Compute the cterm colors given the true colors

The RGB values of the GUI colors are now taken in input to compute the
value of the Xterm colors by using an approximation.

Now, in order to chose colors for the GUI or the TERM, it is enough to
only provide the HSB triplet.

colors/odyssey.vim

@@ -38,8 +38,6 @@ let g:colors_name = 'odyssey'
 " }}}
 " Color Utilities {{{
 
-let s:colors = {}
-
 function! s:hsv2rgb(hue, saturation, value)
   let l:s = a:saturation / 100.0
   let l:v = a:value / 100.0
@@ -77,8 +75,44 @@ function! s:rgb2hex(rgb)
   return printf(l:f, a:rgb[0]) . printf(l:f, a:rgb[1]) . printf(l:f, a:rgb[2])
 endfunction
 
+function! s:rgb2short(rgb)
+  " 216 colors + 24 greys = 240 total ( - 16 because ignoring system color )
+  function! s:xterm2rgb(key, val)
+    let l:incs = [0, 95, 135, 175, 215, 255]
+
+    if a:val < 216
+      return [l:incs[a:val / 36], l:incs[(a:val % 36) / 6], l:incs[a:val % 6]]
+    else
+      let l:coordinate = a:val * 10 - 2152
+      return [l:coordinate, l:coordinate, l:coordinate]
+    endif
+  endfunction
+
+  function! s:distance(lhs, rhs)
+    return abs(a:lhs[0] - a:rhs[0])
+          \ + abs(a:lhs[1] - a:rhs[1])
+          \ + abs(a:lhs[2] - a:rhs[2])
+  endfunction
+
+  let l:xterm = range(0, 239)
+  let l:xterm = map(l:xterm, function('s:xterm2rgb'))
+
+  let l:distances = []
+  for l:value in l:xterm
+    let l:distances += [s:distance(l:value, a:rgb)]
+  endfor
+
+  let l:index = index(l:distances, min(l:distances)) + 16
+
+  return printf("%s", l:index)
+endfunction
+
 function! s:colorize(hue, saturation, value)
-  return s:rgb2hex(s:hsv2rgb(a:hue, a:saturation, a:value))
+  let l:rgb = s:hsv2rgb(a:hue, a:saturation, a:value)
+  let l:gui = s:rgb2hex(l:rgb)
+  let l:cterm = s:rgb2short(l:rgb)
+  let l:res = {'gui': l:gui, 'cterm': l:cterm}
+  return l:res
 endfunction
 
 " }}}
@@ -91,19 +125,21 @@ endfunction
 "
 " The s:colors dictionary is initialized in the Bootstrap section.
 
-let s:colors.beyondback = {'gui': s:colorize(177, 20,  6), 'term': '233'}
-let s:colors.background = {'gui': s:colorize(177, 20, 11), 'term': '234'}
-let s:colors.cursorline = {'gui': s:colorize(177, 25, 16), 'term': '235'}
-let s:colors.foldedline = {'gui': s:colorize(177, 25, 21), 'term': '236'}
-let s:colors.linenumber = {'gui': s:colorize(177,  9, 55), 'term': '245'}
-let s:colors.foreground = {'gui': s:colorize(177, 11, 75), 'term': '250'}
+let s:colors = {}
 
-let s:colors.keyword    = {'gui': s:colorize(170, 45, 70), 'term':  '73'}
-let s:colors.error      = {'gui': s:colorize(  0, 45, 70), 'term': '131'}
-let s:colors.warning    = {'gui': s:colorize( 30, 45, 70), 'term': '137'}
-let s:colors.procedure  = {'gui': s:colorize(140, 45, 70), 'term':  '72'}
-let s:colors.type       = {'gui': s:colorize(200, 45, 70), 'term':  '67'}
-let s:colors.constant   = {'gui': s:colorize(230, 45, 70), 'term':  '61'}
+let s:colors.beyondback = s:colorize(177, 20,  6)
+let s:colors.background = s:colorize(177, 20, 11)
+let s:colors.cursorline = s:colorize(177, 25, 16)
+let s:colors.foldedline = s:colorize(177, 25, 21)
+let s:colors.linenumber = s:colorize(177,  9, 55)
+let s:colors.foreground = s:colorize(177, 11, 75)
+
+let s:colors.keyword    = s:colorize(170, 45, 70)
+let s:colors.error      = s:colorize(  0, 45, 70)
+let s:colors.warning    = s:colorize( 30, 45, 70)
+let s:colors.procedure  = s:colorize(140, 45, 70)
+let s:colors.type       = s:colorize(200, 45, 70)
+let s:colors.constant   = s:colorize(230, 45, 70)
 
 " }}}
 " Highlight Utilities {{{
@@ -111,11 +147,11 @@ let s:colors.constant   = {'gui': s:colorize(230, 45, 70), 'term':  '61'}
 function! s:HL(group, bg, ...)
   let histring = 'highlight ' . a:group . ' '
   let cbg = get(s:colors, a:bg)
-  let histring .= 'guibg=#'. cbg['gui'] . ' ' . 'ctermbg=' . cbg['term']
+  let histring .= 'guibg=#'. cbg['gui'] . ' ' . 'ctermbg=' . cbg['cterm']
 
   if a:0 > 0
     let cfg = get(s:colors, a:1)
-    let histring .= ' guifg=#'. cfg['gui'] . ' ' . 'ctermfg=' . cfg['term']
+    let histring .= ' guifg=#'. cfg['gui'] . ' ' . 'ctermfg=' . cfg['cterm']
   endif
 
   if a:0 > 1