From 3277893cfabf151510aee08b5b6b488e92efea11 Mon Sep 17 00:00:00 2001
From: Shanti Chellaram <shanti@shanti.im>
Date: Mon, 19 Dec 2022 17:34:38 +0900
Subject: [PATCH] Day 15-2

---
 day15.erl | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/day15.erl b/day15.erl
index a48c2cd..3be0dfb 100644
--- a/day15.erl
+++ b/day15.erl
@@ -3,12 +3,29 @@
 -export([solve/1]).
 
 solve(Input) ->
-    {part1(Input), none}.
+    {part1(Input), part2(Input)}.
 
 part1(Input) ->
     Sensors = parse_sensors(Input),
     Ranges = get_row(Sensors, 2000000),
-    sum_ranges(Ranges).
+    BeaconlessRanges = remove_beacons(Sensors, 2000000, Ranges),
+    sum_ranges(BeaconlessRanges).
+
+part2(Input) ->
+    Sensors = parse_sensors(Input),
+    {Sensors, [Answer]} = for(0, 4000000, fun check_row_for_gaps/2, {Sensors, []}),
+    Answer.
+
+check_row_for_gaps(Row, {Sensors, Acc}) ->
+  case rset_remove_after(4000001, rset_remove_before(-1, get_row(Sensors, Row))) of
+      [{_X, _Y}] -> {Sensors, Acc};
+      [{0, NMinusOne}, {NPlusOne, 4000000}] when NMinusOne + 2 == NPlusOne -> {Sensors, [tuning_frequency(NMinusOne+1, Row)|Acc]}
+  end.
+
+tuning_frequency(X, Y) -> X * 4000000 + Y.
+
+for(End, End, Fun, Acc) -> Fun(End, Acc);
+for(Start, End, Fun, Acc) -> for(Start+1, End, Fun, Fun(Start, Acc)).
 
 parse_sensors(Input) -> 
     lists:map(fun parse_sensor/1, binary:split(Input, <<$\n>>, [global, trim_all])).
@@ -17,7 +34,7 @@ parse_sensor(Line) ->
     {{SensorX, SensorY}, {BeaconX, BeaconY}}.
 
 get_row(Sensors, Row) ->
-    remove_beacons(Sensors, Row, get_row(Sensors, Row, rset_new())).
+    get_row(Sensors, Row, rset_new()).
 
 remove_beacons([], _Row, Set) -> Set;
 remove_beacons([{_, {BeaconX, Row}}|Rest], Row, Set) -> remove_beacons(Rest, Row, rset_remove1(BeaconX, Set));
@@ -60,3 +77,13 @@ rset_remove1(Point, [{Point, Point} | Rest]) -> Rest;
 rset_remove1(Point, [{Point, End} | Rest]) -> [{Point+1, End} | Rest];
 rset_remove1(Point, [{Start, Point} | Rest]) -> [{Start, Point-1} | Rest];
 rset_remove1(Point, [{Start, End} | Rest]) -> [{Start, Point-1}, {Point+1, End} | Rest].
+
+rset_remove_before(_Point, []) -> [];
+rset_remove_before(Point, [{_, End}|Rest]) when Point >= End -> rset_remove_before(Point, Rest);
+rset_remove_before(Point, [{Start, _}|_] = List) when Point < Start -> List;
+rset_remove_before(Point, [{_, End}|Rest]) -> [{Point+1, End}|Rest].
+
+rset_remove_after(_Point, []) -> [];
+rset_remove_after(Point, [{Start, _}|_]) when Point =< Start -> [];
+rset_remove_after(Point, [{_, End}=Node | Rest]) when Point > End -> [Node|rset_remove_after(Point, Rest)];
+rset_remove_after(Point, [{Start, _End}|_]) -> [{Start, Point-1}].