ListLink

program ListLink;

uses
crt;

type
  TpListElement = ^TListElement;
  TListElement = record
    info : integer;
    previous : TpListElement;
    next : TpListElement;
  end;

var
  pListFirst : TpListElement;
  pTemp : ^TListElement;
  answer : char;
  element, count : integer;
  listIsEmpty : boolean;

procedure pushAfter();
var
  pTemp, pTempPrev, pTempNext : TpListElement;
  element, afterElement : integer;
begin
  new(pTemp);
  if (count = 0) then
  begin
    write('List is empty. Enter the first element: ');
    readln(element);
    pTemp^.info := element;
    count += 1;
    pListFirst := pTemp;
  end else
  begin
    write('Enter the element: ');
    readln(element);
    write('Enter the element AFTER which you want to add a new one: ');
    readln(afterElement);
    pTempPrev := pListFirst;
    while (pTempPrev^.info <> afterElement) do
    begin
      pTempPrev := pTempPrev^.next;
    end;
    pTemp^.info := element;
    count += 1;
    pTempNext := pTempPrev^.next;
    pTempPrev^.next := pTemp;
    pTemp^.previous := pTempPrev;
    if (pTempNext <> nil) then pTempNext^.previous := pTemp;
    pTemp^.next := pTempNext;
  end;
end;

procedure pushBefore();
var
  pTemp, pTempPrev, pTempNext : TpListElement;
  element, beforeElement : integer;
begin
  new(pTemp);
  if (count = 0) then
  begin
    write('List is empty. Enter the first element: ');
    readln(element);
    pTemp^.info := element;
    count += 1;
    pListFirst := pTemp;
  end else
  begin
    write('Enter the element: ');
    readln(element);
    write('Enter the element BEFORE which you want to add a new one: ');
    readln(beforeElement);
    pTempNext := pListFirst;
    while (pTempNext^.info <> beforeElement) do
    begin
      pTempNext := pTempNext^.next;
    end;
    pTemp^.info := element;
    count += 1;
    pTempPrev := pTempNext^.previous;
    pTemp^.previous := pTempPrev;
    if (pTempPrev <> nil) then pTempPrev^.next := pTemp;
    pTemp^.next := pTempNext;
    if (pTempNext^.previous = nil) then pListFirst := pTemp;
    pTempNext^.previous := pTemp;
  end;
end;

procedure pop();
var
  pTemp, pTempPrev, pTempNext : TpListElement;
  element : integer;
begin
  write('Enter the element to remove: ');
  readln(element);
  pTemp := pListFirst;
  while (pTemp^.info <> element) do pTemp := pTemp^.next;
  pTempPrev := pTemp^.previous;
  pTempNext := pTemp^.next;
  if (pTempNext <> nil) then pTempNext^.previous := pTempPrev;
  if (pTempPrev <> nil) then pTempPrev^.next := pTempNext else pListFirst := pTempNext;
  dispose(pTemp);
  count -= 1;
end;

procedure output;
var
  pTemp : TpListElement;
begin
  pTemp := pListFirst;
  while (pTemp <> nil) do
  begin
    write(pTemp^.info, ' ');
    pTemp := pTemp^.next;
  end;
  writeln;
  writeln('Count: ', count);
end;

begin
  count := 0;

  repeat
    writeln('1. Add element AFTER');
    writeln('2. Add element BEFORE');
    writeln('3. Remove element');
    writeln('4. Output');
    readln(answer);
    case answer of
    '1':
      begin
        pushAfter;
      end;
    '2':
      begin
        pushBefore;
      end;
    '3':
      begin
        pop;
      end;
    '4':
      begin
        output;
      end;
    end;

  until (answer = 'n');

end.