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 735 <body class="article">
 736 <div id="header">
 737 <h1>Trivial merge rules</h1>
 738 <span id="revdate">2024-04-19</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="paragraph"><p>This document describes the outcomes of the trivial merge logic in read-tree.</p></div>
 744 </div>
 745 </div>
 746 <div class="sect1">
 747 <h2 id="_one_way_merge">One-way merge</h2>
 748 <div class="sectionbody">
 749 <div class="paragraph"><p>This replaces the index with a different tree, keeping the stat info
 750 for entries that don&#8217;t change, and allowing -u to make the minimum
 751 required changes to the working tree to have it match.</p></div>
 752 <div class="paragraph"><p>Entries marked <em>+</em> have stat information. Spaces marked <em>*</em> don&#8217;t
 753 affect the result.</p></div>
 754 <div class="literalblock">
 755 <div class="content">
 756 <pre><code>index   tree    result
 757 -----------------------
 758 *       (empty) (empty)
 759 (empty) tree    tree
 760 index+  tree    tree
 761 index+  index   index+</code></pre>
 762 </div></div>
 763 </div>
 764 </div>
 765 <div class="sect1">
 766 <h2 id="_two_way_merge">Two-way merge</h2>
 767 <div class="sectionbody">
 768 <div class="paragraph"><p>It is permitted for the index to lack an entry; this does not prevent
 769 any case from applying.</p></div>
 770 <div class="paragraph"><p>If the index exists, it is an error for it not to match either the old
 771 or the result.</p></div>
 772 <div class="paragraph"><p>If multiple cases apply, the one used is listed first.</p></div>
 773 <div class="paragraph"><p>A result which changes the index is an error if the index is not empty
 774 and not up to date.</p></div>
 775 <div class="paragraph"><p>Entries marked <em>+</em> have stat information. Spaces marked <em>*</em> don&#8217;t
 776 affect the result.</p></div>
 777 <div class="literalblock">
 778 <div class="content">
 779 <pre><code>case  index   old     new     result
 780 -------------------------------------
 781 0/2   (empty) *       (empty) (empty)
 782 1/3   (empty) *       new     new
 783 4/5   index+  (empty) (empty) index+
 784 6/7   index+  (empty) index   index+
 785 10    index+  index   (empty) (empty)
 786 14/15 index+  old     old     index+
 787 18/19 index+  old     index   index+
 788 20    index+  index   new     new</code></pre>
 789 </div></div>
 790 </div>
 791 </div>
 792 <div class="sect1">
 793 <h2 id="_three_way_merge">Three-way merge</h2>
 794 <div class="sectionbody">
 795 <div class="paragraph"><p>It is permitted for the index to lack an entry; this does not prevent
 796 any case from applying.</p></div>
 797 <div class="paragraph"><p>If the index exists, it is an error for it not to match either the
 798 head or (if the merge is trivial) the result.</p></div>
 799 <div class="paragraph"><p>If multiple cases apply, the one used is listed first.</p></div>
 800 <div class="paragraph"><p>A result of "no merge" means that index is left in stage 0, ancest in
 801 stage 1, head in stage 2, and remote in stage 3 (if any of these are
 802 empty, no entry is left for that stage). Otherwise, the given entry is
 803 left in stage 0, and there are no other entries.</p></div>
 804 <div class="paragraph"><p>A result of "no merge" is an error if the index is not empty and not
 805 up to date.</p></div>
 806 <div class="paragraph"><p><strong>empty</strong> means that the tree must not have a directory-file conflict
 807  with the entry.</p></div>
 808 <div class="paragraph"><p>For multiple ancestors, a <em>+</em> means that this case applies even if
 809 only one ancestor or remote fits; a <em>^</em> means all of the ancestors
 810 must be the same.</p></div>
 811 <div class="literalblock">
 812 <div class="content">
 813 <pre><code>case  ancest    head    remote    result
 814 ----------------------------------------
 815 1     (empty)+  (empty) (empty)   (empty)
 816 2ALT  (empty)+  *empty* remote    remote
 817 2     (empty)^  (empty) remote    no merge
 818 3ALT  (empty)+  head    *empty*   head
 819 3     (empty)^  head    (empty)   no merge
 820 4     (empty)^  head    remote    no merge
 821 5ALT  *         head    head      head
 822 6     ancest+   (empty) (empty)   no merge
 823 8     ancest^   (empty) ancest    no merge
 824 7     ancest+   (empty) remote    no merge
 825 10    ancest^   ancest  (empty)   no merge
 826 9     ancest+   head    (empty)   no merge
 827 16    anc1/anc2 anc1    anc2      no merge
 828 13    ancest+   head    ancest    head
 829 14    ancest+   ancest  remote    remote
 830 11    ancest+   head    remote    no merge</code></pre>
 831 </div></div>
 832 <div class="paragraph"><p>Only #2ALT and #3ALT use <strong>empty</strong>, because these are the only cases
 833 where there can be conflicts that didn&#8217;t exist before. Note that we
 834 allow directory-file conflicts between things in different stages
 835 after the trivial merge.</p></div>
 836 <div class="paragraph"><p>A possible alternative for #6 is (empty), which would make it like
 837 #1. This is not used, due to the likelihood that it arises due to
 838 moving the file to multiple different locations or moving and deleting
 839 it in different branches.</p></div>
 840 <div class="paragraph"><p>Case #1 is included for completeness, and also in case we decide to
 841 put on <em>+</em> markings; any path that is never mentioned at all isn&#8217;t
 842 handled.</p></div>
 843 <div class="paragraph"><p>Note that #16 is when both #13 and #14 apply; in this case, we refuse
 844 the trivial merge, because we can&#8217;t tell from this data which is
 845 right. This is a case of a reverted patch (in some direction, maybe
 846 multiple times), and the right answer depends on looking at crossings
 847 of history or common ancestors of the ancestors.</p></div>
 848 <div class="paragraph"><p>Note that, between #6, #7, #9, and #11, all cases not otherwise
 849 covered are handled in this table.</p></div>
 850 <div class="paragraph"><p>For #8 and #10, there is alternative behavior, not currently
 851 implemented, where the result is (empty). As currently implemented,
 852 the automatic merge will generally give this effect.</p></div>
 853 </div>
 854 </div>
 855 </div>
 856 <div id="footnotes"><hr /></div>
 857 <div id="footer">
 858 <div id="footer-text">
 859 Last updated
 860  2020-03-10 15:02:33 PDT
 861 </div>
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