Validate the attr map only once per run

Before, each call to s:attr_map() would make sure that the things being
mapped from and to were valid.  Optimize this by making sure all at once
that all things being mapped from, and all things being mapped to, are
valid, before using any attributes.

plugin/CSApprox.vim

@@ -345,44 +345,50 @@ endfunction
 " can look up, to string values, representing the attribute mapped to or an
 " empty string to disable the given attribute entirely.
 function! s:attr_map(attr)
-  let attr = tolower(a:attr)
+  let rv = get(g:CSApprox_attr_map, a:attr, a:attr)
 
-  if attr == 'inverse'
+  return rv
-    let attr = 'reverse'
+endfunction
-  endif
+
+function! s:NormalizeAttrMap(map)
+  let old = copy(a:map)
+  let new = filter(a:map, '0')
 
   let valid_attrs = [ 'bg', 'fg', 'sp', 'bold', 'italic',
                     \ 'reverse', 'underline', 'undercurl' ]
 
-  if index(valid_attrs, attr) == -1
-    throw "Looking up invalid attribute '" . attr . "'"
-  endif
-
-  if !exists("g:CSApprox_attr_map") || type(g:CSApprox_attr_map) != type({})
-    let g:CSApprox_attr_map = { 'italic' : 'underline', 'sp' : 'fg' }
-  endif
-
-  let rv = get(g:CSApprox_attr_map, attr, attr)
-
-  if index(valid_attrs, rv) == -1 && rv != ''
-    " The user mapped 'attr' to something invalid
-    throw "Bad attr map: '" . attr . "' to unknown attribute '" . rv . "'"
-  endif
-
   let colorattrs = [ 'fg', 'bg', 'sp' ]
-  if rv != '' && !!(index(colorattrs, attr)+1) != !!(index(colorattrs, rv)+1)
-    " The attribute the user mapped to was valid, but of a different type.
-    throw "Bad attr map: Can't map color attr to boolean (".attr."->".rv.")"
-  endif
 
-  if rv == 'inverse'
+  for olhs in keys(old)
-    let rv = 'reverse' " Internally always use 'reverse' instead of 'inverse'
+    if olhs ==? 'inverse'
-  elseif rv == 'sp'
+      let nlhs = 'reverse'
-    " Terminals can't handle the guisp attribute; disable it if it was left on
+    endif
-    let rv = ''
-  endif
 
-  return rv
+    let orhs = old[olhs]
+
+    if orhs ==? 'inverse'
+      let nrhs = 'reverse'
+    endif
+
+    let nlhs = tolower(olhs)
+    let nrhs = tolower(orhs)
+
+    try
+      if index(valid_attrs, nlhs) == -1
+        echomsg "CSApprox: Bad attr map (removing unrecognized attribute " . olhs . ")"
+      elseif nrhs != '' && index(valid_attrs, nrhs) == -1
+        echomsg "CSApprox: Bad attr map (removing unrecognized attribute " . orhs . ")"
+      elseif nrhs != '' && !!(index(colorattrs, nlhs)+1) != !!(index(colorattrs, nrhs)+1)
+        echomsg "CSApprox: Bad attr map (removing " . olhs . "; type mismatch with " . orhs . ")"
+      elseif nrhs == 'sp'
+        echomsg "CSApprox: Bad attr map (removing " . olhs . "; can't map to 'sp')"
+      else
+        let new[nlhs] = nrhs
+      endif
+    catch
+      echo v:exception
+    endtry
+  endfor
 endfunction
 
 " {>2} Normalize the GUI settings of a highlight group
@@ -626,6 +632,12 @@ function! s:CSApprox()
 
     set lz
 
+    if exists("g:CSApprox_attr_map") && type(g:CSApprox_attr_map) == type({})
+      call s:NormalizeAttrMap(g:CSApprox_attr_map)
+    else
+      let g:CSApprox_attr_map = { 'italic' : 'underline', 'sp' : 'fg' }
+    endif
+
     " colors_name must be unset and reset, or vim will helpfully reload the
     " colorscheme when we set the background for the Normal group.
     " See the help entries ':hi-normal-cterm' and 'g:colors_name'