//код иллюстрирует алгоритм подсчета. В жизни НЕ НАДО так работать со строками.
var
  I: Integer;
  S: string;
  LChars1: TStringList;
  LChars2: TStringList;
  LChars3: TStringList;
  LChars4: TStringList;

begin
  with TStringList.Create do
  try
    LoadFromFile('.....README.builtin_functions.txt');
    S := Text;
    S := StringReplace(S, sLineBreak, #32, [rfReplaceAll]);
    S := StringReplace(S, #10, #32, [rfReplaceAll]);
    S := StringReplace(S, #13, #32, [rfReplaceAll]);
    S := StringReplace(S, #9, #32, [rfReplaceAll]);
    while Pos(#32#32, S) > 0 do
      S := StringReplace(S, #32#32, #32, [rfReplaceAll]);

    LChars1 := TStringList.Create;
    LChars2 := TStringList.Create;
    LChars3 := TStringList.Create;
    LChars4 := TStringList.Create;
    try
      LChars1.Sorted := True;
      LChars2.Sorted := True;
      LChars3.Sorted := True;
      LChars4.Sorted := True;

      LChars1.Duplicates := dupIgnore;
      LChars2.Duplicates := dupIgnore;
      LChars3.Duplicates := dupIgnore;
      LChars4.Duplicates := dupIgnore;

      LChars1.CaseSensitive := True;
      LChars2.CaseSensitive := True;
      LChars3.CaseSensitive := True;
      LChars4.CaseSensitive := True;

      for I := 1 to Length(S) do
      begin
        LChars1.Add(Copy(S, I, 1));
        if I >= 2 then
          LChars2.Add(Copy(S, I - 1, 2));
        if I >= 3 then
          LChars3.Add(Copy(S, I - 2, 3));
        if I >= 4 then
          LChars4.Add(Copy(S, I - 3, 4));
      end;

      LChars1.SaveToFile('by1.txt');
      LChars2.SaveToFile('by2.txt');
      LChars3.SaveToFile('by3.txt');
      LChars4.SaveToFile('by4.txt');

    finally
      LChars1.Free;
      LChars2.Free;
      LChars3.Free;
      LChars4.Free;
    end;
  finally
    Free;
  end;