes made under two different schemes would be pointless).</p></div>
1145 </div>
1146 </div>
1147 <div class="sect1">
1148 <h2 id="_commit_lookup_table">Commit lookup table</h2>
1149 <div class="sectionbody">
1150 <div class="paragraph"><p>If the BITMAP_OPT_LOOKUP_TABLE flag is set, the last <code>N * (4 + 8 + 4)</code>
1151 bytes (preceding the name-hash cache and trailing hash) of the <code>.bitmap</code>
1152 file contains a lookup table specifying the information needed to get
1153 the desired bitmap from the entries without parsing previous unnecessary
1154 bitmaps.</p></div>
1155 <div class="paragraph"><p>For a <code>.bitmap</code> containing <code>nr_entries</code> reachability bitmaps, the table
1156 contains a list of <code>nr_entries</code> &lt;commit_pos, offset, xor_row&gt; triplets
1157 (sorted in the ascending order of <code>commit_pos</code>). The content of the i&#8217;th
1158 triplet is -</p></div>
1159 <div class="ulist"><ul>
1160 <li>
1161 <p>
1162 </p>
1163 <div class="dlist"><dl>
1164 <dt class="hdlist1">
1165 commit_pos (4 byte integer, network byte order):
1166 </dt>
1167 <dd>
1168 <p>
1169         It stores the object position of a commit (in the midx or pack
1170         index).
1171 </p>
1172 </dd>
1173 </dl></div>
1174 </li>
1175 <li>
1176 <p>
1177 </p>
1178 <div class="dlist"><dl>
1179 <dt class="hdlist1">
1180 offset (8 byte integer, network byte order):
1181 </dt>
1182 <dd>
1183 <p>
1184         The offset from which that commit&#8217;s bitmap can be read.
1185 </p>
1186 </dd>
1187 </dl></div>
1188 </li>
1189 <li>
1190 <p>
1191 </p>
1192 <div class="dlist"><dl>
1193 <dt class="hdlist1">
1194 xor_row (4 byte integer, network byte order):
1195 </dt>
1196 <dd>
1197 <p>
1198         The position of the triplet whose bitmap is used to compress
1199         this one, or <code>0xffffffff</code> if no such bitmap exists.
1200 </p>
1201 </dd>
1202 </dl></div>
1203 </li>
1204 </ul></div>
1205 </div>
1206 </div>
1207 </div>
1208 <div id="footnotes"><hr /></div>
1209 <div id="footer">
1210 <div id="footer-text">
1211 Last updated
1212  2023-10-24 06:43:46 JST
1213 </div>
1214 </div>
1215 </body>
1216 </html>