How to reset selected lines?

> To put things into context of why I am asking this, git-gui has a 
> feature where you can select parts of a displayed diff, and can 
> stage/unstage those parts. That feature is implemented in git-gui by 
> just generating a diff from the selected lines, and then applying it. 
> Check git-gui/lib/diff.tcl:643 for the implementation.
> 
> Now, I want to add a similar feature, but one that discards/resets the 
> selected lines. And I'd like to avoid the hack that git-gui's 
> apply_range_or_line is. So, is there a cleaner way to do this that does 
> not involve generating a diff and then applying it?

> Git's index and trees only understand whole files, so at some point you
> must generate the final file content. A diff is an easy way to represent
> the changes, apply them to the existing state, and then get that final
> content. But it doesn't _have_ to be. You could make some modifications
> to what is in the working tree and then say "OK, now stage this.".
>
> BUT. That is probably not what the user wants, if the content in the
> index actually has some modifications that are not in the working tree
> (i.e., you wouldn't want to overwrite them). Hence we tend to work with
> diffs, saying "make these changes to what is already in the index, and
> if they conflict, then bail".
>
> So "git add -p", for example, also works by creating diffs, modifying
> them, and feeding the result to "apply". You can see the implementation
> in git-add--interactive.perl, where it literally calls diff and apply
> commands.
>
> And that leads us to the answer to the first question. That script
> implements "add -p", but also "checkout -p" (which is what you want),
> "reset -p", "stash -p", etc. They differ only in what we diff and how we
> apply the result; the main engine of slicing and dicing the diff through
> user interaction is the same. See the %patch_modes hash for the list.

> On Tue, Aug 13, 2019 at 07:48:16PM +0530, Pratyush Yadav wrote:
> 
> > To put things into context of why I am asking this, git-gui has a 
> > feature where you can select parts of a displayed diff, and can 
> > stage/unstage those parts. That feature is implemented in git-gui by 
> > just generating a diff from the selected lines, and then applying it. 
> > Check git-gui/lib/diff.tcl:643 for the implementation.
> > 
> > Now, I want to add a similar feature, but one that discards/resets the 
> > selected lines. And I'd like to avoid the hack that git-gui's 
> > apply_range_or_line is. So, is there a cleaner way to do this that does 
> > not involve generating a diff and then applying it?
> 
> To answer your second question first:
> 
> Git's index and trees only understand whole files, so at some point you
> must generate the final file content. A diff is an easy way to represent
> the changes, apply them to the existing state, and then get that final
> content. But it doesn't _have_ to be. You could make some modifications
> to what is in the working tree and then say "OK, now stage this.".
> 
> BUT. That is probably not what the user wants, if the content in the
> index actually has some modifications that are not in the working tree
> (i.e., you wouldn't want to overwrite them). Hence we tend to work with
> diffs, saying "make these changes to what is already in the index, and
> if they conflict, then bail".
> 
> So "git add -p", for example, also works by creating diffs, modifying
> them, and feeding the result to "apply". You can see the implementation
> in git-add--interactive.perl, where it literally calls diff and apply
> commands.
> 
> And that leads us to the answer to the first question. That script
> implements "add -p", but also "checkout -p" (which is what you want),
> "reset -p", "stash -p", etc. They differ only in what we diff and how we
> apply the result; the main engine of slicing and dicing the diff through
> user interaction is the same. See the %patch_modes hash for the list.

> > So "git add -p", for example, also works by creating diffs, modifying
> > them, and feeding the result to "apply". You can see the implementation
> > in git-add--interactive.perl, where it literally calls diff and apply
> > commands.
> > 
> > And that leads us to the answer to the first question. That script
> > implements "add -p", but also "checkout -p" (which is what you want),
> > "reset -p", "stash -p", etc. They differ only in what we diff and how we
> > apply the result; the main engine of slicing and dicing the diff through
> > user interaction is the same. See the %patch_modes hash for the list.
> 
> Ah, so that means I do have to dive into generating diffs. Too bad, I 
> was hoping for a cleaner (read: easier) way.
> 
> On that note, I don't suppose there is a way to use 
> git-add--interactive's diff engine from a script, is there?  That'd 
> allow me to not write potentially buggy code and solve problems someone 
> already solved.

> Am 13.08.19 um 16:18 schrieb Pratyush Yadav:
> > Now, I want to add a similar feature, but one that discards/resets the 
> > selected lines. And I'd like to avoid the hack that git-gui's 
> > apply_range_or_line is. So, is there a cleaner way to do this that does 
> > not involve generating a diff and then applying it?
> 
> Don't waste your time. I'm using this feature since years. It's
> available in these three commits:
> 
> https://github.com/j6t/git/commit/55276690b2bc6a
> https://github.com/j6t/git/commit/f12b294b68b595
> https://github.com/j6t/git/commit/00860615691604
> 
> Thanks go to Bert Wesarg. The patches are also available in this thread:
> 
> https://public-inbox.org/git/cccd6193cf3bfe170e14270204d735a842bb8563.1326116492.git.bert.wesarg@googlemail.com/#t