unit DblDeclineUnit;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, LResources, Forms, Controls, Graphics, Dialogs,
  StdCtrls, Math, contexthelpunit;

type

  { TDblDeclineFrm }

  TDblDeclineFrm = class(TForm)
    HelpBtn: TButton;
    ResetBtn: TButton;
    ComputeBtn: TButton;
    ReturnBtn: TButton;
    CostEdit: TEdit;
    LifeEdit: TEdit;
    EndEdit: TEdit;
    PeriodEdit: TEdit;
    DeprecEdit: TEdit;
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    Label4: TLabel;
    Label5: TLabel;
    procedure ComputeBtnClick(Sender: TObject);
    procedure FormShow(Sender: TObject);
    procedure HelpBtnClick(Sender: TObject);
    procedure ResetBtnClick(Sender: TObject);
    function DoubleDecliningBalance(Cost, Salvage: Extended; Life, Period: Integer): Extended;
  private
    { private declarations }
  public
    { public declarations }
  end; 

var
  DblDeclineFrm: TDblDeclineFrm;

implementation

{ TDblDeclineFrm }

procedure TDblDeclineFrm.ResetBtnClick(Sender: TObject);
begin
  CostEdit.Text := '';
  LifeEdit.Text := '';
  EndEdit.Text := '';
  PeriodEdit.Text := '';
  DeprecEdit.Text := '';
end;

procedure TDblDeclineFrm.FormShow(Sender: TObject);
begin
  ResetBtnClick(self);
end;

procedure TDblDeclineFrm.HelpBtnClick(Sender: TObject);
begin
  ContextHelpForm.HelpMessage((Sender as TButton).tag);
end;

procedure TDblDeclineFrm.ComputeBtnClick(Sender: TObject);
VAR
     Depreciation, Cost, Salvage : Extended;
     Life, Period : integer;
begin
     Cost := StrToFloat(CostEdit.Text);
     Salvage := StrToFloat(EndEdit.Text);
     Life := StrToInt(LifeEdit.Text);
     Period := StrToInt(PeriodEdit.Text);
     Depreciation := DoubleDecliningBalance(Cost, Salvage, Life, Period);
     DeprecEdit.Text := FloatToStr(Depreciation);
end;

function TDblDeclineFrm.DoubleDecliningBalance(Cost, Salvage: Extended; Life, Period: Integer): Extended;
{ dv := cost * (1 - 2/life)**(period - 1)
  DDB = (2/life) * dv
  if DDB > dv - salvage then DDB := dv - salvage
  if DDB < 0 then DDB := 0
}
var
  DepreciatedVal, Factor: Extended;
begin
  Result := 0;
	if (Period < 1) or (Life < Period) or (Life < 1) or (Cost <= Salvage) then
    Exit;

	{depreciate everything in period 1 if life is only one or two periods}
	if ( Life <= 2 ) then
  begin
		if ( Period = 1 ) then
      DoubleDecliningBalance:=Cost-Salvage
		else
			DoubleDecliningBalance:=0; {all depreciation occurred in first period}
		exit;
  end;
  Factor := 2.0 / Life;

  DepreciatedVal := Cost * IntPower((1.0 - Factor), Period - 1);
	   {DepreciatedVal is Cost-(sum of previous depreciation results)}

  Result := Factor * DepreciatedVal;
	   {Nominal computed depreciation for this period.  The rest of the
			function applies limits to this nominal value. }

	{Only depreciate until total depreciation equals cost-salvage.}
  if Result > DepreciatedVal - Salvage then
    Result := DepreciatedVal - Salvage;

	{No more depreciation after salvage value is reached.  This is mostly a nit.
	 If Result is negative at this point, it's very close to zero.}
  if Result < 0.0 then
    Result := 0.0;
end;

initialization
  {$I dbldeclineunit.lrs}

end.