[fpc-devel] "Blank slate" next version of FPC

[Ben Grasset]

On Fri, Feb 22, 2019 at 1:07 AM Paul van Helden <paul at planetgis.co.za>
wrote:

>  How do you make a (for in) enumerator with a record? I don't use them for
> exactly this reason, and they did seem to be another useful language
> feature that turned out to be poorly implemented by Embarcadero. (Haven't
> checked with FPC).
>

Here's an example (for FPC) that demonstrates it by implementing the
"take-while" pattern:

program TakeWhileExample;

{$mode Delphi}{$H+}{$J-}
{$modeswitch NestedProcVars}
{$ImplicitExceptions Off}
{$PointerMath On}

type
  TSlice<T> = record
  public type
    PT = ^T;
    ArrayType = array of T;
  private
    FFirst, FLast, FCurrent: PT;
    function GetCurrent: T; inline;
  public
    function GetEnumerator: TSlice<T>; inline;
    function MoveNext: Boolean; inline;
    class function TakeWhile(const A: ArrayType; function F(const Val: T):
Boolean): TSlice<T>; static; inline;
    property Current: T read GetCurrent;
  end;

  TTestFunc<T> = function(const Val: T): Boolean;

  function TSlice<T>.GetCurrent: T;
  begin
    Result := FCurrent^;
  end;

  function TSlice<T>.GetEnumerator: TSlice<T>;
  begin
    Result := Self;
    with Result do FCurrent := FFirst - 1;
  end;

  function TSlice<T>.MoveNext: Boolean;
  begin
    Inc(FCurrent);
    Exit((FCurrent <= FLast) and (FFirst <> FLast));
  end;

  function Test(const Val: SizeUInt): Boolean; inline;
  begin
    Exit((Val < 50000000));
  end;

  class function TSlice<T>.TakeWhile(const A: ArrayType; function F(const
Val: T): Boolean): TSlice<T>;
  var
    I: SizeUInt;
    X: TTestFunc<T> absolute F;
    //FPC generates slightly better code for the "absolute" way, not sure
why...
  begin
    with Result do begin
      FFirst := @A[0];
      FLast := @A[0];
      for I := 0 to High(A) do
        case X(A[I]) of
          True: Inc(FLast);
          False: Exit();
        end;
    end;
  end;

var
  I, J: SizeUInt;
  Arr: TSlice<SizeUInt>.ArrayType;

begin
  SetLength(Arr, 100000000);
  for I := 0 to 99999999 do Arr[I] := I;
  I := 0;
  J := 0;
  for I in TSlice<SizeUInt>.TakeWhile(Arr, Test) do J := I;
  WriteLn(J);
end.
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