(* MyQueue: simple queues *)

type 'a cell = {
  content : 'a;
  mutable next : 'a cell;
}

type 'a queue = {
  mutable size : int;
  mutable cell : 'a cell option;
}

let create () = {
  size = 0;
  cell = None;
}

let is_empty q = q.size = 0

let length q = q.size

let push q x =
  begin
    q.size <- q.size + 1;
    match q.cell with
      | None
        -> q.cell <- Some (let rec c = {content = x; next = c} in c)
      | Some cell
        ->
        begin
          cell.next <- {
            content = x;
            next = cell.next;
          };
          q.cell <- Some cell.next;
        end
  end

let take q =
  begin
    match q.cell with
      | None
        -> assert false
      | Some cell
        ->
        let res = cell.next.content in
        begin
          q.size <- q.size - 1;
          if q.size = 0 then
            q.cell <- None
          else
            cell.next <- cell.next.next;
          res
        end
  end