Allow to switch to a random theme

You can now specify the special theme name `random` and vim-airline will
load a random theme from the ones installed. This works from either your
.vimrc as well as when calling `:AirlineTheme` command directly.

closes vim-airline/vim-airline-themes#170

CHANGELOG.md

@@ -11,6 +11,8 @@ This is the Changelog for the vim-airline project.
       Set the `g:airline_statusline_ontop` to enable this experimental feature.
     - If `buffer_idx_mode=2`, up to 89 mappings will be exposed to access more
       buffers directly (issue #1823)
+    - Allow to use `random` as special theme name, which will switch to a random
+      airline theme (at least if a random number can be generated :()
 
 ## [0.10] - 2018-12-15
 - New features

autoload/airline/util.vim

@@ -135,6 +135,6 @@ function! airline#util#doautocmd(event)
 endfunction
 
 function! airline#util#themes(match)
-  let files = split(globpath(&rtp, 'autoload/airline/themes/'.a:match.'*'), "\n")
-  return map(files, 'fnamemodify(v:val, ":t:r")')
+  let files = split(globpath(&rtp, 'autoload/airline/themes/'.a:match.'*.vim'), "\n")
+  return sort(map(files, 'fnamemodify(v:val, ":t:r")') + ['random'])
 endfunction

doc/airline.txt

@@ -249,6 +249,7 @@ COMMANDS                                                  *airline-commands*
 
 :AirlineTheme {theme-name}                                   *:AirlineTheme*
   Displays or changes the current theme.
+  Note: `random` will switch to a random theme.
 
 :AirlineToggleWhitespace                          *:AirlineToggleWhitespace*
   Toggles whitespace detection.

plugin/airline.vim

@@ -21,6 +21,9 @@ function! s:init()
   let s:theme_in_vimrc = exists('g:airline_theme')
   if s:theme_in_vimrc
     try
+      if g:airline_theme is# 'random'
+        let g:airline_theme=s:RandomTheme()
+      endif
       let palette = g:airline#themes#{g:airline_theme}#palette
     catch
       call airline#util#warning(printf('Could not resolve airline theme "%s". Themes have been migrated to github.com/vim-airline/vim-airline-themes.', g:airline_theme))
@@ -171,9 +174,16 @@ endfunction
 function! s:airline_theme(...)
   if a:0
     try
-      call airline#switch_theme(a:1)
+      let theme = a:1
+      if  theme is# 'random'
+        let theme = s:RandomTheme()
+      endif
+      call airline#switch_theme(theme)
     catch " discard error
     endtry
+    if a:1 is# 'random'
+      echo g:airline_theme
+    endif
   else
     echo g:airline_theme
   endif
@@ -215,6 +225,25 @@ function! s:airline_extensions()
   endfor
 endfunction
 
+function! s:Rand(max) abort
+  if has("reltime")
+    let timerstr=reltimestr(reltime())
+    let number=split(timerstr, '\.')[1]+0
+  else
+    " best effort, bash and zsh provide $RANDOM
+    " cmd.exe on windows provides %random%, but expand()
+    " does not seem to be able to expand this correctly.
+    " In the worst case, this always returns zero
+    let number=expand("$RANDOM")+0
+  endif
+  return number % a:max
+endfunction
+
+function! s:RandomTheme() abort
+  let themes=airline#util#themes('')
+  return themes[s:Rand(len(themes))]
+endfunction
+
 command! -bar -nargs=? -complete=customlist,<sid>get_airline_themes AirlineTheme call <sid>airline_theme(<f-args>)
 command! -bar AirlineToggleWhitespace call airline#extensions#whitespace#toggle()
 command! -bar AirlineToggle  call s:airline_toggle()