First draft.

README.mkd

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+## Vim Git Gutter
+
+A Vim plugin which shows a git diff in the 'gutter' (sign column).  It shows whether each line has been added, modified, and where lines have been removed.
+
+This is a port of the [Git Gutter][st2gg] plugin for Sublime Text 2.
+
+
+### Screenshot
+
+![screenshot](https://raw.github.com/airblade/vim-gitgutter/screenshot.png)
+
+In the screenshot above you can see:
+
+* Line 15 has been modified.
+* Lines 21-24 are new.
+* A line or lines were removed between lines 25 and 26.
+
+
+### Installation
+
+If you don't have a preferred installation method, I recommend installing [pathogen.vim][pathogen], and then simply copy and paste:
+
+```
+cd ~/.vim/bundle
+git clone git://github.com/airblade/vim-gitgutter.git
+```
+
+
+### FAQ
+
+> The colours in the sign column are weird.
+
+The syntax highlighting for your sign column is probably set strangely.  Either modify your colorscheme or add this to your `~.vimrc`:
+
+```
+highlight clear SignColumn
+```
+
+> Lines removed below a modified line are not shown.
+
+True.  This plugin uses Vim's signs which require a sign to be on a line (not between two lines) and only permit one sign per line.  Removed lines are signed with an underscore on the line above.  If that line has also been modified, the plugin has to choose whether to show the removed-lines sign or the modified-line sign.  It prefers the latter.
+
+
+### Intellectual Property
+
+Copyright Andrew Stewart, AirBlade Software Ltd.  Released under the MIT licence.
+
+
+  [st2gg]: https://github.com/jisaacks/GitGutter
+  [pathogen]: https://github.com/tpope/vim-pathogen

plugin/gitgutter.vim

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+if exists('g:loaded_gitgutter') || !executable('git') || &cp
+  finish
+endif
+let g:loaded_gitgutter = 1
+
+" Initialisation {{{
+
+function! s:init()
+  if !exists('g:gitgutter_initialised')
+    call s:define_highlights()
+    call s:define_signs()
+    let g:gitgutter_initialised = 1
+  endif
+endfunction
+
+function! s:define_highlights()
+  highlight lineAdded    guifg=#009900 guibg=NONE ctermfg=2 ctermbg=NONE
+  highlight lineModified guifg=#bbbb00 guibg=NONE ctermfg=3 ctermbg=NONE
+  highlight lineRemoved  guifg=#ff2222 guibg=NONE ctermfg=1 ctermbg=NONE
+endfunction
+
+function! s:define_signs()
+  sign define line_added    text=+ texthl=lineAdded
+  sign define line_modified text=~ texthl=lineModified
+  sign define line_removed  text=_ texthl=lineRemoved
+endfunction
+
+" }}}
+
+" Utility {{{
+
+function! s:current_file()
+  return expand('%')
+endfunction
+
+function! s:is_in_a_git_repo()
+  call system('git rev-parse > /dev/null 2>&1')
+  return !v:shell_error
+endfunction
+
+function! s:is_tracked_by_git()
+  call system('git ls-files --error-unmatch > /dev/null 2>&1 ' . shellescape(s:current_file()))
+  return !v:shell_error
+endfunction
+
+" }}}
+
+" Core logic {{{
+
+function! s:run_diff()
+  let cmd = 'git diff --no-ext-diff -U0 ' . shellescape(s:current_file())
+  let diff = system(cmd)
+  return diff
+endfunction
+
+function! s:parse_diff(diff)
+  let hunk_re = '^@@ -\(\d\+\),\?\(\d*\) +\(\d\+\),\?\(\d*\) @@'
+  let hunks = []
+  for line in split(a:diff, '\n')
+    if line =~ '^@@\s'
+      let matches    = matchlist(line, hunk_re)
+      let from_line  = str2nr(matches[1])
+      let from_count = (matches[2] == '') ? 1 : str2nr(matches[2])
+      let to_line    = str2nr(matches[3])
+      let to_count   = (matches[4] == '') ? 1 : str2nr(matches[4])
+      call add(hunks, [from_line, from_count, to_line, to_count])
+    endif
+  endfor
+  return hunks
+endfunction
+
+function! s:process_hunks(hunks)
+  let modified_lines = []
+  for hunk in a:hunks
+    let from_line  = hunk[0]
+    let from_count = hunk[1]
+    let to_line    = hunk[2]
+    let to_count   = hunk[3]
+    " added
+    if from_count == 0 && to_count > 0
+      let offset = 0
+      while offset < to_count
+        let line_number = to_line + offset
+        call add(modified_lines, [line_number, 'added'])
+        let offset += 1
+      endwhile
+    " removed
+    elseif from_count > 0 && to_count == 0
+      " removed lines came after `to_line`.
+      call add(modified_lines, [to_line, 'removed'])
+    " modified
+    else
+      let offset = 0
+      while offset < to_count
+        let line_number = to_line + offset
+        call add(modified_lines, [line_number, 'modified'])
+        let offset += 1
+      endwhile
+    endif
+  endfor
+  return modified_lines
+endfunction
+
+function! s:clear_signs()
+  sign unplace *
+endfunction
+
+function! s:show_signs(modified_lines)
+  let file_name = s:current_file()
+  for line in a:modified_lines
+    let line_number = line[0]
+    let type = line[1]
+    " TODO: eugh
+    if type ==? 'added'
+      let name = 'line_added'
+    elseif type ==? 'removed'
+      let name = 'line_removed'
+    elseif type ==? 'modified'
+      let name = 'line_modified'
+    endif
+    exe ":sign place " . line_number . " line=" . line_number . " name=" . name . " file=" . file_name
+  endfor
+endfunction
+
+" }}}
+
+" Public interface {{{
+
+function! GitGutter()
+  if s:is_in_a_git_repo() && s:is_tracked_by_git()
+    call s:init()
+    let diff = s:run_diff()
+    let hunks = s:parse_diff(diff)
+    let modified_lines = s:process_hunks(hunks)
+    call s:clear_signs()
+    call s:show_signs(modified_lines)
+  endif
+endfunction
+
+" }}}
+
+augroup gitgutter
+  autocmd!
+  autocmd BufReadPost,BufWritePost,FileReadPost,FileWritePost,BufEnter * call GitGutter()
+augroup END
+
+" vim:set et sw=2: