Index: src/natools-smaz_generic-tools.adb
==================================================================
--- src/natools-smaz_generic-tools.adb
+++ src/natools-smaz_generic-tools.adb
@@ -515,23 +515,22 @@
 
 
    function Worst_Index
      (Dict : in Dictionary;
       Counts : in Dictionary_Counts;
-      Method : in Smaz_Tools.Methods.Enum)
+      Method : in Smaz_Tools.Methods.Enum;
+      First, Last : in Dictionary_Code)
      return Dictionary_Code
    is
       use type Smaz_Tools.Score_Value;
 
-      Result : Dictionary_Code := Dictionary_Code'First;
+      Result : Dictionary_Code := First;
       Worst_Score : Smaz_Tools.Score_Value
         := Score (Dict, Counts, Result, Method);
       S : Smaz_Tools.Score_Value;
    begin
-      for I in Dictionary_Code'Succ (Dictionary_Code'First)
-               .. Dict.Last_Code
-      loop
+      for I in Dictionary_Code'Succ (First) .. Last loop
          S := Score (Dict, Counts, I, Method);
 
          if S < Worst_Score then
             Result := I;
             Worst_Score := S;

Index: src/natools-smaz_generic-tools.ads
==================================================================
--- src/natools-smaz_generic-tools.ads
+++ src/natools-smaz_generic-tools.ads
@@ -123,16 +123,27 @@
       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 Smaz_Tools.Methods.Enum;
+      First, Last : in Dictionary_Code)
+     return Dictionary_Code
+     with Pre => Last in First .. Dict.Last_Code;
+      --  Return the element from the given interval with worst score
+
    function Worst_Index
      (Dict : in Dictionary;
       Counts : in Dictionary_Counts;
       Method : in Smaz_Tools.Methods.Enum)
-     return Dictionary_Code;
-      --  Return the element with worst score
+     return Dictionary_Code
+     is (Worst_Index (Dict, Counts, Method,
+                      Dictionary_Code'First, Dict.Last_Code));
+      --  Return the element with worst score in the whole directionary
 
 
    function Score_Encoded
      (Dict : in Dictionary;
       Counts : in Dictionary_Counts;