test.adb

with Ada.Text_IO; use Ada.Text_IO;

procedure Test is
   type UInt16 is mod 2 ** 16
      with Size => 16;
   type UInt32 is mod 2 ** 32
      with Size => 32;
   type Words is array (Positive range <>) of UInt16
      with Component_Size => 16;

   function Checksum (Data : Words)
      return UInt16
   is
      Sum : UInt32 := 0;
   begin
      for D of Data loop
         Sum := Sum + UInt32 (D);
         if Sum > 16#FFFF# then
            Sum := Sum - 16#FFFF#;
         end if;
      end loop;
      return UInt16 (Sum);
   end Checksum;

   function Checksum (Data : String)
      return UInt16
   is
      D : Words (1 .. Data'Length / 2)
         with Address => Data'Address;
   begin
      return Checksum (D);
   end Checksum;

   Sum : UInt16;
begin
   Sum := Checksum ("Test");
   Put_Line (Sum'Image);
end Test;