:- module day1.
:- interface.
:- import_module basics.
:- import_module io.
:- import_module list.

:- pred run(part::in, list(string)::in, io::di, io::uo) is det.

:- implementation.
:- import_module std_util.
:- import_module maybe.
:- import_module string.
:- import_module int.

run(Part, Args, !IO) :-
  if Args = [File] then (
    read_lines_need(File, Lines, !IO),
    Totals = go(Lines),
    ( Part = one,
        format("%d\n", [i(det_head(Totals))], !IO)
    ; Part = two,
        Res = sum(take_upto(3, Totals)),
        format("%d\n", [i(Res)], !IO)
    )
  ) else die("expected one readable file").

:- func go(list(string)) = list(int).
go(Lines) =
  sort(converse(ordering), map(sum, gather(map(to_int_may, Lines)))).

:- func gather(list(maybe(T))) = list(list(T)).
gather(Xs) = Ys :- gather(Xs, Ys).

:- pred gather(list(maybe(T))::in, list(list(T))::out(non_empty_list)) is det.
gather([],            [[]]).
gather([no     | Xs], [[]       | Yss]) :- gather(Xs, Yss).
gather([yes(Y) | Xs], [[Y | Ys] | Yss]) :- gather(Xs, [Ys | Yss]).

:- func sum(list(int)) = int.
sum(Xs) = foldl(plus, Xs, 0).