How to parse git diff of specific file using Ruby?

Question

I have an Android XML (string) file that is edited in a Ruby script. I would like to list and output the changes that were made then. I have tried it with Nokogiri and nokogiri/diff . But it does not have the desired result.

I also have the feeling that it has problems when a new line is added in the middle of it. All in all, I think it would be easiest if I could use git diff .

I've also found ruby-git gem, but I still could not get it to work. Especially because I only need the diff of a specific file.

require 'git'

Git.configure do |config|
 #not sure if I actually need something?
end

g = Git.open(path_to_my_dir, :log => Logger.new(STDOUT))    
g.diff(path_to_file)
#or
g.diff().path(path_to_file)

Can Someone please help me out? :-(

Answer 1

You thought in the right direction.

For this purpose use

require 'git'
g = Git.open('path/to/dir')
g.diff('your.file').patch #=> changes in your.file

For example we had empty files git.rb and smth in our git-repo.

Then we changed them and checked difference:

$ git diff
diff --git a/git.rb b/git.rb
index e69de29..3ff224d 100644
--- a/git.rb
+++ b/git.rb
@@ -0,0 +1,3 @@
+require 'git'
+g = Git.open(__dir__)
+puts g.diff('smth').patch
diff --git a/smth b/smth
index e69de29..7c5bd35 100644
--- a/smth
+++ b/smth
@@ -0,0 +1 @@
+we want to know changes

As already guessed from modified git.rb , now we will see changes only in smth :

$ ruby git.rb
diff --git a/smth b/smth
index e69de29..7c5bd35 100644
--- a/smth
+++ b/smth
@@ -0,0 +1 @@
+we want to know changes

In case there are no changes, you will get empty string "" .

Answer 2

You might want to use the McIlroy-Hunt longest common subsequence (LCS) algorithm directly instead of using derivates/wrappers of it.

See https://github.com/halostatue/diff-lcs

The diff will change if you compare changes of a vs. b as opposed to changes of b vs. a, but you can run it against an array or against a whole file of course.

The gem also has the classic diff tool formatting (used by diff or git) if you prefer that instead of using its direct output.