%% to compile: erlc day3A.erl
%% to run: erl -noshell -s day3 solve
%%
-module(day9).

-export ([solve/0, solve/1, solve/2]).
-export ([read_input/0]).

solve() ->
    solve(['1']),
    solve(['2']),
    init:stop().

solve(A) ->
    solve(A, read_input()).

solve(['1'], D) ->
    io:format("The solution to ~p puzzle1 is: ~p~n", [?MODULE, solve(1, D)]);
solve(1, D) ->
    solution1(D);
solve(['2'], D) ->
    io:format("The solution to ~p puzzle2 is: ~p~n", [?MODULE, solve(2, D)]);
solve(2, D) ->
    solution2(D).

read_input() ->
	{ok, IO} = file:open("input.txt", 'read'),
	Data = read_input(IO),
	file:close(IO),
        Data.

read_input(IO) ->
	read_input(IO, []).

read_input(IO, Input) ->
	case read_line(IO) of
		'eof' -> Input;
		Line -> read_input(IO, Input ++ [Line])
	end.

read_line(IO) ->
	case file:read_line(IO) of
		'eof' -> 'eof';
		{ok, Line} -> [ X - $0 || X <- Line, [X] /= "\n"]
	end.

solution1(Table) ->
  Low_points = low_points(Table),
  lists:foldl(fun(C, Acc) -> Acc + 1 + get_value(C, Table) end, 0, Low_points).

solution2(Table) ->
  Low_points = low_points(Table),
  Basins = get_basins(Low_points, Table),
  [B1,B2,B3|_] = lists:sort(fun(A, B) -> length(A) > length(B) end, Basins),
  length(B1) * length(B2) * length(B3).


get_basins(Low_points, Table) ->
	get_basins(Low_points, Table, []).

get_basins([], _Table, Basins) -> Basins;
get_basins([Low_point|Rest], Table, Basins) ->
  Basin = [Low_point|get_up_slopes([Low_point], Table)],
  get_basins(Rest, Table, [Basin|Basins]).

low_points([H|_] =  Table) ->
  X = length(H),
  Y = length(Table),
  Coords = lists:flatten([[{A,B} || A <- lists:seq(1,X)] || B <- lists:seq(1,Y)]),
  lists:filter(fun(C) -> is_low_point(C, Table) end, Coords).

get_up_slopes(Coords, Table) ->
	get_up_slopes(Coords, Table, []).

get_up_slopes([], _Table, Acc) -> Acc;
get_up_slopes([{X, Y}|Rest], Table, Acc) ->
    V = get_value({X,Y}, Table),
    Neighbours = [{X+1, Y},{X-1, Y},{X, Y+1},{X, Y-1}],
    Up_slopes = lists:filter(fun({X1,Y1}) -> C = get_value({X1,Y1}, Table), C /= 9 andalso C > V end, Neighbours),
    get_up_slopes(lists:usort(Rest ++ Up_slopes), Table, lists:usort(Acc ++ Up_slopes)).

is_low_point({X,Y}, Table) ->
    V = get_value({X,Y}, Table),
    V < get_value({X+1, Y}, Table) andalso
    V < get_value({X-1, Y}, Table) andalso
    V < get_value({X, Y+1}, Table) andalso
    V < get_value({X, Y-1}, Table).

get_value({X,Y}, [H|_] = Table) when X == 0; Y == 0; Y > length(Table); X > length(H) -> 9;

get_value({X,Y}, Table) ->
	lists:nth(X, lists:nth(Y, Table)).