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https://github.com/samoshkin/vim-mergetool

:cake: Efficient way of using Vim as a Git mergetool
https://github.com/samoshkin/vim-mergetool

diff git merge-conflicts mergetool vim vim-plugin vimdiff

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:cake: Efficient way of using Vim as a Git mergetool

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# vim-mergetool



Efficient way of using Vim as a Git mergetool. :cake: With `vim-mergetool` you can have your cake and eat it too. Check out the demo.





  




## Overview



`vim-mergetool` processes `MERGED` file and extracts `ours`, `theirs`, or `common` sides of a conflict by parsing conflict markers left by Git. Then it shows 2-way diff between `ours` and `theirs` versions, with raw conflict markers being already removed.



Unlike simply comparing between `LOCAL` and `REMOTE` history revisions, it takes over where automatic Git merge algorithm gives up. Diffs are present only where Git cannot automatically resolve conflicts, and you're not distracted with diff highlighting of already resolved hunks.



In a screenshot below, `MERGED` file is on the left with conflict markers already removed with optimistic assumption of picking up changes from `ours` side. Then it's compared to the `REMOTE` branch, but focusing only on conflicts. This solution highlights the actual merge conflicts instead of all the diffs.



![Default 2 way diff layout between merged file and remote revision](./screenshots/mergetool_mr_2way_default_layout.png)



Plus, to resolve the conflict you don't need to edit conflict markers directly - just pick either side of a conflict using `:diffget` and `:diffput` commands.



This plugin was initially inspired by [whiteinge/diffconflicts](https://github.com/whiteinge/diffconflicts). Check out [how this plugin is different from existing solutions](#how-is-it-different-from-x).



## Minimal working example (TL;DR)



Use your favorite Vim plugin manager.



Minimal working configuration using [vim-plug](https://github.com/junegunn/vim-plug).



```vim

set nocompatible

filetype plugin indent on



call plug#begin('~/.vim/plugged')

  Plug 'samoshkin/vim-mergetool'

call plug#end()



let g:mergetool_layout = 'mr'

let g:mergetool_prefer_revision = 'local'

```



In Vim, open a file with conflict markers, and start mergetool.



```

:MergetoolStart

```



`vim-mergetool` would show 2-way diff in a new tab with `$MERGED` file on the left. By default, all conflicts are already resolved by picking up `ours/LOCAL` version. You don't need to edit raw conflict markers manually. Either leave hunk as is, or pick `theirs/REMOTE` version with `:diffget` from the right, or edit hunk manually.



Once done, quit merge tool:



```

:MergetoolStop

```



There's a single `:MergetoolToggle` command can both start and stop merge mode. Plus, you can set up a key mapping to toggle merge mode:



```vim

nmap mt (MergetoolToggle)

```



This example only scratches the surface of what `vim-mergetool` can do. Keep reading, if you need more features/customization.



### Conflict markers format



**[ASSUMPTION]**: `vim-mergetool` expects conflict markers in a `MERGED` file to include common `base` ancestor version as well. This is called `diff3` conflict style.



```

" <<<<<<< HEAD

" ours/local revision

" ||||||| base

" common base revision

" =======

" theirs/remote revision

" >>>>>>> feature

```



If you get *"Conflict markers miss common base revision"* error message, put the following in your `~/.gitconfig` to use [diff3](https://git-scm.com/docs/git-config#Documentation/git-config.txt-mergeconflictStyle) conflict style as a default:



```ini

[merge]

conflictStyle = diff3

```



If something goes absolutely wrong, you can always reset conflict markers in a file to their initial state. It's safe to do it only during ongoing merge, otherwise you'd overwrite file in a working tree with version from index.



```

git checkout --conflict=diff3 {file}

```



## Features



- Flexible layouts. You're not limited to default 2-way diff layout. You can use 3-way diff layout, or even setup window of 4 splits. Both horizontal and vertical splits are supported, and mix thereof.

- Toggle between layouts during merge. You can have several layouts and toggle between them during merge. For example, you're using 2-way diff by default, but sometimes you want to quickly recall what's the state of a diff in the `BASE` revision, but don't want to keep 3rd `BASE` split constantly opened.

- Customize layout splits as you like: resize, turn off syntax highlighting, turn off diff mode.

- Choose preferred conflict side. `ours` side is picked up by default but you can also choose `ours`, `theirs` or `base` side of a conflict for the `MERGED` file, or work with raw conflict markers.

- Conventional `LOCAL`, `REMOTE`, `BASE` history revisions are available to compare to as well.

- Can be run as a `git mergetool`, or by opening a file with conflict markers from the running Vim instance.

- Prompts whether merge was successful on quit. If not, rollbacks changes and report non-zero exit status code when run as a `git mergetool`.

- Smart diff exchange commands. Tell direction of a window to `diffget` or `diffput` instead of specifying a buffer number. Especially handy for 3-way diffs. Not limited to merge conflict scenarios, can be used for regular diffs.

- Can tell if we're in merge mode right now. Useful for showing some sort of indicator in a status line.



**NOTE**: `vim-mergetool` does not set up any key mappings for you. It justs exports a handful of commands and `` mappings. You're free to set up key mappings in your `vimrc` as you'd like.



### Preferred conflict side

`vim-mergetool` removes conflict markers from `MERGED` file, and picks up `ours/local` side of a conflict by default. If you prefer another side of a conflict:



```vim

" possible values: 'local' (default), 'remote', 'base'

let g:mergetool_prefer_revision = 'remote'

```



If you don't want `vim-mergetool` to process `MERGED` file and remove raw conflict markers:



```vim

let g:mergetool_prefer_revision = 'unmodified'

```



Alternatively, you can start with `local` or `unmodified` revision, and change your mind later during merge process by running one of these commands:



```vim

:MergetoolPreferLocal

:MergetoolPreferRemote

```



### Available revisions to compare



2-way diff between `local` and `remote` versions derived from conflict markers is a sane default, but you might want to compare `MERGED` file against other revisions:

- `LOCAL`, current branch HEAD.

- `REMOTE`, HEAD of the branch we're going to merge

- `BASE`, common ancestor of two branches, i.e. `git merge-base branchX branchY`

- `local`, `remote`, `base` (in lowercase), those are revisions derived from `MERGED` file by picking up either side of a conflict from conflict markers



### Layout



`vim-mergetool` defaults to two vertical splits layout with `MERGED` file on the left, and `remote` revision on the right. `MERGED` file is processed according to `g:mergetool_prefer_revision` setting as described above.



```vim

" (m) - for working tree version of MERGED file

" (r) - for 'remote' revision

let g:mergetool_layout = 'mr'

```



If you want to use 3-way diff layout as a default, with a `base` revision in the leftmost split:



```vim

let g:mergetool_layout = 'bmr'

```



![3 way diff vertical split layout](./screenshots/bmr_3splits_layout.png)



To show usual `REMOTE`, `LOCAL`, `BASE` history revisions, use uppercase characters:



```vim

let g:mergetool_layout = 'LmR'

```



By the way, this setup is pretty much same to what [vim-fugitive](https://github.com/tpope/vim-fugitive) `:Gdiff` does, except that conflict markers are already removed. You can use `g:mergetool_prefer_revision='unmodified'` to replicate vim-fugitive completely. Indeed, `vim-mergetool` is flexible enough to replicate any existing vim+merge solution.



Vertical splits are used by default. If you prefer working with horizontal splits:



```vim

let g:mergetool_layout = 'm,r'

```



You can mix both approaches. For example, show `MERGED` file and `remote` revision in vertical splits as usual, and have horizontal split with `base` revision at the bottom.



```vim

let g:mergetool_layout = 'mr,b'

```



![Horizontal and vertical mixed layout](./screenshots/bmr_mixed_layout.png)



### Toggle layout during merge

You are not limited to single possible layout. You can switch easily between different layouts during merge process.



For example, you can start with 2-way diff layout, and then temporarily toggle additional split with `base` revision on the left or at the bottom. Or hide `MERGED` file altogether and review `LOCAL`, `BASE` and `REMOTE` history revisions.



```vim

" In 'vimrc'

" Default layout

let g:mergetool_layout = 'mr'



" Later, during merge process:



" View 'base' revision on the left

:MergetoolToggleLayout bmr



" View 'base' revision in horizontal split at the bottom

:MergetoolToggleLayout mr,b



" View history revisions, and hide 'MERGED' file altogether

:MergetoolToggleLayout LBR

```



![Switching layouts](./screenshots/toggle_layouts.png)



In addition to commands, you can set up key mappings for your most common layouts:



```vim

nnoremap  mb :call mergetool#toggle_layout('mr,b')

```



### Advanced layout customization



If you want to further tweak layout or change settings of individual splits, define the callback function, which is called when layout is changed.



Example. When layout is `mr,b`, I want the `base` horizontal split to be pulled of a diff mode and have syntax highlighting enabled. Also, I want it to reduce its height.



```vim

function s:on_mergetool_set_layout(split)

  if a:split["layout"] ==# 'mr,b' && a:split["split"] ==# 'b'

    set nodiff

    set syntax=on



    resize 15

  endif

endfunction



let g:MergetoolSetLayoutCallback = function('s:on_mergetool_set_layout')

```



Callback is called for each split in the layout, with a split being passed as a callback argument.



```

{

    'layout': 'mb,r',         # current layout

    'split': 'b',             # current split

    'filetype': 'javascript', # file type of MERGED file

    'bufnr': 2,               # buffer number of current split

    'winnr': 5                # window number of current split

}

```



Example. I want to turn off syntax and spell checking highlighting for all splits, so it doesn't distract me from diff highlighting.



```vim

function s:on_mergetool_set_layout(split)

  set syntax=off

  set nospell

endfunction



let g:MergetoolSetLayoutCallback = function('s:on_mergetool_set_layout')

```



Here's the end result:



![Layout advanced customization](./screenshots/layout_advanced_customization.png)



### Running as a `git mergetool`



`vim-mergetool` can be configured to run as a `git mergetool`. In your `~/.gitconfig`:



```ini

[merge]

tool = vim_mergetool

conflictstyle = diff3



[mergetool "vim_mergetool"]

cmd = vim -f -c "MergetoolStart" "$MERGED" "$BASE" "$LOCAL" "$REMOTE"

trustExitCode = true

```



Git detects whether merge was successful or not in two ways:

- When `trustExitCode = false`, checks if `MERGED` file was modified.

- When `trustExitCode = true`, checks exit code of merge tool process.



`vim-mergetool` supports both options. On quit, if merge was unsuccessful,  it both discards any unsaved changes to buffer without touching file's `ctime` and returns non-zero exit code.



### Running directly from running Vim instance

You can enter and exit merge mode from running Vim instance by opening a file with conflict markers, and running one of the commands:



```vim

:MergetoolStart

:MergetoolStop

:MergetoolToggle

```



You can set up a key mapping to toggle merge mode:



```vim

nmap mt (MergetoolToggle)

```



When exiting merge mode, if merge was unsuccessful, `vim-mergetool` would discard changes to merged file and rollback to a buffer state as it were right before starting a new merge.



Unlike running as a `git mergetool`, `LOCAL`, `REMOTE` and `BASE` history revisions are not passed from the outside. In this mode, `vim-mergetool` extracts them from the numbered stages of Git index.



```bash

$ git cat-file -p :1:{file} > {file}.base

$ git cat-file -p :2:{file} > {file}.local

$ git cat-file -p :3:{file} > {file}.remote

```



**ASSUMPTION:** Therefore, it's assumed that a git merge is in progress, and `cwd` of running Vim instance is set to repository root dir.



### Smart diff exchange commands



Vim's `:diffget` and `:diffput` commands are convenient and unambiguous as soon as you have only two buffers in diff mode. If you prefer 3-way diff, you're out of lucky, as you need to explicitly tell the buffer number you want to exchange diff with.



`vim-mergetool` comes with `DiffExchange` commands and mapping, that accepts direction of a diff movement: "left", "right", "up", "down". You can set up your own key mappings for diff mode only:



```vim

nmap   &diff? '(MergetoolDiffExchangeLeft)' : ''

nmap   &diff? '(MergetoolDiffExchangeRight)' : ''

nmap   &diff? '(MergetoolDiffExchangeDown)' : ''

nmap   &diff? '(MergetoolDiffExchangeUp)' : ''

```



Commands are available as well:



```vim

:MergetoolDiffExchangeLeft

:MergetoolDiffExchangeRight

:MergetoolDiffExchangeDown

:MergetoolDiffExchangeUp

```



`DiffExchange` logic runs either `:diffget` or `:diffput` with a right buffer number of adjacent window, depending on:

- given direction

- whether window in opposite direction exists or not



![Diff exchange](./screenshots/diff_exchange.png)



It's easier to explain with example.



Suppose, you have 3 split layout: `MERGED` file in the middle, `base` and `remote` revisions are on the sides. Typically, the middle one with a `MERGED` file is an active split. You navigate from hunk to hunk, and decide what to do with a conflict: leave as is, or pick version from left/right splits.

- `` would `diffget` change from the right split into the middle one. If you imagine the diff movement - it goes from right to the left.

- `` would `diffget` change from the left split into the middle one. If you imagine the diff movement - it goes from left to the right.



If the rightmost split were the active one:

- `` would `diffput` change from the current split into the middle one. As soon as there is no adjacent window on the right to get change from, we invert `diffget` operation into `diffput`.

- `` would `diffget` change from middle split.



Same logic applies to "up" and "down" directions. Useful if you prefer horizontal splits.



**Conclusion**: despite how many splits are opened and what's the layout, you don't need to wrap your head around `diffput` vs `diffget` semantics, and you don't need to figure out correct buffer numbers manually. You just tell desired diff movement direction, and `vim-mergetool` handles the details for you.



**Limitation**: `DiffExchange` commands work only in normal mode, and do not support visual mode and working with line ranges.



`DiffExchange` functionality is not specific to resolving merge conflicts, and can be used for regular diffs.



If you like `` mappings from the snippet above, you might also want to map `` and `` keys to navigate diffs, instead of default `[c` and `]c` mappings. They're not used anyway, since you're using `h,j,k,l` for movements, are you? ;-)



```vim

nmap   &diff ? '[c' : ''

nmap   &diff ? ']c' : ''

```



### Merge mode detection

You can detect whether you're in merge mode now, by inspecting `g:mergetool_in_merge_mode` variable.



It can be helpful to show indicator in a status line. Example for [vim-airline](https://github.com/vim-airline/vim-airline):



```vim

function! AirlineDiffmergePart()

  if get(g:, 'mergetool_in_merge_mode', 0)

    return '↸'

  endif



  if &diff

    return '↹'

  endif



  return ''

endfunction



call airline#parts#define_function('_diffmerge', 'AirlineDiffmergePart')

call airline#parts#define_accent('_diffmerge', 'bold')



let g:airline_section_z = airline#section#create(['_diffmerge', ...other_parts])

```



![Status line indicator](./screenshots/airline_merge_indicator.png)



### Quitting merge mode



When exiting merge mode, `vim-mergetool` would prompt you whether merge was successful. If not, it will rollback changes to the buffer, will not save `MERGED` file to disk, and exit with non-zero code, when running as a `git mergetool`.



![Quit prompt](./screenshots/quit_prompt.png)



You can either issue `:MergetoolStop` or `:MergetoolToggle` commands, or use dedicated mapping.



Yet another approach, which I prefer in my personal `vimrc`, is having a `q` key mapped to context-aware `QuitWindow()` function. It detects whether we're in merge mode, and runs `:MergetoolStop` command, or just uses normal "quit" command otherwise.



```vim

function s:QuitWindow()



  " If we're in merge mode, exit

  if get(g:, 'mergetool_in_merge_mode', 0)

    call mergetool#stop()

    return

  endif



  if &diff

    " Quit diff mode intelligently...

  endif



  quit

endfunction



command QuitWindow call s:QuitWindow()

nnoremap  q :QuitWindow

```



## How is it different from X?

Most merge tools use 3-way diff approach by showing several split windows, that include `LOCAL` revision (current branch), `REMOTE` revision (branch we're going to merge), `BASE` revision (common base ancestor), and working tree version of `MERGED` file with or without conflict markers.



Here is the same merge scenario opened with a default `vimdiff` as a mergetool:

![Default vimdiff mergetool](./screenshots/vimdiff_default_tool.png)



  While 3-way diff paradigm is superior to 2-way diff for merging purposes, it does not fit Vim well:



  **Disadvantages**:

  - When several revisions are compared at the same time (i.e. `:diffthis` in all buffers), Vim highlights every diff between each of those files. Usually that means every possible change between BASE-LOCAL-REMOTE-MERGED files, including those which are not relevant to conflict resolution.

  - It highlights even those hunks, which were already automatically resolved by `git merge-file`. It distracts you from focusing purely on unresolved conflicts. Indeed, I don't want to care about already resolved conflicts.

  - Limited window width when three vertical splits are opened. Forces you to scroll horizontally, or wrap lines. Unless you have enough screen width, it's difficult to quickly grasp changes when window width is only ~50 columns or so. Usually, it's not easy to tweak predefined layout.

  - Forces you to pick up conflict side by directly editing conflict markers, instead of choosing change from the left or the right.

  - `:diffget` and `:diffput` Vim commands are convenient only when there're two split windows, otherwise they become ambiguous and you need to tell them the target buffer number, which is a real showstopper. No one wants to think on "What's the Vim's buffer number of the window on the right/left?", when you're already pulling your hair trying to resolve conflicts from long-running "feature" branch.



#### Existing solutions



  - default `vimdiff` merge tool. Shows layout with 3 vertical splits: `LOCAL`, `REMOTE`, `BASE` revisions, and the horizontal split at the bottom with a `MERGED` file, containing raw conflict markers.

  - [vim-fugitive](https://github.com/tpope/vim-fugitive) `:Gdiff` command, which automatically detects conflict markers in a file and switches into 3-way diff. Shows 3 vertical splits: `LOCAL`, `REMOTE` revisions and `MERGED` file in the middle with raw conflict markers. See this [reddit comment](https://www.reddit.com/r/vim/comments/b0jjgw/github_samoshkinvimmergetool_efficient_way_of/eif6oio/) on difference between [vim-fugitive](https://github.com/tpope/vim-fugitive) and this plugin.

  - [whiteinge/diffconflicts](https://github.com/whiteinge/diffconflicts). Parses `MERGED` file and removes conflict markers to pick up one side of a conflict. Default to 2-split layout with `local` and `remote` revisions. `vim-mergetool` uses the same idea, plus brings many additional features.

  - [sjl/splice.vim](https://github.com/sjl/splice.vim). Haven't yet had experience with it.

  - Drop using Vim as a mergetool. Use some GUI program, like [DiffMerge](https://sourcegear.com/diffmerge/) or [Kdiff3](http://kdiff3.sourceforge.net/). Better to use both Vim and some GUI tool as a backup though.