pension2.erl

-module(pension2).

-export([totalBalance/1]).

%% memoization uses a process dictionary. While hackish and not portable
%% through processes, a PD of this kind has the advantage of being
%% garbage collected and being returned in a stack trace. There is also no
%% copying (see: comments over the macros) or locking needed.
-define(memoize(E), lazy_memoize(fun()-> E end)).
lazy_memoize(F) when is_function(F) ->
    case erlang:get(F) of
        undefined ->
            erlang:put(F,F()),
            erlang:get(F);
        X -> X
    end.

%% total of what you own:
totalBalance(T) -> caBalance(T) + saBalance(T).

%% balance of cash account - is constant since all money not spend is invested:
caBalance(0) -> caBalanceStart();
caBalance(T) ->
    ?memoize(caBalance(T-1)) +
    ?memoize(salary(T-1)) -
    ?memoize(expenses(T-1)) -
    ?memoize(tradingFees(T-1)).

%% balance of security account - rises while working - can rise while not working if yield > expenses
saBalance(0) -> saBalanceStart();
saBalance(T) -> ?memoize(saBalance(T-1)) * yieldRate() + ?memoize(investments(T-1)).

%% everything spend on securities - in case of not working fees and taxes are directly offset by sells
tradingFees(T) ->
    case working(T) of
        true -> investments(T) + transactionFee();
        false -> investments(T) + tax(expenses(T),T) + transactionFee()
    end.

%% what is actually spend on stocks:
investments(T) ->
    case working(T) of
        true-> salary(T) - expenses(T) - transactionFee();
        false-> -expenses(T) - tax(expenses(T),T) - transactionFee()
    end.

%% salary:
salary(0) -> netIncome();
salary(T) ->
    case working(T) of
        true -> salary(T-1)*inflationRate();
        false -> 0
    end.

%% expenses
expenses(0) -> expensesStart();
expenses(T) -> ?memoize(expenses(T-1))*inflationRate().

%% still working?
working(T) -> T<workingTimeUnits().

%% lots of constants - in respect to months
transactionFee() -> 10.
workingTimeUnits() -> 60. %%months
caBalanceStart() -> 10000.
saBalanceStart() -> 10000.
netIncome() -> 3000.
expensesStart() -> 2000.
inflationRate() -> 1.0017.
yieldRate() -> 1.004.
taxRate() -> 0.2638.

%% ------------HERE STARTS THE HEAVY LIFTING-----------------

%% used to calc taxes payed on profits
tax(X,T) -> ?memoize(profit(X,T,?memoize(portfolio(T))))*taxRate().

%% what is in the security account at time T:
portfolio(T) ->
    case working(T) of
        true -> [{?memoize(investments(X)),X} || X <- lists:seq(0,T)];
        false -> remove(?memoize(investments(T-1)),T,?memoize(portfolio(T-1)))
    end.

%% a helper method for removing positions from the portfolio
remove(X,T,[{Amount,Time}|Tail]) ->
    case currentValue(Amount,T-Time) > X of
        true -> [{Amount-?memoize(originalValue(X,T-Time)),T}|Tail];
        false -> remove(X-currentValue(Amount,T-Time),T,Tail)
    end.

%% tells you what is the actual profit for an amount/revenue generated by selling at a time T
profit(X,T,[{Amount,Time}|Tail]) ->
    case currentValue(Amount,T-Time) > X of
        true -> X - ?memoize(originalValue(X,T-Time));
        false -> X - (Amount + ?memoize(profit((X - ?memoize(currentValue(Amount,T-Time))),T,Tail)))
    end.

%% helper functions for calculation the current and original value of a position
currentValue(X,TD) -> X * math:pow(yieldRate(),TD).
originalValue(X,TD) -> X / math:pow(yieldRate(),TD)

Can you write "money denomination" code in erlang Language ?