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735 <body class="article">
736 <div id="header">
737 <h1>How to separate topic branches</h1>
738 <span id="revdate">2024-01-12</span>
739 </div>
740 <div id="content">
741 <div id="preamble">
742 <div class="sectionbody">
743 <div class="paragraph"><p>This text was originally a footnote to a discussion about the
744 behaviour of the git diff commands.</p></div>
745 <div class="paragraph"><p>Often I find myself doing that [running diff against something other
746 than HEAD] while rewriting messy development history. For example, I
747 start doing some work without knowing exactly where it leads, and end
748 up with a history like this:</p></div>
749 <div class="literalblock">
750 <div class="content">
751 <pre><code> "master"
752 o---o
753 \ "topic"
754 o---o---o---o---o---o</code></pre>
755 </div></div>
756 <div class="paragraph"><p>At this point, "topic" contains something I know I want, but it
757 contains two concepts that turned out to be completely independent.
758 And often, one topic component is larger than the other. It may
759 contain more than two topics.</p></div>
760 <div class="paragraph"><p>In order to rewrite this mess to be more manageable, I would first do
761 "diff master..topic", to extract the changes into a single patch, start
762 picking pieces from it to get logically self-contained units, and
763 start building on top of "master":</p></div>
764 <div class="literalblock">
765 <div class="content">
766 <pre><code>$ git diff master..topic &gt;P.diff
767 $ git checkout -b topicA master
768 ... pick and apply pieces from P.diff to build
769 ... commits on topicA branch.</code></pre>
770 </div></div>
771 <div class="literalblock">
772 <div class="content">
773 <pre><code> o---o---o
774 / "topicA"
775 o---o"master"
776 \ "topic"
777 o---o---o---o---o---o</code></pre>
778 </div></div>
779 <div class="paragraph"><p>Before doing each commit on "topicA" HEAD, I run "diff HEAD"
780 before update-index the affected paths, or "diff --cached HEAD"
781 after. Also I would run "diff --cached master" to make sure
782 that the changes are only the ones related to "topicA". Usually
783 I do this for smaller topics first.</p></div>
784 <div class="paragraph"><p>After that, I&#8217;d do the remainder of the original "topic", but
785 for that, I do not start from the patchfile I extracted by
786 comparing "master" and "topic" I used initially. Still on
787 "topicA", I extract "diff topic", and use it to rebuild the
788 other topic:</p></div>
789 <div class="literalblock">
790 <div class="content">
791 <pre><code>$ git diff -R topic &gt;P.diff ;# --cached also would work fine
792 $ git checkout -b topicB master
793 ... pick and apply pieces from P.diff to build
794 ... commits on topicB branch.</code></pre>
795 </div></div>
796 <div class="literalblock">
797 <div class="content">
798 <pre><code> "topicB"
799 o---o---o---o---o
801 /o---o---o
802 |/ "topicA"
803 o---o"master"
804 \ "topic"
805 o---o---o---o---o---o</code></pre>
806 </div></div>
807 <div class="paragraph"><p>After I am done, I&#8217;d try a pretend-merge between "topicA" and
808 "topicB" in order to make sure I have not missed anything:</p></div>
809 <div class="literalblock">
810 <div class="content">
811 <pre><code>$ git pull . topicA ;# merge it into current "topicB"
812 $ git diff topic
813 "topicB"
814 o---o---o---o---o---* (pretend merge)
816 /o---o---o----------'
817 |/ "topicA"
818 o---o"master"
819 \ "topic"
820 o---o---o---o---o---o</code></pre>
821 </div></div>
822 <div class="paragraph"><p>The last diff better not to show anything other than cleanups
823 for cruft. Then I can finally clean things up:</p></div>
824 <div class="literalblock">
825 <div class="content">
826 <pre><code>$ git branch -D topic
827 $ git reset --hard HEAD^ ;# nuke pretend merge</code></pre>
828 </div></div>
829 <div class="literalblock">
830 <div class="content">
831 <pre><code> "topicB"
832 o---o---o---o---o
834 /o---o---o
835 |/ "topicA"
836 o---o"master"</code></pre>
837 </div></div>
838 </div>
839 </div>
840 </div>
841 <div id="footnotes"><hr /></div>
842 <div id="footer">
843 <div id="footer-text">
844 Last updated
845 2024-01-12 16:27:45 PST
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