MyFirstObjectWalk.html

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 441 <body class="article">
 442 <div id="header">
 443 <h1>My First Object Walk</h1>
 444 <div class="details">
 445 <span id="revdate">2024-11-01</span>
 446 </div>
 447 </div>
 448 <div id="content">
 449 <div class="sect1">
 450 <h2 id="_whats_an_object_walk">What&#8217;s an Object Walk?</h2>
 451 <div class="sectionbody">
 452 <div class="paragraph">
 453 <p>The object walk is a key concept in Git - this is the process that underpins
 454 operations like object transfer and fsck. Beginning from a given commit, the
 455 list of objects is found by walking parent relationships between commits (commit
 456 X based on commit W) and containment relationships between objects (tree Y is
 457 contained within commit X, and blob Z is located within tree Y, giving our
 458 working tree for commit X something like <code>y/z.txt</code>).</p>
 459 </div>
 460 <div class="paragraph">
 461 <p>A related concept is the revision walk, which is focused on commit objects and
 462 their parent relationships and does not delve into other object types. The
 463 revision walk is used for operations like <code>git</code> <code>log</code>.</p>
 464 </div>
 465 <div class="sect2">
 466 <h3 id="_related_reading">Related Reading</h3>
 467 <div class="ulist">
 468 <ul>
 469 <li>
 470 <p><code>Documentation/user-manual.txt</code> under "Hacking Git" contains some coverage of
 471 the revision walker in its various incarnations.</p>
 472 </li>
 473 <li>
 474 <p><code>revision.h</code></p>
 475 </li>
 476 <li>
 477 <p><a href="https://eagain.net/articles/git-for-computer-scientists/">Git for Computer Scientists</a>
 478 gives a good overview of the types of objects in Git and what your object
 479 walk is really describing.</p>
 480 </li>
 481 </ul>
 482 </div>
 483 </div>
 484 </div>
 485 </div>
 486 <div class="sect1">
 487 <h2 id="_setting_up">Setting Up</h2>
 488 <div class="sectionbody">
 489 <div class="paragraph">
 490 <p>Create a new branch from <code>master</code>.</p>
 491 </div>
 492 <div class="listingblock">
 493 <div class="content">
 494 <pre>git checkout -b revwalk origin/master</pre>
 495 </div>
 496 </div>
 497 <div class="paragraph">
 498 <p>We&#8217;ll put our fiddling into a new command. For fun, let&#8217;s name it <code>git</code> <code>walken</code>.
 499 Open up a new file <code>builtin/walken.c</code> and set up the command handler:</p>
 500 </div>
 501 <div class="listingblock">
 502 <div class="content">
 503 <pre>/*
 504  * "git walken"
 505  *
 506  * Part of the "My First Object Walk" tutorial.
 507  */
 508
 509 #include "builtin.h"
 510 #include "trace.h"
 511
 512 int cmd_walken(int argc, const char **argv, const char *prefix)
 513 {
 514         trace_printf(_("cmd_walken incoming...\n"));
 515         return 0;
 516 }</pre>
 517 </div>
 518 </div>
 519 <div class="admonitionblock note">
 520 <table>
 521 <tr>
 522 <td class="icon">
 523 <div class="title">Note</div>
 524 </td>
 525 <td class="content">
 526 <code>trace_printf</code>(), defined in <code>trace.h</code>, differs from <code>printf</code>() in
 527 that it can be turned on or off at runtime. For the purposes of this
 528 tutorial, we will write <code>walken</code> as though it is intended for use as
 529 a "plumbing" command: that is, a command which is used primarily in
 530 scripts, rather than interactively by humans (a "porcelain" command).
 531 So we will send our debug output to <code>trace_printf</code>() instead.
 532 When running, enable trace output by setting the environment variable <code>GIT_TRACE</code>.
 533 </td>
 534 </tr>
 535 </table>
 536 </div>
 537 <div class="paragraph">
 538 <p>Add usage text and <code>-h</code> handling, like all subcommands should consistently do
 539 (our test suite will notice and complain if you fail to do so).
 540 We&#8217;ll need to include the <code>parse-options.h</code> header.</p>
 541 </div>
 542 <div class="listingblock">
 543 <div class="content">
 544 <pre>#include "parse-options.h"
 545
 546 ...
 547
 548 int cmd_walken(int argc, const char **argv, const char *prefix)
 549 {
 550         const char * const walken_usage[] = {
 551                 N_("git walken"),
 552                 NULL,
 553         };
 554         struct option options[] = {
 555                 OPT_END()
 556         };
 557
 558         argc = parse_options(argc, argv, prefix, options, walken_usage, 0);
 559
 560         ...
 561 }</pre>
 562 </div>
 563 </div>
 564 <div class="paragraph">
 565 <p>Also add the relevant line in <code>builtin.h</code> near <code>cmd_whatchanged</code>():</p>
 566 </div>
 567 <div class="listingblock">
 568 <div class="content">
 569 <pre>int cmd_walken(int argc, const char **argv, const char *prefix);</pre>
 570 </div>
 571 </div>
 572 <div class="paragraph">
 573 <p>Include the command in <code>git.c</code> in <code>commands</code>[] near the entry for <code>whatchanged</code>,
 574 maintaining alphabetical ordering:</p>
 575 </div>
 576 <div class="listingblock">
 577 <div class="content">
 578 <pre>{ "walken", cmd_walken, RUN_SETUP },</pre>
 579 </div>
 580 </div>
 581 <div class="paragraph">
 582 <p>Add it to the <code>Makefile</code> near the line for <code>builtin/worktree.o</code>:</p>
 583 </div>
 584 <div class="listingblock">
 585 <div class="content">
 586 <pre>BUILTIN_OBJS += builtin/walken.o</pre>
 587 </div>
 588 </div>
 589 <div class="paragraph">
 590 <p>Build and test out your command, without forgetting to ensure the <code>DEVELOPER</code>
 591 flag is set, and with <code>GIT_TRACE</code> enabled so the debug output can be seen:</p>
 592 </div>
 593 <div class="listingblock">
 594 <div class="content">
 595 <pre>$ echo DEVELOPER=1 &gt;&gt;config.mak
 596 $ make
 597 $ GIT_TRACE=1 ./bin-wrappers/git walken</pre>
 598 </div>
 599 </div>
 600 <div class="admonitionblock note">
 601 <table>
 602 <tr>
 603 <td class="icon">
 604 <div class="title">Note</div>
 605 </td>
 606 <td class="content">
 607 For a more exhaustive overview of the new command process, take a look at
 608 <code>Documentation/MyFirstContribution.txt</code>.
 609 </td>
 610 </tr>
 611 </table>
 612 </div>
 613 <div class="admonitionblock note">
 614 <table>
 615 <tr>
 616 <td class="icon">
 617 <div class="title">Note</div>
 618 </td>
 619 <td class="content">
 620 A reference implementation can be found at
 621 <a href="https://github.com/nasamuffin/git/tree/revwalk" class="bare">https://github.com/nasamuffin/git/tree/revwalk</a>.
 622 </td>
 623 </tr>
 624 </table>
 625 </div>
 626 <div class="sect2">
 627 <h3 id="_struct_rev_cmdline_info"><code>struct</code> <code>rev_cmdline_info</code></h3>
 628 <div class="paragraph">
 629 <p>The definition of <code>struct</code> <code>rev_cmdline_info</code> can be found in <code>revision.h</code>.</p>
 630 </div>
 631 <div class="paragraph">
 632 <p>This struct is contained within the <code>rev_info</code> struct and is used to reflect
 633 parameters provided by the user over the CLI.</p>
 634 </div>
 635 <div class="paragraph">
 636 <p><code>nr</code> represents the number of <code>rev_cmdline_entry</code> present in the array.</p>
 637 </div>
 638 <div class="paragraph">
 639 <p><code>alloc</code> is used by the <code>ALLOC_GROW</code> macro. Check <code>alloc.h</code> - this variable is
 640 used to track the allocated size of the list.</p>
 641 </div>
 642 <div class="paragraph">
 643 <p>Per entry, we find:</p>
 644 </div>
 645 <div class="paragraph">
 646 <p><code>item</code> is the object provided upon which to base the object walk. Items in Git
 647 can be blobs, trees, commits, or tags. (See <code>Documentation/gittutorial-2.txt</code>.)</p>
 648 </div>
 649 <div class="paragraph">
 650 <p><code>name</code> is the object ID (OID) of the object - a hex string you may be familiar
 651 with from using Git to organize your source in the past. Check the tutorial
 652 mentioned above towards the top for a discussion of where the OID can come
 653 from.</p>
 654 </div>
 655 <div class="paragraph">
 656 <p><code>whence</code> indicates some information about what to do with the parents of the
 657 specified object. We&#8217;ll explore this flag more later on; take a look at
 658 <code>Documentation/revisions.txt</code> to get an idea of what could set the <code>whence</code>
 659 value.</p>
 660 </div>
 661 <div class="paragraph">
 662 <p><code>flags</code> are used to hint the beginning of the revision walk and are the first
 663 block under the #include`s <code>in</code> `revision.<code>h</code>. The most likely ones to be set in
 664 the <code>rev_cmdline_info</code> are <code>UNINTERESTING</code> and <code>BOTTOM</code>, but these same flags
 665 can be used during the walk, as well.</p>
 666 </div>
 667 </div>
 668 <div class="sect2">
 669 <h3 id="_struct_rev_info"><code>struct</code> <code>rev_info</code></h3>
 670 <div class="paragraph">
 671 <p>This one is quite a bit longer, and many fields are only used during the walk
 672 by <code>revision.c</code> - not configuration options. Most of the configurable flags in
 673 <code>struct</code> <code>rev_info</code> have a mirror in <code>Documentation/rev-list-options.txt</code>. It&#8217;s a
 674 good idea to take some time and read through that document.</p>
 675 </div>
 676 </div>
 677 </div>
 678 </div>
 679 <div class="sect1">
 680 <h2 id="_basic_commit_walk">Basic Commit Walk</h2>
 681 <div class="sectionbody">
 682 <div class="paragraph">
 683 <p>First, let&#8217;s see if we can replicate the output of <code>git</code> <code>log</code> <code>--oneline</code>. We&#8217;ll
 684 refer back to the implementation frequently to discover norms when performing
 685 an object walk of our own.</p>
 686 </div>
 687 <div class="paragraph">
 688 <p>To do so, we&#8217;ll first find all the commits, in order, which preceded the current
 689 commit. We&#8217;ll extract the name and subject of the commit from each.</p>
 690 </div>
 691 <div class="paragraph">
 692 <p>Ideally, we will also be able to find out which ones are currently at the tip of
 693 various branches.</p>
 694 </div>
 695 <div class="sect2">
 696 <h3 id="_setting_up_2">Setting Up</h3>
 697 <div class="paragraph">
 698 <p>Preparing for your object walk has some distinct stages.</p>
 699 </div>
 700 <div class="olist arabic">
 701 <ol class="arabic">
 702 <li>
 703 <p>Perform default setup for this mode, and others which may be invoked.</p>
 704 </li>
 705 <li>
 706 <p>Check configuration files for relevant settings.</p>
 707 </li>
 708 <li>
 709 <p>Set up the <code>rev_info</code> struct.</p>
 710 </li>
 711 <li>
 712 <p>Tweak the initialized <code>rev_info</code> to suit the current walk.</p>
 713 </li>
 714 <li>
 715 <p>Prepare the <code>rev_info</code> for the walk.</p>
 716 </li>
 717 <li>
 718 <p>Iterate over the objects, processing each one.</p>
 719 </li>
 720 </ol>
 721 </div>
 722 <div class="sect3">
 723 <h4 id="_default_setups">Default Setups</h4>
 724 <div class="paragraph">
 725 <p>Before examining configuration files which may modify command behavior, set up
 726 default state for switches or options your command may have. If your command
 727 utilizes other Git components, ask them to set up their default states as well.
 728 For instance, <code>git</code> <code>log</code> takes advantage of <code>grep</code> and <code>diff</code> functionality, so
 729 its <code>init_log_defaults</code>() sets its own state (<code>decoration_style</code>) and asks
 730 <code>grep</code> and <code>diff</code> to initialize themselves by calling each of their
 731 initialization functions.</p>
 732 </div>
 733 </div>
 734 <div class="sect3">
 735 <h4 id="_configuring_from_gitconfig">Configuring From .<code>gitconfig</code></h4>
 736 <div class="paragraph">
 737 <p>Next, we should have a look at any relevant configuration settings (i.e.,
 738 settings readable and settable from <code>git</code> <code>config</code>). This is done by providing a
 739 callback to <code>git_config</code>(); within that callback, you can also invoke methods
 740 from other components you may need that need to intercept these options. Your
 741 callback will be invoked once per each configuration value which Git knows about
 742 (global, local, worktree, etc.).</p>
 743 </div>
 744 <div class="paragraph">
 745 <p>Similarly to the default values, we don&#8217;t have anything to do here yet
 746 ourselves; however, we should call <code>git_default_config</code>() if we aren&#8217;t calling
 747 any other existing config callbacks.</p>
 748 </div>
 749 <div class="paragraph">
 750 <p>Add a new function to <code>builtin/walken.c</code>.
 751 We&#8217;ll also need to include the <code>config.h</code> header:</p>
 752 </div>
 753 <div class="listingblock">
 754 <div class="content">
 755 <pre>#include "config.h"
 756
 757 ...
 758
 759 static int git_walken_config(const char *var, const char *value,
 760                              const struct config_context *ctx, void *cb)
 761 {
 762         /*
 763          * For now, we don't have any custom configuration, so fall back to
 764          * the default config.
 765          */
 766         return git_default_config(var, value, ctx, cb);
 767 }</pre>
 768 </div>
 769 </div>
 770 <div class="paragraph">
 771 <p>Make sure to invoke <code>git_config</code>() with it in your <code>cmd_walken</code>():</p>
 772 </div>
 773 <div class="listingblock">
 774 <div class="content">
 775 <pre>int cmd_walken(int argc, const char **argv, const char *prefix)
 776 {
 777         ...
 778
 779         git_config(git_walken_config, NULL);
 780
 781         ...
 782 }</pre>
 783 </div>
 784 </div>
 785 </div>
 786 <div class="sect3">
 787 <h4 id="_setting_up_rev_info">Setting Up <code>rev_info</code></h4>
 788 <div class="paragraph">
 789 <p>Now that we&#8217;ve gathered external configuration and options, it&#8217;s time to
 790 initialize the <code>rev_info</code> object which we will use to perform the walk. This is
 791 typically done by calling <code>repo_init_revisions</code>() with the repository you intend
 792 to target, as well as the <code>prefix</code> argument of <code>cmd_walken</code> and your <code>rev_info</code>
 793 struct.</p>
 794 </div>
 795 <div class="paragraph">
 796 <p>Add the <code>struct</code> <code>rev_info</code> and the <code>repo_init_revisions</code>() call.
 797 We&#8217;ll also need to include the <code>revision.h</code> header:</p>
 798 </div>
 799 <div class="listingblock">
 800 <div class="content">
 801 <pre>#include "revision.h"
 802
 803 ...
 804
 805 int cmd_walken(int argc, const char **argv, const char *prefix)
 806 {
 807         /* This can go wherever you like in your declarations.*/
 808         struct rev_info rev;
 809         ...
 810
 811         /* This should go after the git_config() call. */
 812         repo_init_revisions(the_repository, &amp;rev, prefix);
 813
 814         ...
 815 }</pre>
 816 </div>
 817 </div>
 818 </div>
 819 <div class="sect3">
 820 <h4 id="_tweaking_rev_info_for_the_walk">Tweaking <code>rev_info</code> For the Walk</h4>
 821 <div class="paragraph">
 822 <p>We&#8217;re getting close, but we&#8217;re still not quite ready to go. Now that <code>rev</code> is
 823 initialized, we can modify it to fit our needs. This is usually done within a
 824 helper for clarity, so let&#8217;s add one:</p>
 825 </div>
 826 <div class="listingblock">
 827 <div class="content">
 828 <pre>static void final_rev_info_setup(struct rev_info *rev)
 829 {
 830         /*
 831          * We want to mimic the appearance of `git log --oneline`, so let's
 832          * force oneline format.
 833          */
 834         get_commit_format("oneline", rev);
 835
 836         /* Start our object walk at HEAD. */
 837         add_head_to_pending(rev);
 838 }</pre>
 839 </div>
 840 </div>
 841 <div class="admonitionblock note">
 842 <table>
 843 <tr>
 844 <td class="icon">
 845 <div class="title">Note</div>
 846 </td>
 847 <td class="content">
 848 <div class="paragraph">
 849 <p>Instead of using the shorthand <code>add_head_to_pending</code>(), you could do
 850 something like this:</p>
 851 </div>
 852 <div class="listingblock">
 853 <div class="content">
 854 <pre>        struct setup_revision_opt opt;
 855
 856         memset(&amp;opt, 0, sizeof(opt));
 857         opt.def = "HEAD";
 858         opt.revarg_opt = REVARG_COMMITTISH;
 859         setup_revisions(argc, argv, rev, &amp;opt);</pre>
 860 </div>
 861 </div>
 862 <div class="paragraph">
 863 <p>Using a <code>setup_revision_opt</code> gives you finer control over your walk&#8217;s starting
 864 point.</p>
 865 </div>
 866 </td>
 867 </tr>
 868 </table>
 869 </div>
 870 <div class="paragraph">
 871 <p>Then let&#8217;s invoke <code>final_rev_info_setup</code>() after the call to
 872 <code>repo_init_revisions</code>():</p>
 873 </div>
 874 <div class="listingblock">
 875 <div class="content">
 876 <pre>int cmd_walken(int argc, const char **argv, const char *prefix)
 877 {
 878         ...
 879
 880         final_rev_info_setup(&amp;rev);
 881
 882         ...
 883 }</pre>
 884 </div>
 885 </div>
 886 <div class="paragraph">
 887 <p>Later, we may wish to add more arguments to <code>final_rev_info_setup</code>(). But for
 888 now, this is all we need.</p>
 889 </div>
 890 </div>
 891 <div class="sect3">
 892 <h4 id="_preparing_rev_info_for_the_walk">Preparing <code>rev_info</code> For the Walk</h4>
 893 <div class="paragraph">
 894 <p>Now that <code>rev</code> is all initialized and configured, we&#8217;ve got one more setup step
 895 before we get rolling. We can do this in a helper, which will both prepare the
 896 <code>rev_info</code> for the walk, and perform the walk itself. Let&#8217;s start the helper
 897 with the call to <code>prepare_revision_walk</code>(), which can return an error without
 898 dying on its own:</p>
 899 </div>
 900 <div class="listingblock">
 901 <div class="content">
 902 <pre>static void walken_commit_walk(struct rev_info *rev)
 903 {
 904         if (prepare_revision_walk(rev))
 905                 die(_("revision walk setup failed"));
 906 }</pre>
 907 </div>
 908 </div>
 909 <div class="admonitionblock note">
 910 <table>
 911 <tr>
 912 <td class="icon">
 913 <div class="title">Note</div>
 914 </td>
 915 <td class="content">
 916 <code>die</code>() prints to <code>stderr</code> and exits the program. Since it will print to
 917 <code>stderr</code> it&#8217;s likely to be seen by a human, so we will localize it.
 918 </td>
 919 </tr>
 920 </table>
 921 </div>
 922 </div>
 923 <div class="sect3">
 924 <h4 id="_performing_the_walk">Performing the Walk!</h4>
 925 <div class="paragraph">
 926 <p>Finally! We are ready to begin the walk itself. Now we can see that <code>rev_info</code>
 927 can also be used as an iterator; we move to the next item in the walk by using
 928 <code>get_revision</code>() repeatedly. Add the listed variable declarations at the top and
 929 the walk loop below the <code>prepare_revision_walk</code>() call within your
 930 <code>walken_commit_walk</code>():</p>
 931 </div>
 932 <div class="listingblock">
 933 <div class="content">
 934 <pre>#include "pretty.h"
 935
 936 ...
 937
 938 static void walken_commit_walk(struct rev_info *rev)
 939 {
 940         struct commit *commit;
 941         struct strbuf prettybuf = STRBUF_INIT;
 942
 943         ...
 944
 945         while ((commit = get_revision(rev))) {
 946                 strbuf_reset(&amp;prettybuf);
 947                 pp_commit_easy(CMIT_FMT_ONELINE, commit, &amp;prettybuf);
 948                 puts(prettybuf.buf);
 949         }
 950         strbuf_release(&amp;prettybuf);
 951 }</pre>
 952 </div>
 953 </div>
 954 <div class="admonitionblock note">
 955 <table>
 956 <tr>
 957 <td class="icon">
 958 <div class="title">Note</div>
 959 </td>
 960 <td class="content">
 961 <code>puts</code>() prints a <code>char</code>* to <code>stdout</code>. Since this is the part of the
 962 command we expect to be machine-parsed, we&#8217;re sending it directly to stdout.
 963 </td>
 964 </tr>
 965 </table>
 966 </div>
 967 <div class="paragraph">
 968 <p>Give it a shot.</p>
 969 </div>
 970 <div class="listingblock">
 971 <div class="content">
 972 <pre>$ make
 973 $ ./bin-wrappers/git walken</pre>
 974 </div>
 975 </div>
 976 <div class="paragraph">
 977 <p>You should see all of the subject lines of all the commits in
 978 your tree&#8217;s history, in order, ending with the initial commit, "Initial revision
 979 of "git", the information manager from hell". Congratulations! You&#8217;ve written
 980 your first revision walk. You can play with printing some additional fields
 981 from each commit if you&#8217;re curious; have a look at the functions available in
 982 <code>commit.h</code>.</p>
 983 </div>
 984 </div>
 985 </div>
 986 <div class="sect2">
 987 <h3 id="_adding_a_filter">Adding a Filter</h3>
 988 <div class="paragraph">
 989 <p>Next, let&#8217;s try to filter the commits we see based on their author. This is
 990 equivalent to running <code>git</code> <code>log</code> <code>--author=</code><em>&lt;pattern&gt;</em>. We can add a filter by
 991 modifying <code>rev_info.grep_filter</code>, which is a <code>struct</code> <code>grep_opt</code>.</p>
 992 </div>
 993 <div class="paragraph">
 994 <p>First some setup. Add <code>grep_config</code>() to <code>git_walken_config</code>():</p>
 995 </div>
 996 <div class="listingblock">
 997 <div class="content">
 998 <pre>static int git_walken_config(const char *var, const char *value,
 999                              const struct config_context *ctx, void *cb)
1000 {
1001         grep_config(var, value, ctx, cb);
1002         return git_default_config(var, value, ctx, cb);
1003 }</pre>
1004 </div>
1005 </div>
1006 <div class="paragraph">
1007 <p>Next, we can modify the <code>grep_filter</code>. This is done with convenience functions
1008 found in <code>grep.h</code>. For fun, we&#8217;re filtering to only commits from folks using a
1009 <code>gmail.com</code> email address - a not-very-precise guess at who may be working on
1010 Git as a hobby. Since we&#8217;re checking the author, which is a specific line in the
1011 header, we&#8217;ll use the <code>append_header_grep_pattern</code>() helper. We can use
1012 the <code>enum</code> <code>grep_header_field</code> to indicate which part of the commit header we want
1013 to search.</p>
1014 </div>
1015 <div class="paragraph">
1016 <p>In <code>final_rev_info_setup</code>(), add your filter line:</p>
1017 </div>
1018 <div class="listingblock">
1019 <div class="content">
1020 <pre>static void final_rev_info_setup(int argc, const char **argv,
1021                 const char *prefix, struct rev_info *rev)
1022 {
1023         ...
1024
1025         append_header_grep_pattern(&amp;rev-&gt;grep_filter, GREP_HEADER_AUTHOR,
1026                 "gmail");
1027         compile_grep_patterns(&amp;rev-&gt;grep_filter);
1028
1029         ...
1030 }</pre>
1031 </div>
1032 </div>
1033 <div class="paragraph">
1034 <p><code>append_header_grep_pattern</code>() adds your new "gmail" pattern to <code>rev_info</code>, but
1035 it won&#8217;t work unless we compile it with <code>compile_grep_patterns</code>().</p>
1036 </div>
1037 <div class="admonitionblock note">
1038 <table>
1039 <tr>
1040 <td class="icon">
1041 <div class="title">Note</div>
1042 </td>
1043 <td class="content">
1044 If you are using <code>setup_revisions</code>() (for example, if you are passing a
1045 <code>setup_revision_opt</code> instead of using <code>add_head_to_pending</code>()), you don&#8217;t need
1046 to call <code>compile_grep_patterns</code>() because <code>setup_revisions</code>() calls it for you.
1047 </td>
1048 </tr>
1049 </table>
1050 </div>
1051 <div class="admonitionblock note">
1052 <table>
1053 <tr>
1054 <td class="icon">
1055 <div class="title">Note</div>
1056 </td>
1057 <td class="content">
1058 We could add the same filter via the <code>append_grep_pattern</code>() helper if we
1059 wanted to, but <code>append_header_grep_pattern</code>() adds the <code>enum</code> <code>grep_context</code> and
1060 <code>enum</code> <code>grep_pat_token</code> for us.
1061 </td>
1062 </tr>
1063 </table>
1064 </div>
1065 </div>
1066 <div class="sect2">
1067 <h3 id="_changing_the_order">Changing the Order</h3>
1068 <div class="paragraph">
1069 <p>There are a few ways that we can change the order of the commits during a
1070 revision walk. Firstly, we can use the <code>enum</code> <code>rev_sort_order</code> to choose from some
1071 typical orderings.</p>
1072 </div>
1073 <div class="paragraph">
1074 <p><code>topo_order</code> is the same as <code>git</code> <code>log</code> <code>--topo-order</code>: we avoid showing a parent
1075 before all of its children have been shown, and we avoid mixing commits which
1076 are in different lines of history. (<code>git</code> <code>help</code> <code>log</code>'s section on <code>--topo-order</code>
1077 has a very nice diagram to illustrate this.)</p>
1078 </div>
1079 <div class="paragraph">
1080 <p>Let&#8217;s see what happens when we run with <code>REV_SORT_BY_COMMIT_DATE</code> as opposed to
1081 <code>REV_SORT_BY_AUTHOR_DATE</code>. Add the following:</p>
1082 </div>
1083 <div class="listingblock">
1084 <div class="content">
1085 <pre>static void final_rev_info_setup(int argc, const char **argv,
1086                 const char *prefix, struct rev_info *rev)
1087 {
1088         ...
1089
1090         rev-&gt;topo_order = 1;
1091         rev-&gt;sort_order = REV_SORT_BY_COMMIT_DATE;
1092
1093         ...
1094 }</pre>
1095 </div>
1096 </div>
1097 <div class="paragraph">
1098 <p>Let&#8217;s output this into a file so we can easily diff it with the walk sorted by
1099 author date.</p>
1100 </div>
1101 <div class="listingblock">
1102 <div class="content">
1103 <pre>$ make
1104 $ ./bin-wrappers/git walken &gt; commit-date.txt</pre>
1105 </div>
1106 </div>
1107 <div class="paragraph">
1108 <p>Then, let&#8217;s sort by author date and run it again.</p>
1109 </div>
1110 <div class="listingblock">
1111 <div class="content">
1112 <pre>static void final_rev_info_setup(int argc, const char **argv,
1113                 const char *prefix, struct rev_info *rev)
1114 {
1115         ...
1116
1117         rev-&gt;topo_order = 1;
1118         rev-&gt;sort_order = REV_SORT_BY_AUTHOR_DATE;
1119
1120         ...
1121 }</pre>
1122 </div>
1123 </div>
1124 <div class="listingblock">
1125 <div class="content">
1126 <pre>$ make
1127 $ ./bin-wrappers/git walken &gt; author-date.txt</pre>
1128 </div>
1129 </div>
1130 <div class="paragraph">
1131 <p>Finally, compare the two. This is a little less helpful without object names or
1132 dates, but hopefully we get the idea.</p>
1133 </div>
1134 <div class="listingblock">
1135 <div class="content">
1136 <pre>$ diff -u commit-date.txt author-date.txt</pre>
1137 </div>
1138 </div>
1139 <div class="paragraph">
1140 <p>This display indicates that commits can be reordered after they&#8217;re written, for
1141 example with <code>git</code> <code>rebase</code>.</p>
1142 </div>
1143 <div class="paragraph">
1144 <p>Let&#8217;s try one more reordering of commits. <code>rev_info</code> exposes a <code>reverse</code> flag.
1145 Set that flag somewhere inside of <code>final_rev_info_setup</code>():</p>
1146 </div>
1147 <div class="listingblock">
1148 <div class="content">
1149 <pre>static void final_rev_info_setup(int argc, const char **argv, const char *prefix,
1150                 struct rev_info *rev)
1151 {
1152         ...
1153
1154         rev-&gt;reverse = 1;
1155
1156         ...
1157 }</pre>
1158 </div>
1159 </div>
1160 <div class="paragraph">
1161 <p>Run your walk again and note the difference in order. (If you remove the grep
1162 pattern, you should see the last commit this call gives you as your current
1163 HEAD.)</p>
1164 </div>
1165 </div>
1166 </div>
1167 </div>
1168 <div class="sect1">
1169 <h2 id="_basic_object_walk">Basic Object Walk</h2>
1170 <div class="sectionbody">
1171 <div class="paragraph">
1172 <p>So far we&#8217;ve been walking only commits. But Git has more types of objects than
1173 that! Let&#8217;s see if we can walk <em>all</em> objects, and find out some information
1174 about each one.</p>
1175 </div>
1176 <div class="paragraph">
1177 <p>We can base our work on an example. <code>git</code> <code>pack-objects</code> prepares all kinds of
1178 objects for packing into a bitmap or packfile. The work we are interested in
1179 resides in <code>builtin/pack-objects.c:get_object_list</code>(); examination of that
1180 function shows that the all-object walk is being performed by
1181 <code>traverse_commit_list</code>() or <code>traverse_commit_list_filtered</code>(). Those two
1182 functions reside in <code>list-objects.c</code>; examining the source shows that, despite
1183 the name, these functions traverse all kinds of objects. Let&#8217;s have a look at
1184 the arguments to <code>traverse_commit_list</code>().</p>
1185 </div>
1186 <div class="ulist">
1187 <ul>
1188 <li>
1189 <p><code>struct</code> <code>rev_info</code> *revs: This is the <code>rev_info</code> used for the walk. If
1190 its <code>filter</code> member is not <code>NULL</code>, then <code>filter</code> contains information for
1191 how to filter the object list.</p>
1192 </li>
1193 <li>
1194 <p><code>show_commit_fn</code> <code>show_commit</code>: A callback which will be used to handle each
1195 individual commit object.</p>
1196 </li>
1197 <li>
1198 <p><code>show_object_fn</code> <code>show_object</code>: A callback which will be used to handle each
1199 non-commit object (so each blob, tree, or tag).</p>
1200 </li>
1201 <li>
1202 <p><code>void</code> *show_data: A context buffer which is passed in turn to <code>show_commit</code>
1203 and <code>show_object</code>.</p>
1204 </li>
1205 </ul>
1206 </div>
1207 <div class="paragraph">
1208 <p>In addition, <code>traverse_commit_list_filtered</code>() has an additional parameter:</p>
1209 </div>
1210 <div class="ulist">
1211 <ul>
1212 <li>
1213 <p><code>struct</code> <code>oidset</code> *omitted: A linked-list of object IDs which the provided
1214 filter caused to be omitted.</p>
1215 </li>
1216 </ul>
1217 </div>
1218 <div class="paragraph">
1219 <p>It looks like these methods use callbacks we provide instead of needing us
1220 to call it repeatedly ourselves. Cool! Let&#8217;s add the callbacks first.</p>
1221 </div>
1222 <div class="paragraph">
1223 <p>For the sake of this tutorial, we&#8217;ll simply keep track of how many of each kind
1224 of object we find. At file scope in <code>builtin/walken.c</code> add the following
1225 tracking variables:</p>
1226 </div>
1227 <div class="listingblock">
1228 <div class="content">
1229 <pre>static int commit_count;
1230 static int tag_count;
1231 static int blob_count;
1232 static int tree_count;</pre>
1233 </div>
1234 </div>
1235 <div class="paragraph">
1236 <p>Commits are handled by a different callback than other objects; let&#8217;s do that
1237 one first:</p>
1238 </div>
1239 <div class="listingblock">
1240 <div class="content">
1241 <pre>static void walken_show_commit(struct commit *cmt, void *buf)
1242 {
1243         commit_count++;
1244 }</pre>
1245 </div>
1246 </div>
1247 <div class="paragraph">
1248 <p>The <code>cmt</code> argument is fairly self-explanatory. But it&#8217;s worth mentioning that
1249 the <code>buf</code> argument is actually the context buffer that we can provide to the
1250 traversal calls - <code>show_data</code>, which we mentioned a moment ago.</p>
1251 </div>
1252 <div class="paragraph">
1253 <p>Since we have the <code>struct</code> <code>commit</code> object, we can look at all the same parts that
1254 we looked at in our earlier commit-only walk. For the sake of this tutorial,
1255 though, we&#8217;ll just increment the commit counter and move on.</p>
1256 </div>
1257 <div class="paragraph">
1258 <p>The callback for non-commits is a little different, as we&#8217;ll need to check
1259 which kind of object we&#8217;re dealing with:</p>
1260 </div>
1261 <div class="listingblock">
1262 <div class="content">
1263 <pre>static void walken_show_object(struct object *obj, const char *str, void *buf)
1264 {
1265         switch (obj-&gt;type) {
1266         case OBJ_TREE:
1267                 tree_count++;
1268                 break;
1269         case OBJ_BLOB:
1270                 blob_count++;
1271                 break;
1272         case OBJ_TAG:
1273                 tag_count++;
1274                 break;
1275         case OBJ_COMMIT:
1276                 BUG("unexpected commit object in walken_show_object\n");
1277         default:
1278                 BUG("unexpected object type %s in walken_show_object\n",
1279                         type_name(obj-&gt;type));
1280         }
1281 }</pre>
1282 </div>
1283 </div>
1284 <div class="paragraph">
1285 <p>Again, <code>obj</code> is fairly self-explanatory, and we can guess that <code>buf</code> is the same
1286 context pointer that <code>walken_show_commit</code>() receives: the <code>show_data</code> argument
1287 to <code>traverse_commit_list</code>() and <code>traverse_commit_list_filtered</code>(). Finally,
1288 <code>str</code> contains the name of the object, which ends up being something like
1289 <code>foo.txt</code> (blob), <code>bar/baz</code> (tree), or <code>v1.2.3</code> (tag).</p>
1290 </div>
1291 <div class="paragraph">
1292 <p>To help assure us that we aren&#8217;t double-counting commits, we&#8217;ll include some
1293 complaining if a commit object is routed through our non-commit callback; we&#8217;ll
1294 also complain if we see an invalid object type. Since those two cases should be
1295 unreachable, and would only change in the event of a semantic change to the Git
1296 codebase, we complain by using <code>BUG</code>() - which is a signal to a developer that
1297 the change they made caused unintended consequences, and the rest of the
1298 codebase needs to be updated to understand that change. <code>BUG</code>() is not intended
1299 to be seen by the public, so it is not localized.</p>
1300 </div>
1301 <div class="paragraph">
1302 <p>Our main object walk implementation is substantially different from our commit
1303 walk implementation, so let&#8217;s make a new function to perform the object walk. We
1304 can perform setup which is applicable to all objects here, too, to keep separate
1305 from setup which is applicable to commit-only walks.</p>
1306 </div>
1307 <div class="paragraph">
1308 <p>We&#8217;ll start by enabling all types of objects in the <code>struct</code> <code>rev_info</code>.  We&#8217;ll
1309 also turn on <code>tree_blobs_in_commit_order</code>, which means that we will walk a
1310 commit&#8217;s tree and everything it points to immediately after we find each commit,
1311 as opposed to waiting for the end and walking through all trees after the commit
1312 history has been discovered. With the appropriate settings configured, we are
1313 ready to call <code>prepare_revision_walk</code>().</p>
1314 </div>
1315 <div class="listingblock">
1316 <div class="content">
1317 <pre>static void walken_object_walk(struct rev_info *rev)
1318 {
1319         rev-&gt;tree_objects = 1;
1320         rev-&gt;blob_objects = 1;
1321         rev-&gt;tag_objects = 1;
1322         rev-&gt;tree_blobs_in_commit_order = 1;
1323
1324         if (prepare_revision_walk(rev))
1325                 die(_("revision walk setup failed"));
1326
1327         commit_count = 0;
1328         tag_count = 0;
1329         blob_count = 0;
1330         tree_count = 0;</pre>
1331 </div>
1332 </div>
1333 <div class="paragraph">
1334 <p>Let&#8217;s start by calling just the unfiltered walk and reporting our counts.
1335 Complete your implementation of <code>walken_object_walk</code>().
1336 We&#8217;ll also need to include the <code>list-objects.h</code> header.</p>
1337 </div>
1338 <div class="listingblock">
1339 <div class="content">
1340 <pre>#include "list-objects.h"
1341
1342 ...
1343
1344         traverse_commit_list(rev, walken_show_commit, walken_show_object, NULL);
1345
1346         printf("commits %d\nblobs %d\ntags %d\ntrees %d\n", commit_count,
1347                 blob_count, tag_count, tree_count);
1348 }</pre>
1349 </div>
1350 </div>
1351 <div class="admonitionblock note">
1352 <table>
1353 <tr>
1354 <td class="icon">
1355 <div class="title">Note</div>
1356 </td>
1357 <td class="content">
1358 This output is intended to be machine-parsed. Therefore, we are not
1359 sending it to <code>trace_printf</code>(), and we are not localizing it - we need scripts
1360 to be able to count on the formatting to be exactly the way it is shown here.
1361 If we were intending this output to be read by humans, we would need to localize
1362 it with <code>_</code>().
1363 </td>
1364 </tr>
1365 </table>
1366 </div>
1367 <div class="paragraph">
1368 <p>Finally, we&#8217;ll ask <code>cmd_walken</code>() to use the object walk instead. Discussing
1369 command line options is out of scope for this tutorial, so we&#8217;ll just hardcode
1370 a branch we can change at compile time. Where you call <code>final_rev_info_setup</code>()
1371 and <code>walken_commit_walk</code>(), instead branch like so:</p>
1372 </div>
1373 <div class="listingblock">
1374 <div class="content">
1375 <pre>        if (1) {
1376                 add_head_to_pending(&amp;rev);
1377                 walken_object_walk(&amp;rev);
1378         } else {
1379                 final_rev_info_setup(argc, argv, prefix, &amp;rev);
1380                 walken_commit_walk(&amp;rev);
1381         }</pre>
1382 </div>
1383 </div>
1384 <div class="admonitionblock note">
1385 <table>
1386 <tr>
1387 <td class="icon">
1388 <div class="title">Note</div>
1389 </td>
1390 <td class="content">
1391 For simplicity, we&#8217;ve avoided all the filters and sorts we applied in
1392 <code>final_rev_info_setup</code>() and simply added <code>HEAD</code> to our pending queue. If you
1393 want, you can certainly use the filters we added before by moving
1394 <code>final_rev_info_setup</code>() out of the conditional and removing the call to
1395 <code>add_head_to_pending</code>().
1396 </td>
1397 </tr>
1398 </table>
1399 </div>
1400 <div class="paragraph">
1401 <p>Now we can try to run our command! It should take noticeably longer than the
1402 commit walk, but an examination of the output will give you an idea why. Your
1403 output should look similar to this example, but with different counts:</p>
1404 </div>
1405 <div class="listingblock">
1406 <div class="content">
1407 <pre>Object walk completed. Found 55733 commits, 100274 blobs, 0 tags, and 104210 trees.</pre>
1408 </div>
1409 </div>
1410 <div class="paragraph">
1411 <p>This makes sense. We have more trees than commits because the Git project has
1412 lots of subdirectories which can change, plus at least one tree per commit. We
1413 have no tags because we started on a commit (<code>HEAD</code>) and while tags can point to
1414 commits, commits can&#8217;t point to tags.</p>
1415 </div>
1416 <div class="admonitionblock note">
1417 <table>
1418 <tr>
1419 <td class="icon">
1420 <div class="title">Note</div>
1421 </td>
1422 <td class="content">
1423 You will have different counts when you run this yourself! The number of
1424 objects grows along with the Git project.
1425 </td>
1426 </tr>
1427 </table>
1428 </div>
1429 <div class="sect2">
1430 <h3 id="_adding_a_filter_2">Adding a Filter</h3>
1431 <div class="paragraph">
1432 <p>There are a handful of filters that we can apply to the object walk laid out in
1433 <code>Documentation/rev-list-options.txt</code>. These filters are typically useful for
1434 operations such as creating packfiles or performing a partial clone. They are
1435 defined in <code>list-objects-filter-options.h</code>. For the purposes of this tutorial we
1436 will use the "tree:1" filter, which causes the walk to omit all trees and blobs
1437 which are not directly referenced by commits reachable from the commit in
1438 <code>pending</code> when the walk begins. (<code>pending</code> is the list of objects which need to
1439 be traversed during a walk; you can imagine a breadth-first tree traversal to
1440 help understand. In our case, that means we omit trees and blobs not directly
1441 referenced by <code>HEAD</code> or <code>HEAD</code>'s history, because we begin the walk with only
1442 <code>HEAD</code> in the <code>pending</code> list.)</p>
1443 </div>
1444 <div class="paragraph">
1445 <p>For now, we are not going to track the omitted objects, so we&#8217;ll replace those
1446 parameters with <code>NULL</code>. For the sake of simplicity, we&#8217;ll add a simple
1447 build-time branch to use our filter or not. Preface the line calling
1448 <code>traverse_commit_list</code>() with the following, which will remind us which kind of
1449 walk we&#8217;ve just performed:</p>
1450 </div>
1451 <div class="listingblock">
1452 <div class="content">
1453 <pre>        if (0) {
1454                 /* Unfiltered: */
1455                 trace_printf(_("Unfiltered object walk.\n"));
1456         } else {
1457                 trace_printf(
1458                         _("Filtered object walk with filterspec 'tree:1'.\n"));
1459
1460                 parse_list_objects_filter(&amp;rev-&gt;filter, "tree:1");
1461         }
1462         traverse_commit_list(rev, walken_show_commit,
1463                              walken_show_object, NULL);</pre>
1464 </div>
1465 </div>
1466 <div class="paragraph">
1467 <p>The <code>rev-</code>&gt;<code>filter</code> member is usually built directly from a command
1468 line argument, so the module provides an easy way to build one from a string.
1469 Even though we aren&#8217;t taking user input right now, we can still build one with
1470 a hardcoded string using <code>parse_list_objects_filter</code>().</p>
1471 </div>
1472 <div class="paragraph">
1473 <p>With the filter spec "tree:1", we are expecting to see <em>only</em> the root tree for
1474 each commit; therefore, the tree object count should be less than or equal to
1475 the number of commits. (For an example of why that&#8217;s true: <code>git</code> <code>commit</code> <code>--revert</code>
1476 points to the same tree object as its grandparent.)</p>
1477 </div>
1478 </div>
1479 <div class="sect2">
1480 <h3 id="_counting_omitted_objects">Counting Omitted Objects</h3>
1481 <div class="paragraph">
1482 <p>We also have the capability to enumerate all objects which were omitted by a
1483 filter, like with <code>git</code> <code>log</code> <code>--filter=</code><em>&lt;spec&gt;</em> <code>--filter-print-omitted</code>. To do this,
1484 change <code>traverse_commit_list</code>() to <code>traverse_commit_list_filtered</code>(), which is
1485 able to populate an <code>omitted</code> list.  Asking for this list of filtered objects
1486 may cause performance degradations, however, because in this case, despite
1487 filtering objects, the possibly much larger set of all reachable objects must
1488 be processed in order to populate that list.</p>
1489 </div>
1490 <div class="paragraph">
1491 <p>First, add the <code>struct</code> <code>oidset</code> and related items we will use to iterate it:</p>
1492 </div>
1493 <div class="listingblock">
1494 <div class="content">
1495 <pre>#include "oidset.h"
1496
1497 ...
1498
1499 static void walken_object_walk(
1500         ...
1501
1502         struct oidset omitted;
1503         struct oidset_iter oit;
1504         struct object_id *oid = NULL;
1505         int omitted_count = 0;
1506         oidset_init(&amp;omitted, 0);
1507
1508         ...</pre>
1509 </div>
1510 </div>
1511 <div class="paragraph">
1512 <p>Replace the call to <code>traverse_commit_list</code>() with
1513 <code>traverse_commit_list_filtered</code>() and pass a pointer to the <code>omitted</code> oidset
1514 defined and initialized above:</p>
1515 </div>
1516 <div class="listingblock">
1517 <div class="content">
1518 <pre>        ...
1519
1520                 traverse_commit_list_filtered(rev,
1521                         walken_show_commit, walken_show_object, NULL, &amp;omitted);
1522
1523         ...</pre>
1524 </div>
1525 </div>
1526 <div class="paragraph">
1527 <p>Then, after your traversal, the <code>oidset</code> traversal is pretty straightforward.
1528 Count all the objects within and modify the print statement:</p>
1529 </div>
1530 <div class="listingblock">
1531 <div class="content">
1532 <pre>        /* Count the omitted objects. */
1533         oidset_iter_init(&amp;omitted, &amp;oit);
1534
1535         while ((oid = oidset_iter_next(&amp;oit)))
1536                 omitted_count++;
1537
1538         printf("commits %d\nblobs %d\ntags %d\ntrees %d\nomitted %d\n",
1539                 commit_count, blob_count, tag_count, tree_count, omitted_count);</pre>
1540 </div>
1541 </div>
1542 <div class="paragraph">
1543 <p>By running your walk with and without the filter, you should find that the total
1544 object count in each case is identical. You can also time each invocation of
1545 the <code>walken</code> subcommand, with and without <code>omitted</code> being passed in, to confirm
1546 to yourself the runtime impact of tracking all omitted objects.</p>
1547 </div>
1548 </div>
1549 <div class="sect2">
1550 <h3 id="_changing_the_order_2">Changing the Order</h3>
1551 <div class="paragraph">
1552 <p>Finally, let&#8217;s demonstrate that you can also reorder walks of all objects, not
1553 just walks of commits. First, we&#8217;ll make our handlers chattier - modify
1554 <code>walken_show_commit</code>() and <code>walken_show_object</code>() to print the object as they
1555 go:</p>
1556 </div>
1557 <div class="listingblock">
1558 <div class="content">
1559 <pre>#include "hex.h"
1560
1561 ...
1562
1563 static void walken_show_commit(struct commit *cmt, void *buf)
1564 {
1565         trace_printf("commit: %s\n", oid_to_hex(&amp;cmt-&gt;object.oid));
1566         commit_count++;
1567 }
1568
1569 static void walken_show_object(struct object *obj, const char *str, void *buf)
1570 {
1571         trace_printf("%s: %s\n", type_name(obj-&gt;type), oid_to_hex(&amp;obj-&gt;oid));
1572
1573         ...
1574 }</pre>
1575 </div>
1576 </div>
1577 <div class="admonitionblock note">
1578 <table>
1579 <tr>
1580 <td class="icon">
1581 <div class="title">Note</div>
1582 </td>
1583 <td class="content">
1584 Since we will be examining this output directly as humans, we&#8217;ll use
1585 <code>trace_printf</code>() here. Additionally, since this change introduces a significant
1586 number of printed lines, using <code>trace_printf</code>() will allow us to easily silence
1587 those lines without having to recompile.
1588 </td>
1589 </tr>
1590 </table>
1591 </div>
1592 <div class="paragraph">
1593 <p>(Leave the counter increment logic in place.)</p>
1594 </div>
1595 <div class="paragraph">
1596 <p>With only that change, run again (but save yourself some scrollback):</p>
1597 </div>
1598 <div class="listingblock">
1599 <div class="content">
1600 <pre>$ GIT_TRACE=1 ./bin-wrappers/git walken 2&gt;&amp;1 | head -n 10</pre>
1601 </div>
1602 </div>
1603 <div class="paragraph">
1604 <p>Take a look at the top commit with <code>git</code> <code>show</code> and the object ID you printed; it
1605 should be the same as the output of <code>git</code> <code>show</code> <code>HEAD</code>.</p>
1606 </div>
1607 <div class="paragraph">
1608 <p>Next, let&#8217;s change a setting on our <code>struct</code> <code>rev_info</code> within
1609 <code>walken_object_walk</code>(). Find where you&#8217;re changing the other settings on <code>rev</code>,
1610 such as <code>rev-</code>&gt;<code>tree_objects</code> and <code>rev-</code>&gt;<code>tree_blobs_in_commit_order</code>, and add the
1611 <code>reverse</code> setting at the bottom:</p>
1612 </div>
1613 <div class="listingblock">
1614 <div class="content">
1615 <pre>        ...
1616
1617         rev-&gt;tree_objects = 1;
1618         rev-&gt;blob_objects = 1;
1619         rev-&gt;tag_objects = 1;
1620         rev-&gt;tree_blobs_in_commit_order = 1;
1621         rev-&gt;reverse = 1;
1622
1623         ...</pre>
1624 </div>
1625 </div>
1626 <div class="paragraph">
1627 <p>Now, run again, but this time, let&#8217;s grab the last handful of objects instead
1628 of the first handful:</p>
1629 </div>
1630 <div class="listingblock">
1631 <div class="content">
1632 <pre>$ make
1633 $ GIT_TRACE=1 ./bin-wrappers/git walken 2&gt;&amp;1 | tail -n 10</pre>
1634 </div>
1635 </div>
1636 <div class="paragraph">
1637 <p>The last commit object given should have the same OID as the one we saw at the
1638 top before, and running <code>git</code> <code>show</code> <em>&lt;oid&gt;</em> with that OID should give you again
1639 the same results as <code>git</code> <code>show</code> <code>HEAD</code>. Furthermore, if you run and examine the
1640 first ten lines again (with <code>head</code> instead of <code>tail</code> like we did before applying
1641 the <code>reverse</code> setting), you should see that now the first commit printed is the
1642 initial commit, <code>e83c5163</code>.</p>
1643 </div>
1644 </div>
1645 </div>
1646 </div>
1647 <div class="sect1">
1648 <h2 id="_wrapping_up">Wrapping Up</h2>
1649 <div class="sectionbody">
1650 <div class="paragraph">
1651 <p>Let&#8217;s review. In this tutorial, we:</p>
1652 </div>
1653 <div class="ulist">
1654 <ul>
1655 <li>
1656 <p>Built a commit walk from the ground up</p>
1657 </li>
1658 <li>
1659 <p>Enabled a grep filter for that commit walk</p>
1660 </li>
1661 <li>
1662 <p>Changed the sort order of that filtered commit walk</p>
1663 </li>
1664 <li>
1665 <p>Built an object walk (tags, commits, trees, and blobs) from the ground up</p>
1666 </li>
1667 <li>
1668 <p>Learned how to add a filter-spec to an object walk</p>
1669 </li>
1670 <li>
1671 <p>Changed the display order of the filtered object walk</p>
1672 </li>
1673 </ul>
1674 </div>
1675 </div>
1676 </div>
1677 </div>
1678 <div id="footer">
1679 <div id="footer-text">
1680 Last updated 2024-04-09 14:45:01 -0700
1681 </div>
1682 </div>
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