Index: src/natools-smaz-tools.adb
==================================================================
--- src/natools-smaz-tools.adb
+++ src/natools-smaz-tools.adb
@@ -917,18 +917,19 @@
 
 
    function Worst_Index
      (Dict : in Dictionary;
       Counts : in Dictionary_Counts;
-      Method : in Methods.Enum)
+      Method : in Methods.Enum;
+      First, Last : in Ada.Streams.Stream_Element)
      return Ada.Streams.Stream_Element
    is
-      Result : Ada.Streams.Stream_Element := 0;
-      Worst_Score : Score_Value := Score (Dict, Counts, 0, Method);
+      Result : Ada.Streams.Stream_Element := First;
+      Worst_Score : Score_Value := Score (Dict, Counts, First, Method);
       S : Score_Value;
    begin
-      for I in 1 .. Dict.Dict_Last loop
+      for I in First + 1 .. Last loop
          S := Score (Dict, Counts, I, Method);
 
          if S < Worst_Score then
             Result := I;
             Worst_Score := S;

Index: src/natools-smaz-tools.ads
==================================================================
--- src/natools-smaz-tools.ads
+++ src/natools-smaz-tools.ads
@@ -204,16 +204,26 @@
       Counts : in out Dictionary_Counts);
       --  Compress all strings of Corpus, returning the total number of
       --  compressed bytes and the number of uses for each dictionary
       --  element.
 
+   function Worst_Index
+     (Dict : in Dictionary;
+      Counts : in Dictionary_Counts;
+      Method : in Methods.Enum;
+      First, Last : in Ada.Streams.Stream_Element)
+     return Ada.Streams.Stream_Element
+     with Pre => Last in First .. Dict.Dict_Last;
+      --  Return the element with worst score in the whole directionary
+
    function Worst_Index
      (Dict : in Dictionary;
       Counts : in Dictionary_Counts;
       Method : in Methods.Enum)
-     return Ada.Streams.Stream_Element;
-      --  Return the element with worst score
+     return Ada.Streams.Stream_Element
+     is (Worst_Index (Dict, Counts, Method, 0, Dict.Dict_Last));
+      --  Return the element with worst score in the whole directionary
 
 
    type Score_Value is range 0 .. 2 ** 31 - 1;
 
    function Score_Encoded