aoc2024/src/day_4.erl

-module(day_4).
-export([part_1/0, part_2/0]).
-export([map_from_matrix/1, mget/3, check_if_cross/3, read_input/1]).


part_1() ->
    Filename = filename:absname("./priv/input/4.txt"),
    part_1(Filename).

part_1(Filename) ->
    Input = read_input(Filename),

    Xmases = [
              find_xmas_in_matrix(Input),
              find_xmas_in_matrix(transpose(Input)),
              find_xmas_in_matrix(transpose(skew(Input))),
              find_xmas_in_matrix(transpose(skew(Input, reversed)))
             ],

    lists:sum(Xmases).


part_2() ->
    Filename = filename:absname("./priv/input/4.txt"),
    part_2(Filename).

part_2(Filename) ->
    Input = read_input(Filename),
    Matrix = map_from_matrix(Input),
    Coords = maps:fold(
           fun(Y, Line, Acc) ->
                   As = maps:fold(
                     fun(X, C, AccLine) ->
                             case C of
                                 $A -> [{X, Y} | AccLine];
                                 _ -> AccLine
                             end
                     end, [], Line),
                   [As | Acc]
           end,
           [],
           Matrix),
    ACoords = lists:append(Coords),
    Xmases = lists:filter(fun({X,Y}) -> check_if_cross(X, Y, Matrix) end, ACoords),
    length(Xmases).


%% Private functions

read_input(Filename) ->
    {ok, Filehandle} = file:open(Filename, read),
    read_input(Filehandle, []).

read_input(Filehandle, Lines) ->
    case file:read_line(Filehandle) of
        {ok, Line} ->
            Trimmed = string:trim(Line),
            read_input(Filehandle, [Trimmed | Lines]);
         eof ->
            lists:reverse(Lines)
    end.

find_xmas_in_matrix(Matrix) ->
    lists:foldl(
      fun(Line, Sum) ->
              Xmas = find_xmas_in_line(list_to_binary(Line)),
              Samx = find_xmas_in_line(list_to_binary(string:reverse(Line))),
              Sum + Xmas + Samx
      end,
      0,
      Matrix
     ).

find_xmas_in_line(<<"XMAS", Rest/binary>>) ->
    1 + find_xmas_in_line(<<Rest/binary>>);
find_xmas_in_line(<<_,Rest/binary>>) ->
    find_xmas_in_line(Rest);
find_xmas_in_line(<<>>) ->
    0.



transpose([[]|_]) -> [];
transpose(Matrix) ->
    [lists:map(fun hd/1, Matrix) | transpose(lists:map(fun tl/1, Matrix))].

%% rotate([[]|_]) -> [];
%% rotate(Matrix) ->
%%     lists:map(fun lists:reverse/1, transpose(Matrix)).

skew(Matrix) ->
    Length = length(Matrix),
    {Skewed, _} =
        lists:mapfoldl(
          fun(Line, Index) ->
                  %% {Init, Tail} = lists:split(Index, Line),
                  %% {lists:append([Tail, Init]) , Index + 1}
                  {
                   lists:append(
                     [
                      lists:duplicate(Index, " "),
                      Line,
                      lists:duplicate(Length - Index - 1, " ")
                     ]
                    ),
                   Index + 1
                  }
          end,
          0,
          Matrix),
    Skewed.

skew(Matrix, reversed) ->
    Length = length(Matrix),
    {Skewed, _} =
        lists:mapfoldl(
          fun(Line, Index) ->
                  %% {Init, Tail} = lists:split(Length - Index, Line),
                  %% {lists:append([Tail, Init]) , Index + 1}
                  {
                   lists:append(
                     [
                      lists:duplicate(Length - Index - 1, " "),
                      Line,
                      lists:duplicate(Index, " ")
                     ]
                    ),
                   Index + 1
                  }
          end,
          0,
          Matrix),
    Skewed.

%% Part 2

map_from_matrix(Matrix) ->
    {List, _Length} = lists:mapfoldl(
                        fun(Line, Index) -> {{Index, map_from_line(Line)}, Index + 1} end,
                        0,
                        Matrix),
    maps:from_list(List).

map_from_line(Line) ->
    {List, _Length} = lists:mapfoldl(
                        fun(Char, Index) -> {{Index, Char}, Index + 1} end,
                        0,
                        Line),
    maps:from_list(List).

mget(Matrix, X, Y) ->
    maps:get(X, maps:get(Y, Matrix)).

check_if_cross(0, _, _Matrix) ->
    false;
check_if_cross(_, 0, _Matrix) ->
    false;
check_if_cross(X, Y, Matrix) ->
    maybe
        true ?= maps:size(Matrix) - 1 > Y,
        Line = maps:get(Y,Matrix),
        true ?= maps:size(Line) - 1 > X,
        "MS" ?= lists:sort([
                        mget(Matrix, X - 1, Y - 1),
                        mget(Matrix, X + 1, Y + 1)
                       ]),
        "MS" ?= lists:sort([
                        mget(Matrix, X - 1, Y + 1),
                        mget(Matrix, X + 1, Y - 1)
                       ]),
        true
    else
        _ -> false
    end.

%% TESTS

-ifdef(TEST).
-include_lib("eunit/include/eunit.hrl").

find_xmas_in_line_test() ->
    ?assertEqual(0, find_xmas_in_line(<<>>)),
    ?assertEqual(0, find_xmas_in_line(<<"ABC">>)),
    ?assertEqual(1, find_xmas_in_line(<<"XMASAAAAA">>)),
    ?assertEqual(1, find_xmas_in_line(<<"XXMASS">>)),
    ?assertEqual(1, find_xmas_in_line(<<"SAMXMAS">>)),
    ?assertEqual(1, find_xmas_in_line(<<"XMASAMX">>)).


transpose_test() ->
    ?assertEqual(
       ["100",
        "010",
        "001"
       ],
       transpose(["100",
                  "010",
                  "001"
                 ])).

skew_test() ->
    ?assertEqual(
       lists:map(fun list_to_binary/1,
       ["100  ",
        " 100 ",
        "  100"
       ]),
       lists:map(fun list_to_binary/1, skew(
         ["100",
          "100",
          "100"
         ]))),
    ?assertEqual(
       lists:map(fun list_to_binary/1,
       ["  100",
        " 100 ",
        "100  "
       ]),
       lists:map(fun list_to_binary/1, skew(
         ["100",
          "100",
          "100"
         ], reversed))).

part_1_test() ->
    Testfile = filename:absname("./priv/test_input/4.txt"),
    ?assertEqual(18, part_1(Testfile)).

part_2_test() ->
    Testfile = filename:absname("./priv/test_input/4.txt"),
    ?assertEqual(9, part_2(Testfile)).

-endif.