fix strcoll() workaround

The strcoll() workaround we had in place for Kobo devices was (or has
become) ineffective. We had set self.strcoll to nil on Kobo devices -
but this was the instance variable. Setting it to nil effectively makes
the instance variable vanish, so when trying to access it later, it
was not there and got looked up via the metatable - which had the original
reference. Setting it to nil had no effect whatsoever.

We simplify that approach and set the replacement function where before we
had set this to nil.

This is a partial fix for issue #1183 (and explains a comment in issue #686
which says that the old fix did not work).

However, to really fix #1183 - if we want to do so - we would need a collate
function that normalizes uppercase/lowercase before compare.

frontend/ui/widget/filechooser.lua

@@ -8,12 +8,12 @@ local DEBUG = require("dbg")
 local _ = require("gettext")
 local ffi = require("ffi")
 ffi.cdef[[
-int strcoll (char *str1, char *str2);
+int strcoll (const char *str1, const char *str2);
 ]]
 
 -- string sort function respecting LC_COLLATE
 local function strcoll(str1, str2)
-    return ffi.C.strcoll(ffi.cast("char*", str1), ffi.cast("char*", str2)) <= 0
+    return ffi.C.strcoll(str1, str2) < 0
 end
 
 local FileChooser = Menu:extend{
@@ -38,8 +38,10 @@ function FileChooser:init()
         end
         return true
     end
-    -- disable string collating in Kobo devices. See issue koreader/koreader#686
-    if Device:isKobo() then self.strcoll = nil end
+    -- circumvent string collating in Kobo devices. See issue koreader/koreader#686
+    if Device:isKobo() then
+        self.strcoll = function(a, b) return a < b end
+    end
     self.item_table = self:genItemTableFromPath(self.path)
     Menu.init(self) -- call parent's init()
 end
@@ -71,11 +73,9 @@ function FileChooser:genItemTableFromPath(path)
     local sorting = nil
     local reverse = self.reverse_collate
     if self.collate == "strcoll" then
-        sorting = self.strcoll and function(a, b)
-                return self.strcoll(a.name, b.name) == not reverse
-            end or function(a, b)
-                return (a.name < b.name) == not reverse
-            end
+        sorting = function(a, b)
+            return self.strcoll(a.name, b.name) == not reverse
+        end
     elseif self.collate == "access" then
         sorting = function(a, b)
             if reverse then