Vigenere Cipher Algorithm - Delphi Implementation (Comments by Alan Lloyd)

in Vigenere Cipher :: A long time Delphi developer, Alan Lloyd had a few comments on the recently published Vigenere Cipher - Delphi Implementation - Fancy Delphi Application Contest Entry #49.

Alan suggests:

While we have loads of computation power & memory available today, I think as a professional it is better to code quicker than slower, unless it makes code unclear.

In the posted Vigenere code . . .

1. String memory allocation/re-allocation is slow so avoid ConCat(), String + String (even though its quicker than ConCat), and Delete(). Better to SetLength for strings and use index accessing for characters.
2. Use 'for' loops rather than 'while' loops.
3. Reduce the code for every character. Check for bEncrypt before entering repeated similar code in a 'for' loop for each character action.
4. IMO sTable should be initialised before use by := '' or SetLength(). AIUI variables in functions are not zeroed, and you might be using garbage.
5. Crypt by re-referencing the key as given rather that using multiple keys up to the source length.

Here's the Vigenere crypto algorithm as suggested by Allan:

function Vigenere(Src, Key : string; Encrypt : boolean) : string;
const
  OrdMinChar : integer = Ord('A');
  OrdMaxChar : integer = Ord('Z');
  IncludeChars : set of char = ['A'..'Z'];
var
  CharRangeCount, i, j, KeyLen, KeyInc, SrcOrd, CryptOrd : integer;
  SrcA : string;
begin
  CharRangeCount := OrdMaxChar - OrdMinChar + 1;
  KeyLen := Length(Key);
  SetLength(SrcA, Length(Src));
  If Encrypt then 
  begin
    // transfer only included characters to SrcA for encryption
    j := 1;
    for i := 1 to Length(Src) do
      begin
      if (Src[i] in IncludeChars) then 
      begin
        SrcA[j] := Src[i];
        inc(j);
      end;
    end;
    SetLength(SrcA, j - 1);
  end;
  SetLength(Result, Length(SrcA));
  if Encrypt then
    begin
    // Encrypt to Result
    for i := 1 to Length(SrcA) do 
    begin
      SrcOrd := Ord(Src[i]) - OrdMinChar;
      KeyInc := Ord(Key[((i - 1 ) mod KeyLen)+ 1]) - OrdMinChar;
      CryptOrd := ((SrcOrd + KeyInc) mod CharRangeCount) + OrdMinChar;
      Result[i] := Char(CryptOrd);
    end;
  end;
  else
  begin
    // Decrypt to Result
    for i := 1 to Length(SrcA) do 
    begin
      SrcOrd := Ord(Src[i]) - OrdMinChar;
      KeyInc := Ord(Key[((i - 1 ) mod KeyLen)+ 1]) - OrdMinChar;
      CryptOrd := ((SrcOrd - KeyInc + CharRangeCount) mod CharRangeCount) + OrdMinChar;
      // KeyInc may be larger than SrcOrd
      Result[i] := Char(CryptOrd);
    end;
  end;
end;