# Can I convince the diff algorithm to behave better? 4 messages from 2021-03-03 to 2021-03-04. Participants: Tom Ritter, Thomas Braun, Jonathan Tan, Christian Couder. Thread: https://gitlist.dev/t/55244 ## Tom Ritter, 2021-03-03 02:03 Subject: Can I convince the diff algorithm to behave better? Message-ID: URL: https://gitlist.dev/e/CA%2BcU71%3DFfReSG411Feo%3Dvmkw4MdK4KDgokP1jH6uwOkC_0AbYA%40mail.gmail.com ``` (For a specific, nuanced, and personal definition of better...) I have a frequent behavior that arises when I am copy/pasting chunks of code, typically in tests. Here is an example: My Original code: def function(): line 1 line 2 line 3 line 4 line 5 line 6 -------------------------------- I add, after it: def function2(): line 1 line 2 line 3 line 4 line 5 line 6 -------------------------------- My diff is: + line 3 + line 4 + line 5 + line 6 + +def function2(): + line 1 + line 2 -------------------------------- I'd like my diff to be + +def function2(): + line 1 + line 2 + line 3 + line 4 + line 5 + line 6 Obviously there's nothing incorrect about the former diff, I just wish it was the latter rather than the former. I know that git includes four diff algorithms; in my testing patience or histogram exacerbated the problem; and none of them improved upon it. If anyone has suggestions I'd be curious to know if there's anything that could be done... Thanks, -tom ``` ## Thomas Braun, 2021-03-03 12:41 Subject: Re: Can I convince the diff algorithm to behave better? Message-ID: <10c330f1-b3ae-38ab-1a8b-23c0b46f1557@virtuell-zuhause.de> URL: https://gitlist.dev/e/10c330f1-b3ae-38ab-1a8b-23c0b46f1557%40virtuell-zuhause.de In-Reply-To: ``` On 3/3/2021 3:03 AM, Tom Ritter wrote: Hi Tom, > (For a specific, nuanced, and personal definition of better...) > > I have a frequent behavior that arises when I am copy/pasting chunks > of code, typically in tests. Here is an example: > > My Original code: > > def function(): > line 1 > line 2 > line 3 > line 4 > line 5 > line 6 > > -------------------------------- > I add, after it: > > def function2(): > line 1 > line 2 > line 3 > line 4 > line 5 > line 6 > > -------------------------------- > My diff is: > > + line 3 > + line 4 > + line 5 > + line 6 > + > +def function2(): > + line 1 > + line 2 > > -------------------------------- > I'd like my diff to be > > + > +def function2(): > + line 1 > + line 2 > + line 3 > + line 4 > + line 5 > + line 6 I tried to reproduce and got exactly the diff you wanted to have. I need to add a newline after the first "line 4" to get the not-sought-for diff. Commit: +++ b/test.py @@ -0,0 +1,7 @@ +def function(): + line 1 + line 2 + line 3 + line 4 + line 5 + line 6 and then the following change: --- a/test.py +++ b/test.py @@ -3,5 +3,14 @@ def function(): line 2 line 3 line 4 + + line 5 + line 6 + +def function2(): + line 1 + line 2 + line 3 + line 4 line 5 line 6 I usually play around with --anchored when I want to solve an issue like that. The documentation of anchored says If a line exists in both the source and destination, exists only once, and starts with this text, this algorithm attempts to prevent it from appearing as a deletion or addition in the output. It uses the "patience diff" algorithm internally. But I can't get it working here as the "exists only once" premise is broken. Stepping back: It might also make sense to rethink the code as repeating the same 6 lines in every function might not be the best possible design. Thomas [...] ``` ## Jonathan Tan, 2021-03-03 23:45 Subject: Re: Can I convince the diff algorithm to behave better? Message-ID: <20210303234530.3122368-1-jonathantanmy@google.com> URL: https://gitlist.dev/e/20210303234530.3122368-1-jonathantanmy%40google.com In-Reply-To: ``` > I know that git includes four diff algorithms; in my testing patience > or histogram exacerbated the problem; and none of them improved upon > it. If anyone has suggestions I'd be curious to know if there's > anything that could be done... In your particular case, I can't think of anything, but in general, if one of the lines weren't repeated, you might be able to use the --anchored option. ``` ## Christian Couder, 2021-03-04 09:52 Subject: Re: Can I convince the diff algorithm to behave better? Message-ID: URL: https://gitlist.dev/e/CAP8UFD1gFA2DyyjyfJ7pKNRyqO2%3DY2BOyF0Aeni3GXvgeL8Wtg%40mail.gmail.com In-Reply-To: ``` On Thu, Mar 4, 2021 at 8:37 AM Tom Ritter wrote: [...] > Obviously there's nothing incorrect about the former diff, I just wish > it was the latter rather than the former. > > I know that git includes four diff algorithms; in my testing patience > or histogram exacerbated the problem; and none of them improved upon > it. If anyone has suggestions I'd be curious to know if there's > anything that could be done... It's not so easy to implement good diff algorithms. You might want to take a look at the "v2.11 new diff heuristic?" article in: https://git.github.io/rev_news/2016/12/14/edition-22/ Best, Christian. ```