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<body class="article">
<div id="header">
<h1>Trivial merge rules</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>This document describes the outcomes of the trivial merge logic in read-tree.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_one_way_merge">One-way merge</h2>
<div class="sectionbody">
<div class="paragraph"><p>This replaces the index with a different tree, keeping the stat info
for entries that don&#8217;t change, and allowing -u to make the minimum
required changes to the working tree to have it match.</p></div>
<div class="paragraph"><p>Entries marked <em>+</em> have stat information. Spaces marked <em>*</em> don&#8217;t
affect the result.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>index   tree    result
-----------------------
*       (empty) (empty)
(empty) tree    tree
index+  tree    tree
index+  index   index+</code></pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_two_way_merge">Two-way merge</h2>
<div class="sectionbody">
<div class="paragraph"><p>It is permitted for the index to lack an entry; this does not prevent
any case from applying.</p></div>
<div class="paragraph"><p>If the index exists, it is an error for it not to match either the old
or the result.</p></div>
<div class="paragraph"><p>If multiple cases apply, the one used is listed first.</p></div>
<div class="paragraph"><p>A result which changes the index is an error if the index is not empty
and not up-to-date.</p></div>
<div class="paragraph"><p>Entries marked <em>+</em> have stat information. Spaces marked <em>*</em> don&#8217;t
affect the result.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>case  index   old     new     result
-------------------------------------
0/2   (empty) *       (empty) (empty)
1/3   (empty) *       new     new
4/5   index+  (empty) (empty) index+
6/7   index+  (empty) index   index+
10    index+  index   (empty) (empty)
14/15 index+  old     old     index+
18/19 index+  old     index   index+
20    index+  index   new     new</code></pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_three_way_merge">Three-way merge</h2>
<div class="sectionbody">
<div class="paragraph"><p>It is permitted for the index to lack an entry; this does not prevent
any case from applying.</p></div>
<div class="paragraph"><p>If the index exists, it is an error for it not to match either the
head or (if the merge is trivial) the result.</p></div>
<div class="paragraph"><p>If multiple cases apply, the one used is listed first.</p></div>
<div class="paragraph"><p>A result of "no merge" means that index is left in stage 0, ancest in
stage 1, head in stage 2, and remote in stage 3 (if any of these are
empty, no entry is left for that stage). Otherwise, the given entry is
left in stage 0, and there are no other entries.</p></div>
<div class="paragraph"><p>A result of "no merge" is an error if the index is not empty and not
up-to-date.</p></div>
<div class="paragraph"><p><strong>empty</strong> means that the tree must not have a directory-file conflict
 with the entry.</p></div>
<div class="paragraph"><p>For multiple ancestors, a <em>+</em> means that this case applies even if
only one ancestor or remote fits; a <em>^</em> means all of the ancestors
must be the same.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>case  ancest    head    remote    result
----------------------------------------
1     (empty)+  (empty) (empty)   (empty)
2ALT  (empty)+  *empty* remote    remote
2     (empty)^  (empty) remote    no merge
3ALT  (empty)+  head    *empty*   head
3     (empty)^  head    (empty)   no merge
4     (empty)^  head    remote    no merge
5ALT  *         head    head      head
6     ancest+   (empty) (empty)   no merge
8     ancest^   (empty) ancest    no merge
7     ancest+   (empty) remote    no merge
10    ancest^   ancest  (empty)   no merge
9     ancest+   head    (empty)   no merge
16    anc1/anc2 anc1    anc2      no merge
13    ancest+   head    ancest    head
14    ancest+   ancest  remote    remote
11    ancest+   head    remote    no merge</code></pre>
</div></div>
<div class="paragraph"><p>Only #2ALT and #3ALT use <strong>empty</strong>, because these are the only cases
where there can be conflicts that didn&#8217;t exist before. Note that we
allow directory-file conflicts between things in different stages
after the trivial merge.</p></div>
<div class="paragraph"><p>A possible alternative for #6 is (empty), which would make it like
#1. This is not used, due to the likelihood that it arises due to
moving the file to multiple different locations or moving and deleting
it in different branches.</p></div>
<div class="paragraph"><p>Case #1 is included for completeness, and also in case we decide to
put on <em>+</em> markings; any path that is never mentioned at all isn&#8217;t
handled.</p></div>
<div class="paragraph"><p>Note that #16 is when both #13 and #14 apply; in this case, we refuse
the trivial merge, because we can&#8217;t tell from this data which is
right. This is a case of a reverted patch (in some direction, maybe
multiple times), and the right answer depends on looking at crossings
of history or common ancestors of the ancestors.</p></div>
<div class="paragraph"><p>Note that, between #6, #7, #9, and #11, all cases not otherwise
covered are handled in this table.</p></div>
<div class="paragraph"><p>For #8 and #10, there is alternative behavior, not currently
implemented, where the result is (empty). As currently implemented,
the automatic merge will generally give this effect.</p></div>
</div>
</div>
</div>
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Last updated 2013-06-10 20:01:55 UTC
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