Better algorightm, to get msdos colors

currently, full 255 colors were returned, while in fact
msdos console only supports 16 colors. So fix this.

autoload/airline/highlighter.vim

@@ -9,12 +9,11 @@ let s:accents = {}
 function! s:gui2cui(rgb, fallback)
   if a:rgb == ''
     return a:fallback
-  elseif match(a:rgb, 'NONE\|[fb]g')
+  elseif match(a:rgb, '^\%(NONE\|[fb]g\)$') > -1
     return a:rgb
   endif
-  let rgb = map(matchlist(a:rgb, '#\(..\)\(..\)\(..\)')[1:3], '0 + ("0x".v:val)')
-  let rgb = [rgb[0] > 127 ? 4 : 0, rgb[1] > 127 ? 2 : 0, rgb[2] > 127 ? 1 : 0]
-  return rgb[0]+rgb[1]+rgb[2]
+  let rgb = map(split(a:rgb[1:], '..\zs'), '0 + ("0x".v:val)')
+  return airline#msdos#round_msdos_colors(rgb)
 endfunction
 
 function! s:get_syn(group, what)

autoload/airline/msdos.vim

@@ -0,0 +1,57 @@
+" MIT License. Copyright (c) 2013-2016 Bailey Ling.
+" vim: et ts=2 sts=2 sw=2
+
+" basic 16 msdos from MSDOS
+" see output of color, should be
+"     0    Black
+"     1    DarkBlue
+"     2    DarkGreen
+"     3    DarkCyan
+"     4    DarkRed
+"     5    DarkMagenta
+"     6    Brown
+"     7    LightGray
+"     8    DarkGray
+"     9    Blue
+"     10   Green
+"     11   Cyan
+"     12   Red
+"     13   Magenta
+"     14   Yellow
+"     15   White
+
+let s:basic16 = [
+  \ [ 0x00, 0x00, 0x00 ], 
+  \ [ 0x00, 0x00, 0x80 ],
+  \ [ 0x00, 0x80, 0x00 ],
+  \ [ 0x00, 0x80, 0x80 ],
+  \ [ 0x80, 0x00, 0x00 ],
+  \ [ 0x80, 0x00, 0x80 ],
+  \ [ 0x80, 0x80, 0x00 ],
+  \ [ 0xC0, 0xC0, 0xC0 ],
+  \ [ 0x80, 0x80, 0x80 ],
+  \ [ 0x00, 0x00, 0xFF ],
+  \ [ 0x00, 0xFF, 0x00 ],
+  \ [ 0x00, 0xFF, 0xFF ],
+  \ [ 0xFF, 0x00, 0x00 ],
+  \ [ 0xFF, 0x00, 0xFF ],
+  \ [ 0xFF, 0xFF, 0x00 ],
+  \ [ 0xFF, 0xFF, 0xFF ]
+  \ ]
+
+function! airline#msdos#round_msdos_colors(rgblist)
+  " Check for values from MSDOS 16 color terminal
+  let best = []
+  let min  = 100000
+  let list = s:basic16
+  for value in list
+    let t = abs(value[0] - a:rgblist[0]) +
+          \ abs(value[1] - a:rgblist[1]) +
+          \ abs(value[2] - a:rgblist[2])
+    if min > t
+      let min = t
+      let best = value
+    endif
+  endfor
+  return index(s:basic16, best)
+endfunction