94 lines
2.5 KiB
Erlang
94 lines
2.5 KiB
Erlang
%% to compile: erlc day3A.erl
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%% to run: erl -noshell -s day3 solve
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%%
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-module(day9).
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-export ([solve/0, solve/1, solve/2]).
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-export ([read_input/0]).
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solve() ->
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solve(['1']),
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solve(['2']),
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init:stop().
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solve(A) ->
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solve(A, read_input()).
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solve(['1'], D) ->
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io:format("The solution to ~p puzzle1 is: ~p~n", [?MODULE, solve(1, D)]);
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solve(1, D) ->
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solution1(D);
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solve(['2'], D) ->
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io:format("The solution to ~p puzzle2 is: ~p~n", [?MODULE, solve(2, D)]);
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solve(2, D) ->
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solution2(D).
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read_input() ->
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{ok, IO} = file:open("input.txt", 'read'),
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Data = read_input(IO),
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file:close(IO),
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Data.
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read_input(IO) ->
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read_input(IO, []).
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read_input(IO, Input) ->
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case read_line(IO) of
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'eof' -> Input;
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Line -> read_input(IO, Input ++ [Line])
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end.
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read_line(IO) ->
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case file:read_line(IO) of
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'eof' -> 'eof';
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{ok, Line} -> [ X - $0 || X <- Line, [X] /= "\n"]
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end.
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solution1(Table) ->
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Low_points = low_points(Table),
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lists:foldl(fun(C, Acc) -> Acc + 1 + get_value(C, Table) end, 0, Low_points).
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solution2(Table) ->
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Low_points = low_points(Table),
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Basins = get_basins(Low_points, Table),
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[B1,B2,B3|_] = lists:sort(fun(A, B) -> length(A) > length(B) end, Basins),
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length(B1) * length(B2) * length(B3).
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get_basins(Low_points, Table) ->
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get_basins(Low_points, Table, []).
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get_basins([], _Table, Basins) -> Basins;
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get_basins([Low_point|Rest], Table, Basins) ->
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Basin = [Low_point|get_up_slopes([Low_point], Table)],
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get_basins(Rest, Table, [Basin|Basins]).
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low_points([H|_] = Table) ->
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X = length(H),
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Y = length(Table),
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Coords = lists:flatten([[{A,B} || A <- lists:seq(1,X)] || B <- lists:seq(1,Y)]),
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lists:filter(fun(C) -> is_low_point(C, Table) end, Coords).
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get_up_slopes(Coords, Table) ->
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get_up_slopes(Coords, Table, []).
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get_up_slopes([], _Table, Acc) -> Acc;
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get_up_slopes([{X, Y}|Rest], Table, Acc) ->
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V = get_value({X,Y}, Table),
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Neighbours = [{X+1, Y},{X-1, Y},{X, Y+1},{X, Y-1}],
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Up_slopes = lists:filter(fun({X1,Y1}) -> C = get_value({X1,Y1}, Table), C /= 9 andalso C > V end, Neighbours),
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get_up_slopes(lists:usort(Rest ++ Up_slopes), Table, lists:usort(Acc ++ Up_slopes)).
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is_low_point({X,Y}, Table) ->
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V = get_value({X,Y}, Table),
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V < get_value({X+1, Y}, Table) andalso
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V < get_value({X-1, Y}, Table) andalso
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V < get_value({X, Y+1}, Table) andalso
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V < get_value({X, Y-1}, Table).
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get_value({X,Y}, [H|_] = Table) when X == 0; Y == 0; Y > length(Table); X > length(H) -> 9;
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get_value({X,Y}, Table) ->
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lists:nth(X, lists:nth(Y, Table)).
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