(* For ML lecture 4 *)

(* Question 2: simple bank account module *)

module BankImpl = 
  struct
    type account = string * int ref
    exception Underflow
    exception InvalidPassword
    exception InvalidOperation

    let accounts = ref []
    let banktreasure = ref 10000

    let do_cashier (_, cur : account) amount = 
      cur := !cur + amount;
      banktreasure := !banktreasure + amount

    let pass_check (p, _ : account) pass = 
      if (p = pass) then () else raise InvalidPassword

    let balance_int (_, cur : account) = 
      !cur

    let new_account password initial =
      if initial <= 0 then raise InvalidOperation else begin
	let instance = (password, ref 0 : account) in
	accounts := instance :: !accounts;
	do_cashier instance initial;
	instance
      end

    let balance acc pass = 
      pass_check acc pass;
      balance_int acc

    let deposit acc amount =
      if amount <= 0 then raise InvalidOperation
      else do_cashier acc amount

    let withdraw acc pass amount = 
      if amount <= 0 then raise InvalidOperation
      else begin
	pass_check acc pass;
	if balance_int acc < amount then raise Underflow
	else do_cashier acc (-amount)
      end

    let bank_statistics () = 
      List.length !accounts, !banktreasure
  end

(*
  # open BankImpl;;
  # let a1 = new_account "pass1" 50;;
  val a1 : BankImpl.account = ("pass1", {contents = 50})
  # bank_statistics ();;
  - : int * int = (1, 10050)
  # withdraw a1 "foobar" 30;;
  Exception: BankImpl.InvalidPassword.
  # withdraw a1 "pass1" 30;;
  - : unit = ()
  # withdraw a1 "pass1" 100;;
  Exception: BankImpl.Underflow.
  # deposit a1 50;;
  - : unit = ()
  # balance a1 "pass1";;
  - : int = 70
  # let a2 = new_account "pass2" 1000;;
  val a2 : BankImpl.account = ("pass2", {contents = 1000})
  # bank_statistics ();;
  - : int * int = (2, 11070)
*)