function! gitgutter#highlight#define_sign_column_highlight() abort
  if g:gitgutter_override_sign_column_highlight
    highlight! link SignColumn LineNr
  else
    highlight default link SignColumn LineNr
  endif
endfunction

function! gitgutter#highlight#define_highlights() abort
  let [guibg, ctermbg] = gitgutter#highlight#get_background_colors('SignColumn')

  " Highlights used by the signs.

  execute "highlight GitGutterAddDefault    guifg=#009900 guibg=" . guibg . " ctermfg=2 ctermbg=" . ctermbg
  execute "highlight GitGutterChangeDefault guifg=#bbbb00 guibg=" . guibg . " ctermfg=3 ctermbg=" . ctermbg
  execute "highlight GitGutterDeleteDefault guifg=#ff2222 guibg=" . guibg . " ctermfg=1 ctermbg=" . ctermbg
  highlight default link GitGutterChangeDeleteDefault GitGutterChangeDefault

  execute "highlight GitGutterAddInvisible    guifg=bg guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
  execute "highlight GitGutterChangeInvisible guifg=bg guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
  execute "highlight GitGutterDeleteInvisible guifg=bg guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
  highlight default link GitGutterChangeDeleteInvisible GitGutterChangeInvisble

  highlight default link GitGutterAdd          GitGutterAddDefault
  highlight default link GitGutterChange       GitGutterChangeDefault
  highlight default link GitGutterDelete       GitGutterDeleteDefault
  highlight default link GitGutterChangeDelete GitGutterChangeDeleteDefault

  " Highlights used for the whole line.

  highlight default link GitGutterAddLine          DiffAdd
  highlight default link GitGutterChangeLine       DiffChange
  highlight default link GitGutterDeleteLine       DiffDelete
  highlight default link GitGutterChangeDeleteLine GitGutterChangeLine
endfunction

function! gitgutter#highlight#define_signs() abort
  sign define GitGutterLineAdded
  sign define GitGutterLineModified
  sign define GitGutterLineRemoved
  sign define GitGutterLineRemovedFirstLine
  sign define GitGutterLineModifiedRemoved
  sign define GitGutterDummy

  call gitgutter#highlight#define_sign_text()
  call gitgutter#highlight#define_sign_text_highlights()
  call gitgutter#highlight#define_sign_line_highlights()
endfunction

function! gitgutter#highlight#define_sign_text() abort
  execute "sign define GitGutterLineAdded            text=" . g:gitgutter_sign_added
  execute "sign define GitGutterLineModified         text=" . g:gitgutter_sign_modified
  execute "sign define GitGutterLineRemoved          text=" . g:gitgutter_sign_removed
  execute "sign define GitGutterLineRemovedFirstLine text=" . g:gitgutter_sign_removed_first_line
  execute "sign define GitGutterLineModifiedRemoved  text=" . g:gitgutter_sign_modified_removed
endfunction

function! gitgutter#highlight#define_sign_text_highlights() abort
  " Once a sign's text attribute has been defined, it cannot be undefined or
  " set to an empty value.  So to make signs' text disappear (when toggling
  " off or disabling) we make them invisible by setting their foreground colours
  " to the background's.
  if g:gitgutter_signs
    sign define GitGutterLineAdded            texthl=GitGutterAdd
    sign define GitGutterLineModified         texthl=GitGutterChange
    sign define GitGutterLineRemoved          texthl=GitGutterDelete
    sign define GitGutterLineRemovedFirstLine texthl=GitGutterDelete
    sign define GitGutterLineModifiedRemoved  texthl=GitGutterChangeDelete
  else
    sign define GitGutterLineAdded            texthl=GitGutterAddInvisible
    sign define GitGutterLineModified         texthl=GitGutterChangeInvisible
    sign define GitGutterLineRemoved          texthl=GitGutterDeleteInvisible
    sign define GitGutterLineRemovedFirstLine texthl=GitGutterDeleteInvisible
    sign define GitGutterLineModifiedRemoved  texthl=GitGutterChangeDeleteInvisible
  endif
endfunction

function! gitgutter#highlight#define_sign_line_highlights() abort
  if g:gitgutter_highlight_lines
    sign define GitGutterLineAdded            linehl=GitGutterAddLine
    sign define GitGutterLineModified         linehl=GitGutterChangeLine
    sign define GitGutterLineRemoved          linehl=GitGutterDeleteLine
    sign define GitGutterLineRemovedFirstLine linehl=GitGutterDeleteLine
    sign define GitGutterLineModifiedRemoved  linehl=GitGutterChangeDeleteLine
  else
    sign define GitGutterLineAdded            linehl=
    sign define GitGutterLineModified         linehl=
    sign define GitGutterLineRemoved          linehl=
    sign define GitGutterLineRemovedFirstLine linehl=
    sign define GitGutterLineModifiedRemoved  linehl=
  endif
endfunction

function! gitgutter#highlight#get_background_colors(group) abort
  redir => highlight
  silent execute 'silent highlight ' . a:group
  redir END

  let link_matches = matchlist(highlight, 'links to \(\S\+\)')
  if len(link_matches) > 0 " follow the link
    return gitgutter#highlight#get_background_colors(link_matches[1])
  endif

  let ctermbg = gitgutter#highlight#match_highlight(highlight, 'ctermbg=\([0-9A-Za-z]\+\)')
  let guibg   = gitgutter#highlight#match_highlight(highlight, 'guibg=\([#0-9A-Za-z]\+\)')
  return [guibg, ctermbg]
endfunction

function! gitgutter#highlight#match_highlight(highlight, pattern) abort
  let matches = matchlist(a:highlight, a:pattern)
  if len(matches) == 0
    return 'NONE'
  endif
  return matches[1]
endfunction