TOOLS/lua/autoload: further optimize the natural sorting

Commits 257dbdf06f uses a algorithm of splitting strings
into a table of number and string values to achieve natural sorting.
This approach works well in most cases, but it does not work well
in some specific scenarios.
Now let's implement natural sorting with a stronger algorithm.

Also fixes indentation.

Ref: 3cf323f3c4
Closes https://github.com/mpv-player/mpv/issues/8969

TOOLS/lua/autoload.lua

@@ -94,47 +94,31 @@ table.filter = function(t, iter)
     end
 end
 
--- splitbynum and alnumcomp from alphanum.lua (C) Andre Bogus
--- Released under the MIT License
--- http://www.davekoelle.com/files/alphanum.lua
+-- alphanum sorting for humans in Lua
+-- http://notebook.kulchenko.com/algorithms/alphanumeric-natural-sorting-for-humans-in-lua
 
--- split a string into a table of number and string values
-function splitbynum(s)
-    local result = {}
-    for x, y in (s or ""):gmatch("(%d*)(%D*)") do
-        if x ~= "" then table.insert(result, tonumber(x)) end
-        if y ~= "" then table.insert(result, y) end
+function alphanumsort(o)
+    local function padnum(d)
+        local dec, n = string.match(d, "(%.?)0*(.+)")
+        return #dec > 0 and ("%.12f"):format(d) or ("%s%03d%s"):format(dec, #n, n)
     end
-    return result
-end
-
-function clean_key(k)
-    k = (' '..k..' '):gsub("%s+", " "):sub(2, -2):lower()
-    return splitbynum(k)
-end
-
--- compare two strings
-function alnumcomp(x, y)
-    local xt, yt = clean_key(x), clean_key(y)
-    for i = 1, math.min(#xt, #yt) do
-        local xe, ye = xt[i], yt[i]
-        if type(xe) == "string" then ye = tostring(ye)
-        elseif type(ye) == "string" then xe = tostring(xe) end
-        if xe ~= ye then return xe < ye end
-    end
-    return #xt < #yt
+    table.sort(o, function(a,b)
+        return tostring(a):lower():gsub("%.?%d+",padnum)..("%3d"):format(#b)
+             < tostring(b):lower():gsub("%.?%d+",padnum)..("%3d"):format(#a) 
+    end)
+    return o
 end
 
 local autoloaded = nil
 
 function get_playlist_filenames()
-  local filenames = {}
-  for n = 0, pl_count - 1, 1 do
-    local filename = mp.get_property('playlist/'..n..'/filename')
-    local _, file = utils.split_path(filename)
-    filenames[file] = true
-  end
-  return filenames
+    local filenames = {}
+    for n = 0, pl_count - 1, 1 do
+        local filename = mp.get_property('playlist/'..n..'/filename')
+        local _, file = utils.split_path(filename)
+        filenames[file] = true
+    end
+    return filenames
 end
 
 function find_and_add_entries()
@@ -181,7 +165,7 @@ function find_and_add_entries()
         end
         return EXTENSIONS[string.lower(ext)]
     end)
-    table.sort(files, alnumcomp)
+    alphanumsort(files)
 
     if dir == "." then
         dir = ""