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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Copying string slices before calling subroutines?
Newsgroups: comp.lang.ada
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Date: Sat, 5 May 2007 09:41:57 +0200
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Date: 2007-05-05T09:41:22+02:00
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On Fri, 04 May 2007 23:27:57 +0100, Simon Wright wrote:

> My reworking begins
> 
>    function Index
>      (Source  : String;
>       Pattern : String;
>       Going   : Ada.Strings.Direction := Ada.Strings.Forward;
>       Mapping : Ada.Strings.Maps.Character_Mapping
>         := Ada.Strings.Maps.Identity) return Natural
>    is
>       Cur_Index       : Natural;
>       Potential_Match : Boolean;
>       use Ada.Strings;
>       use Ada.Strings.Maps;
>    begin
>       if Pattern = "" then
>          raise Pattern_Error;
>       end if;
> 
>       --  Forwards case
> 
>       if Going = Forward then
>          for J in 1 .. Source'Length - Pattern'Length + 1 loop
>             Cur_Index := Source'First + J - 1;
>             Potential_Match := True;
>             for K in Pattern'Range loop
>                if Pattern (K) /=
>                  Value (Mapping, Source (Cur_Index + K - 1)) then
>                   Potential_Match := False;
>                   exit;
>                end if;
>             end loop;
>             if Potential_Match then
>                return Cur_Index;
>             end if;
>          end loop;
> 
> which calls Ada.Strings.Maps.Value rather more often than I suppose it
> could.

You can save remapping of Source, just there is no need to do it in advance
and allocate a full copy of Source. Instead of that, you make a ring buffer
of mod Pattern'Length where you store Value (Mapping, Source (i)). Once you
advance the main string index you "rotate" the buffer.

Then Pattern'Length = 1 should be a special case, as well as identity
mapping. Though, I don't know how efficient the latter could be.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de