# [BUG] `git diff` treats an unchanged line as modified 3 messages from 2019-12-24 to 2019-12-25. Participants: Hao Lee, Junio C Hamano. Thread: https://gitlist.dev/t/52516 ## Hao Lee, 2019-12-24 17:50 Subject: [BUG] `git diff` treats an unchanged line as modified Message-ID: URL: https://gitlist.dev/e/CA%2BPpKPm0_zXr978Mw8h7owHBChx-uqVCdjrtuh45uOS-A_0qrA%40mail.gmail.com ``` Here are two files `old` and `new`. The content of `old` file: ˋˋˋ kasan_poison_slab(page);// shuffle = shuffle_freelist(s, page);// if (!shuffle) { for_each_object_idx(p, idx, s, start, page->objects) { setup_object(s, page, p); if (likely(idx < page->objects)) ˋˋˋ The content of `new` file: ˋˋˋ kasan_poison_slab(page); shuffle = shuffle_freelist(s, page); ˋˋˋ When using git to compare these two files, the result is: ˋˋˋ diff --git a/old b/new index 474ab07..d432a67 100644 --- a/old +++ b/new @@ -1,11 +1,6 @@ - kasan_poison_slab(page);// - - shuffle = shuffle_freelist(s, page);// - - if (!shuffle) { - for_each_object_idx(p, idx, s, start, page->objects) { - setup_object(s, page, p); - if (likely(idx < page->objects)) + kasan_poison_slab(page); + + shuffle = shuffle_freelist(s, page); ˋˋˋ However, the Linux diff command gives the following result: ˋˋˋ --- old 2019-12-25 00:56:40.000000000 +0800 +++ new 2019-12-25 01:12:48.000000000 +0800 @@ -1,11 +1,6 @@ - kasan_poison_slab(page);// + kasan_poison_slab(page); - shuffle = shuffle_freelist(s, page);// - - if (!shuffle) { - for_each_object_idx(p, idx, s, start, page->objects) { - setup_object(s, page, p); - if (likely(idx < page->objects)) + shuffle = shuffle_freelist(s, page); ˋˋˋ I think the latter is correct because the fifth line is not changed and it's still a line which only contains a whitespace. Steps to Reproduce: curl -L https://git.io/JeFNy | base64 -d > file.tgz tar -xf file.tgz git diff --no-index old new ``` ## Junio C Hamano, 2019-12-24 19:34 Subject: Re: [BUG] `git diff` treats an unchanged line as modified Message-ID: URL: https://gitlist.dev/e/xmqqy2v14iwc.fsf%40gitster-ct.c.googlers.com In-Reply-To: ``` Hao Lee writes: > I think the latter is correct because the fifth line is not changed > and it's still a line which only contains a whitespace. As long as lines that begin with "-" and " " match what the 'old' file has, and replacing them with lines that begin with " " and "+" in the 'old' file yields the 'new' file, any patch is "correct". IOW, there is no *single* diff output that is correct. Among many "correct" diff output, there are ones that readers find them easier to understand and those that look suboptimal. The differences are often personal tastes. "git diff" has options to tweak heuristics it uses to choose among many "correct" diff outputs, like --patience, --histogram, etc. Perhaps using one of them would produce one that match your taste better? ``` ## Hao Lee, 2019-12-25 15:26 Subject: Re: [BUG] `git diff` treats an unchanged line as modified Message-ID: URL: https://gitlist.dev/e/CA%2BPpKPnCrSciiNinC9g6BVX7DHXPDq8k2zegK%2B-9Yg2CPzF_bQ%40mail.gmail.com In-Reply-To: ``` On Wed, 25 Dec 2019 at 03:34, Junio C Hamano wrote: > IOW, there is no *single* diff output that is correct. > > Among many "correct" diff output, there are ones that readers find > them easier to understand and those that look suboptimal. The > differences are often personal tastes. Thanks, this explanation is clear for me. > "git diff" has options to tweak heuristics it uses to choose among > many "correct" diff outputs, like --patience, --histogram, etc. > Perhaps using one of them would produce one that match your taste > better? > After understanding the correctness of diff algorithm, the default algorithm also suits my needs now, although the patience algorithm is more understandable in some special circumstances. Regards, Hao Lee ```