unit SumYrsDepUnit;

{$mode objfpc}{$H+}

interface

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

type

  { TSumYrsDepFrm }

  TSumYrsDepFrm = class(TForm)
    HelpBtn: TButton;
    ResetBtn: TButton;
    ComputeBtn: TButton;
    ReturnBtn: TButton;
    CostEdit: TEdit;
    SalvageEdit: TEdit;
    LifeEdit: TEdit;
    PeriodEdit: TEdit;
    DepreciationEdit: 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);
  private
    { private declarations }
    function SYDDepreciation(Cost, Salvage: Extended; Life, Period: Integer): Extended;
  public
    { public declarations }
  end; 

var
  SumYrsDepFrm: TSumYrsDepFrm;

implementation

{ TSumYrsDepFrm }

procedure TSumYrsDepFrm.ResetBtnClick(Sender: TObject);
begin
     CostEdit.Text := '';
     SalvageEdit.Text := '';
     LifeEdit.Text := '';
     DepreciationEdit.Text := '';
     PeriodEdit.Text := '';
end;

procedure TSumYrsDepFrm.ComputeBtnClick(Sender: TObject);
VAR
     Cost, Depreciation, Salvage : Extended;
     Life, Period : integer;

begin

     Cost := StrToFloat(CostEdit.Text);
     Salvage := StrToFloat(SalvageEdit.Text);
     Life := StrToInt(LifeEdit.Text);
     Period := StrToInt(PeriodEdit.Text);
     Depreciation := SYDDepreciation(Cost, Salvage, Life, Period);
     DepreciationEdit.Text := FloatToStr(Depreciation);
end;

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

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

function TSumYrsDepFrm.SYDDepreciation(Cost, Salvage: Extended; Life, Period: Integer): Extended;
{ SYD = (cost - salvage) * (life - period + 1) / (life*(life + 1)/2) }
{ Note: life*(life+1)/2 = 1+2+3+...+life "sum of years"
				The depreciation factor varies from life/sum_of_years in first period = 1
																			 downto  1/sum_of_years in last period = life.
				Total depreciation over life is cost-salvage.}
var
  X1, X2: Extended;
begin
  Result := 0;
	if (Period < 1) or (Life < Period) or (Cost <= Salvage) then Exit;
  X1 := 2 * (Life - Period + 1);
  X2 := Life * (Life + 1);
  Result := (Cost - Salvage) * X1 / X2
end;

initialization
  {$I sumyrsdepunit.lrs}

end.