Programmers day in Erlang

pday.erl

-module(pday).
-export([main/1, pday/1, ordinal_to_date/2, date_to_string/1]).


ordinal_to_date(Year, N) ->
    {Month, Day} = submod(N, months(Year)),
    {Year, Month + 1, Day}.


date_to_string({Year, Month, Day}) ->
    lists:flatten(io_lib:format("~4..0b-~2..0b-~2..0b", [Year, Month, Day])).


pday(Year) ->
    ordinal_to_date(Year, 256).


months(Year) ->
    Leap = is_leap(Year),
    Feb = if Leap -> 29; true -> 28 end,
    [31, Feb, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31].


is_leap(Year) when Year rem 400 == 0 ->
    true;
is_leap(Year) when Year rem 100 == 0 ->
    false;
is_leap(Year) when Year rem 4 == 0 ->
    true;
is_leap(_) ->
    false.


submod(X, Ys) ->
    submod(X, 0, Ys).


submod(X, Sub, [Y|_]) when X =< Y ->
    {Sub, X};
submod(X, Sub, [Y|Ys]) ->
    submod(X - Y, Sub + 1, Ys).


main([Year]) ->
    D = pday(list_to_integer(Year)),
    io:format("~s~n", [date_to_string(D)]);
main(_) ->
    io:format("usage: escript pday.erl YEAR~n").