Fun with enumerators, part 6 - generators

[Part 1 - Introduction, Part 2 - Additional enumerators, Part 3 - Parameterized enumerators, Part 4 - External enumerators, Part 5 - Class helper enumerators.]

This was a long and interesting trip but it has to end some day. And it will end today. Just one last small topic to cover ...

in Part 4 I mentioned that we could abuse enumerators to the point where they are not working on any external structure. My example for such generator which is providing its own enumeration data was

for i in Fibonacci(10) do
  Writeln(i);

and I promised to show you how to write it.

Again, we will start with a 'standard' enumerator factory, the one that we used a lot in parts 4 and 5.

  IEnumFibonacciFactory = interface
    function GetEnumerator: TEnumFibonacci;
  end;

  TEnumFibonacciFactory = class(TInterfacedObject, IEnumFibonacciFactory)
  private
    FUpperBound: integer;
  public
    constructor Create(upperBound: integer);
    function GetEnumerator: TEnumFibonacci;
  end;

  function Fibonacci(upperBound: integer): IEnumFibonacciFactory;

Enumerator is slightly trickier this time - it prepares requested data in the constructor and uses MoveNext to move over this data.

  TEnumFibonacci = class
  private
    FFibArray: array of integer;
    FIndex: integer;
  public
    constructor Create(upperBound: integer);
    function GetCurrent: integer;
    function MoveNext: boolean;
    property Current: integer read GetCurrent;
  end;

constructor TEnumFibonacci.Create(upperBound: integer);
var
  i: integer;
begin
  SetLength(FFibArray, upperBound);
  if upperBound >= 1 then
    FFibArray[0] := 1;
  if upperBound >= 2 then
    FFibArray[1] := 1;
  for i := 2 to upperBound - 1 do
    FFibArray[i] := FFibArray[i-1] + FFibArray[i-2];
  FIndex := -1;
end;

function TEnumFibonacci.GetCurrent: integer;
begin
  Result := FFibArray[FIndex];
end;

function TEnumFibonacci.MoveNext: boolean;
begin
  Result := FIndex < High(FFibArray);
  if Result then
    Inc(FIndex);
end;

Test code follows the well-learned pattern.

procedure TfrmFunWithEnumerators.btnFibonacciClick(Sender: TObject);
var
  i : integer;
  ln: string;
begin
  ln := '';
  for i in Fibonacci(10) do begin
    if ln <> '' then
      ln := ln + ', ';
    ln := ln + IntToStr(i);
  end;
  lbLog.Items.Add('Generator: ' + ln);
end; 

And now it really is a time to say goodbye. At least to this series - I intend to write many more blog entries. It was an interesting experience for me and I hope an interesting reading for you, dear reader. If you liked it, tell that to others so that they may enjoy it too.

[A full source code of the demo program including all enumerators is available at http://17slon.com/blogs/gabr/files/FunWithEnumerators.zip]

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