{$mode objfpc}
unit u2listC; // кольцевые двусвязные списки
interface

type
	P2list=^T2list;
	T2list=record
		key:integer;
		pprev:p2list;
		pnext:p2list;
	end;

function createElement(key:integer):p2list;
procedure addfirstelement(var PH:p2list; p:p2list);
procedure addlastelement(var PH:p2list; p:p2list);
procedure InsertElementBefore(var El:p2list; p:p2list);
procedure InsertElementAfter(var El:p2list; p:p2list);

function deleteElement(var El:p2list):p2list;
procedure disposeList(var pH:p2list);
function countElements(pH:p2list):integer;

type
 TlistAction=procedure(key:integer);
procedure listAction(p:p2list; proc:TListAction; 
					 backward:boolean=false); 

implementation

function createElement(key:integer):p2list;
begin
new(result);
result^.key:=key;
result^.pnext:=nil;
result^.pprev:=nil;
end;



procedure addfirstelement(var PH:p2list; p:p2list);
begin
 addLastElement(ph,p);
 PH:=p;
end;


procedure addlastelement(var PH:p2list; p:p2list);
begin
 if ph=nil then begin
  ph:=p;
  ph^.pnext:=ph;
  ph^.pprev:=ph;
  exit;
 end;
 p^.pprev:=ph^.pprev;
 ph^.pprev^.pnext:=p;
 p^.pnext:=PH;
 ph^.pprev:=p;
end;

{
procedure InsertElementBefore(var PH:p2list; var El:p2list; p:p2list);
begin
 if ph=nil then 
  addlastelement(ph,p)
 else 
  addlastelement(El,p);
end;

procedure InsertElementAfter(var PH:p2list; var El:p2list; p:p2list);
begin
 if ph=nil then
  InsertElementBefore(ph,el,p)
 else
  InsertElementBefore(ph,el^.pnext,p)
end;
}

procedure InsertElementBefore(var El:p2list; p:p2list);
begin
  addlastelement(El,p);
end;

procedure InsertElementAfter(var El:p2list; p:p2list);
begin
 if ph=nil then
  InsertElementBefore(el,p)
 else
  InsertElementBefore(el^.pnext,p)
end;

// ------------------- TODO -----------------------

function deleteElement(var El:p2list):p2list;
begin
result:=El;
if El=nil then exit;

if (el^.pprev=nil)and(el^.pnext=nil) then begin
 // единственный элемент
 el:=nil;
 exit;
end;

if el^.pnext=nil then begin
 // последний элемент
 el^.pprev^.pnext:=nil;
 el^.pprev:=nil;
 exit; 
end;

if el^.pprev=nil then begin
 // первый элемент
 el^.pnext^.pprev:=nil;
 el:=el^.pnext;
 result^.pnext:=nil;
 exit; 
end;

// в середине списка
el^.pprev^.pnext:=el^.pnext;
el^.pnext^.pprev:=el^.pprev;
el^.pprev:=nil;
el^.pnext:=nil;
end;


procedure disposeList(var pH:p2list);
begin
 if pH=nil then exit;
 disposeList(pH^.pnext);
 dispose(pH);
 pH:=nil;
end;


function countElements(pH:plist):integer;
 var c:plist;
begin
 c:=pH;
 result:=0;
 while c <>nil do begin
  c:=c^.pnext;
  inc(result);
 end;
end;


procedure listAction(p:p2list; proc:TListAction;
					 backward:boolean=false); 
var c:p2list;
begin
 c:=p;
 while c<>nil do begin 
  proc(c^.key); 
  if backward then c:=c^.pprev else c:=c^.pnext; 
 end;
end;

end.