day1

.gitignore

@@ -2,3 +2,4 @@ Mercury/
 *.mh
 *.err
 aoc
+input/

aoc.m

@@ -31,7 +31,8 @@ main2(!IO) :-
 :- inst solution == (pred(in, in, di, uo) is det).
 
 :- pred solution(int::in, solution::out(solution)) is semidet.
-solution(_, _) :- fail.
+:- import_module day1.
+solution(1, day1.run).
 
 :- pred run(int::in, part::in, list(string)::in, io::di, io::uo) is det.
 

day1.m

@@ -0,0 +1,46 @@
+:- 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 die.
+:- import_module std_util.
+:- import_module string.
+:- import_module int.
+
+run(Part, Args, !IO) :-
+  if Args = [File] then (
+    read_named_file_as_lines(File, LinesM, !IO),
+    need(LinesM, Lines),
+    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(int_sum, gather(Lines))).
+
+:- func gather(list(string)::in) = (list(list(string))::out(non_empty_list)).
+gather([]) = [[]].
+gather([X | Xs]) = Groups :-
+  [Ys | Yss] = gather(Xs),
+  (if X = "" then Groups = [[], Ys | Yss]
+             else Groups = [[X | Ys] | Yss]).
+
+:- func sum(list(int)) = int.
+sum(Xs) = foldl(func(X, Y) = X + Y, Xs, 0).
+
+:- func int_sum(list(string)) = int.
+int_sum(Xs) = sum(map(det_to_int, Xs)).
+
+:- pred need(io.res(T)::in, T::out) is det.
+need(ok(X),    X).
+need(error(E), _) :- die(error_message(E)).