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89 lines
3.6 KiB
89 lines
3.6 KiB
-module(day15).
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-export([solve/1]).
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solve(Input) ->
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{part1(Input), part2(Input)}.
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part1(Input) ->
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Sensors = parse_sensors(Input),
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Ranges = get_row(Sensors, 2000000),
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BeaconlessRanges = remove_beacons(Sensors, 2000000, Ranges),
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sum_ranges(BeaconlessRanges).
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part2(Input) ->
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Sensors = parse_sensors(Input),
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{Sensors, [Answer]} = for(0, 4000000, fun check_row_for_gaps/2, {Sensors, []}),
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Answer.
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check_row_for_gaps(Row, {Sensors, Acc}) ->
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case rset_remove_after(4000001, rset_remove_before(-1, get_row(Sensors, Row))) of
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[{_X, _Y}] -> {Sensors, Acc};
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[{0, NMinusOne}, {NPlusOne, 4000000}] when NMinusOne + 2 == NPlusOne -> {Sensors, [tuning_frequency(NMinusOne+1, Row)|Acc]}
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end.
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tuning_frequency(X, Y) -> X * 4000000 + Y.
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for(End, End, Fun, Acc) -> Fun(End, Acc);
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for(Start, End, Fun, Acc) -> for(Start+1, End, Fun, Fun(Start, Acc)).
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parse_sensors(Input) ->
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lists:map(fun parse_sensor/1, binary:split(Input, <<$\n>>, [global, trim_all])).
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parse_sensor(Line) ->
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{ok, [SensorX, SensorY, BeaconX, BeaconY], _Rest} = io_lib:fread("Sensor at x=~d, y=~d: closest beacon is at x=~d, y=~d", binary_to_list(Line)),
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{{SensorX, SensorY}, {BeaconX, BeaconY}}.
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get_row(Sensors, Row) ->
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get_row(Sensors, Row, rset_new()).
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remove_beacons([], _Row, Set) -> Set;
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remove_beacons([{_, {BeaconX, Row}}|Rest], Row, Set) -> remove_beacons(Rest, Row, rset_remove1(BeaconX, Set));
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remove_beacons([_|Rest], Row, Set) -> remove_beacons(Rest, Row, Set).
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get_row([], _Row, Acc) -> Acc;
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get_row([Sensor | Rest], Row, Acc) ->
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NewAcc =
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case get_intersection(Sensor, Row) of
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none -> Acc;
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Range -> rset_append(Range, Acc)
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end,
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get_row(Rest, Row, NewAcc).
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get_intersection({{SensorX, SensorY} = Sensor, NearestBeacon}, Row) ->
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case vec_mag(vec_sub(Sensor, NearestBeacon)) - abs(Row - SensorY) of
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TooFar when TooFar < 0 -> none;
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Signal -> {SensorX - Signal, SensorX + Signal}
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end.
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vec_sub({LX, LY}, {RX, RY}) -> {LX - RX, LY - RY}.
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% vector magnitude
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vec_mag({X, Y}) -> abs(X) + abs(Y).
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sum_ranges([]) -> 0;
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sum_ranges([{Start, End} | Rest]) -> End - Start + 1 + sum_ranges(Rest).
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rset_new() -> [].
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% rangeset is a sorted set of discrete ranges. Adjacent ranges are merged.
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rset_append(Entry, []) -> [Entry];
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rset_append({_, End} = Entry, [{HeadStart, _} | _] = Set) when End < HeadStart - 1 -> [Entry | Set];
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rset_append({Start, _} = Entry, [{_, HeadEnd} = Head | Rest]) when Start > HeadEnd + 1 -> [ Head | rset_append(Entry, Rest)];
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rset_append({Start, End}, [{HeadStart, HeadEnd} | Rest]) -> rset_append({min(Start, HeadStart), max(End, HeadEnd)}, Rest).
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rset_remove1(_Point, []) -> [];
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rset_remove1(Point, [{Start, _End} | _Rest] = Set) when Point < Start -> Set;
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rset_remove1(Point, [{_Start, End} = Head | Rest]) when Point > End -> [Head | rset_remove1(Point, Rest)];
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rset_remove1(Point, [{Point, Point} | Rest]) -> Rest;
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rset_remove1(Point, [{Point, End} | Rest]) -> [{Point+1, End} | Rest];
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rset_remove1(Point, [{Start, Point} | Rest]) -> [{Start, Point-1} | Rest];
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rset_remove1(Point, [{Start, End} | Rest]) -> [{Start, Point-1}, {Point+1, End} | Rest].
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rset_remove_before(_Point, []) -> [];
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rset_remove_before(Point, [{_, End}|Rest]) when Point >= End -> rset_remove_before(Point, Rest);
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rset_remove_before(Point, [{Start, _}|_] = List) when Point < Start -> List;
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rset_remove_before(Point, [{_, End}|Rest]) -> [{Point+1, End}|Rest].
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rset_remove_after(_Point, []) -> [];
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rset_remove_after(Point, [{Start, _}|_]) when Point =< Start -> [];
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rset_remove_after(Point, [{_, End}=Node | Rest]) when Point > End -> [Node|rset_remove_after(Point, Rest)];
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rset_remove_after(Point, [{Start, _End}|_]) -> [{Start, Point-1}].
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