# git merge heuristic 2 messages from 2013-03-22 to 2013-03-22. Participants: Senthil Natarajan, Jeff King. Thread: https://gitlist.dev/t/33260 ## Senthil Natarajan, 2013-03-22 18:53 Subject: git merge heuristic Message-ID: URL: https://gitlist.dev/e/ECEA5ACFB4B2BC4A81A93F032519ABCAD47F79%40minsk.us.stoke.com ``` Hi all, I want to learn about how Git compares patches while doing a merge.  For example, if a patch has been cherry-picked from branch A to branch B, and then downstream we do a "git merge" from A to B, how does Git know to skip the cherry-picked patch?  It would have a different SHA-1, so what is the comparison algorithm/heuristic?  What happens if the comment is different, but the actual patch is identical? I searched around, but couldn't find information on this.  I would appreciate it if someone could point me in the right direction. Thanks! -Senthil ``` ## Jeff King, 2013-03-22 22:33 Subject: Re: git merge heuristic Message-ID: <20130322223330.GB13207@sigill.intra.peff.net> URL: https://gitlist.dev/e/20130322223330.GB13207%40sigill.intra.peff.net In-Reply-To: ``` On Fri, Mar 22, 2013 at 11:53:04AM -0700, Senthil Natarajan wrote: > I want to learn about how Git compares patches while doing a merge.  > For example, if a patch has been cherry-picked from branch A to branch > B, and then downstream we do a "git merge" from A to B, how does Git > know to skip the cherry-picked patch? It doesn't. Git's 3-way merge only looks at three things: where each side of the merge ended, and what their common ancestor looked like. So when you cherry-pick a commit, as long as the content in the file ended up the same, there is no conflict. And it doesn't matter if it happened by cherry-picking, or if you just happened to make a sequence of commits that ended in the same state. However, we do perform such detection during a rebase, in which we try to skip patches that have already been applied upstream. > It would have a different SHA-1, so what is the comparison > algorithm/heuristic?  What happens if the comment is different, but the > actual patch is identical? Yes, the commit will have a different sha1. For that, we use the "patch-id", which is basically a sha of the contents of the diff of the commit against its parent. See the manual for git-patch-id, git-cherry, and the --cherry-* options of "git log". It will not find all duplicate commits (e.g., it will miss ones where there was a conflict during cherry-pick, or even where the context is slightly different). However, in many cases, rebase can also realize while applying a patch that it has already been applied, and skip it then. -Peff ```