Solve day 2

src/aoc2024.app.src

@@ -7,7 +7,10 @@
         stdlib
     ]},
     {env, []},
-    {modules, []},
+    {modules, [
+               day_1,
+               day_2
+    ]},
     {licenses, ["Apache-2.0"]},
     {links, []}
  ]}.

src/day_2.erl

@@ -0,0 +1,125 @@
+-module(day_2).
+
+-export([part_1/0, part_2/0]).
+
+part_1() ->
+    Filename = open_file(),
+    part_1(Filename).
+
+part_1(Filename) ->
+    Reports = parse_input(Filename),
+    SafeReports = lists:filter(fun is_safe/1, Reports),
+    length(SafeReports).
+
+part_2() ->
+    Filename = open_file(),
+    part_2(Filename).
+
+part_2(Filename) ->
+    Reports = parse_input(Filename),
+    SafeReports = lists:filter(fun is_tolerably_safe/1, Reports),
+    length(SafeReports).
+
+%% Private Functions
+
+open_file() ->
+    Filename = filename:absname("./priv/input/2.txt"),
+    open_file(Filename).
+
+open_file(Filename) ->
+    {ok, File} = file:open(Filename, read),
+    File.
+
+parse_input(Filename) ->
+    parse_input(Filename, []).
+
+parse_input(Filename, Reports) ->
+    case file:read_line(Filename) of
+        {ok, Line} ->
+            Trimmed = string:trim(Line),
+            Chunks = string:split(Trimmed, " ", all),
+            NonEmpty = lists:filter(fun(L) -> length(L) > 0 end, Chunks),
+            Report = lists:map(fun list_to_integer/1, NonEmpty),
+            parse_input(Filename, [Report | Reports]);
+        eof ->
+            Reports
+    end.
+
+check_report([_Whatever | []]) ->
+    safe;
+check_report([A | [B | _] = Rest]) when A < B andalso B - A < 4 ->
+    check_report(Rest, asc);
+check_report([A | [B | _] = Rest]) when A > B andalso A - B < 4 ->
+    check_report(Rest, desc);
+check_report(Rest) ->
+    unsafe.
+
+check_report([_A], _Whatever) ->
+    safe;
+check_report([A, B], asc) when A < B andalso B - A < 4 ->
+    safe;
+check_report([A, B], desc) when A > B andalso A - B < 4 ->
+    safe;
+check_report([A | [B | _] = Rest], asc) when A < B andalso B - A < 4 ->
+    check_report(Rest, asc);
+check_report([A | [B | _] = Rest], desc) when A > B andalso A - B < 4 ->
+    check_report(Rest, desc);
+check_report(_Rest, _Direction) ->
+    unsafe.
+
+is_safe(Report) ->
+    safe == check_report(Report).
+
+check_report_with_tolerance(Report) ->
+    case check_report(Report) of
+        safe ->
+            safe;
+        unsafe ->
+            check_report_by_removing_levels(Report)
+    end.
+
+check_report_by_removing_levels([H | Tail]) ->
+    case check_report(Tail) of
+        safe -> safe;
+        unsafe ->
+            check_report_by_removing_levels([H], Tail)
+    end.
+
+check_report_by_removing_levels(_Init, []) ->
+    unsafe;
+check_report_by_removing_levels(Init, [H | Tail]) ->
+    case check_report(Init ++ Tail) of
+        safe ->
+            safe;
+        unsafe ->
+            check_report_by_removing_levels(Init ++ [H], Tail)
+    end.
+
+
+is_tolerably_safe(Report) ->
+    safe == check_report_with_tolerance(Report).
+
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+
+part_1_test() ->
+    TestFile = test_data(),
+    ?assertEqual(2, part_1(TestFile)).
+
+part_2_test() ->
+    TestFile = test_data(),
+    ?assertEqual(4, part_2(TestFile)).
+
+is_safe_4_test() -> ?assertEqual(safe, check_report([7, 6, 4, 2, 1])).
+is_safe_5_test() -> ?assertEqual(unsafe, check_report([1, 2, 7, 8, 9])).
+is_safe_6_test() -> ?assertEqual(unsafe, check_report([9, 7, 6, 2, 1])).
+is_safe_7_test() -> ?assertEqual(unsafe, check_report([1, 3, 2, 4, 5])).
+is_safe_8_test() -> ?assertEqual(unsafe, check_report([8, 6, 4, 4, 1])).
+is_safe_9_test() -> ?assertEqual(safe, check_report([1, 3, 6, 7, 9])).
+is_safe_10_test() -> ?assertEqual(safe, check_report([40, 42, 44, 46, 49, 51])).
+
+
+test_data() ->
+    open_file("./priv/test_input/2.txt").
+
+-endif.