day24 solution

day24/Makefile

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+all: day24.beam
+	erl -noshell -s day24 start -s init stop
+
+day24.beam:
+	erlc day24.erl
+
+clean:
+	rm -f day24.beam

day24/day24.erl

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+-module(day24).
+-export([start/0]).
+
+read_input(Filename)
+  -> { ok, Input } = file:read_file(Filename),
+     lists:map(fun(X) ->
+                   [A,B|_] = re:split(X, "/"),
+                   { AInt, _ } = string:to_integer(A),
+                   { BInt, _ } = string:to_integer(B),
+                   { AInt, BInt }
+               end, re:split(string:chomp(Input), "\n")).
+
+strongest_bridge(Input)
+  -> strongest_bridge(Input, 0, 0, []).
+
+strongest_bridge([], _LastEdge, Sum, Train)
+  -> { Sum, Train };
+strongest_bridge(Input, LastEdge, Sum, Train)
+  -> { Matching, _ } = lists:partition(fun(X) ->
+                                              { A, B } = X,
+                                              (A == LastEdge) or (B == LastEdge)
+                                          end, Input),
+     if length(Matching) == 0 -> { Sum, Train };
+        true ->
+          MatchingSums = lists:map(fun(X) ->
+                                      { _, Remaining } = lists:partition(fun(Y) -> Y == X end, Input),
+                                      { A, B } = X,
+                                      NewTrain = lists:append(Train, [X]),
+                                      NewSum = Sum + A + B,
+                                      if A == LastEdge -> strongest_bridge(Remaining, B, NewSum, NewTrain);
+                                         true -> strongest_bridge(Remaining, A, NewSum, NewTrain)
+                                      end
+                                   end, Matching),
+          [FirstSum|_] = lists:sort(fun(X, Y) ->
+                                        { A, _ } = X,
+                                        { B, _ } = Y,
+                                        B =< A
+                                    end, MatchingSums),
+          FirstSum
+     end.
+
+longest_bridge(Input)
+  -> longest_bridge(Input, 0, 0, []).
+
+longest_bridge([], _LastEdge, Sum, Train)
+  -> { Sum, Train };
+longest_bridge(Input, LastEdge, Sum, Train)
+  -> { Matching, _ } = lists:partition(fun(X) ->
+                                              { A, B } = X,
+                                              (A == LastEdge) or (B == LastEdge)
+                                          end, Input),
+     if length(Matching) == 0 -> { Sum, Train };
+        true ->
+          MatchingSums = lists:map(fun(X) ->
+                                      { _, Remaining } = lists:partition(fun(Y) -> Y == X end, Input),
+                                      { A, B } = X,
+                                      NewTrain = lists:append(Train, [X]),
+                                      NewSum = Sum + A + B,
+                                      if A == LastEdge -> longest_bridge(Remaining, B, NewSum, NewTrain);
+                                         true -> longest_bridge(Remaining, A, NewSum, NewTrain)
+                                      end
+                                   end, Matching),
+          [FirstSum|_] = lists:sort(fun(X, Y) ->
+                                        { A, I } = X,
+                                        { B, J } = Y,
+                                        if length(I) == length(J) -> B =< A;
+                                           true -> length(J) =< length(I)
+                                        end
+                                    end, MatchingSums),
+          FirstSum
+     end.
+
+start()
+  -> SampleInput = read_input("sample"),
+     io:fwrite("~p~n", [strongest_bridge(SampleInput)]),
+     io:fwrite("~p~n", [longest_bridge(SampleInput)]),
+     ProblemInput = read_input("problem"),
+     io:fwrite("~p~n", [strongest_bridge(ProblemInput)]),
+     io:fwrite("~p~n", [longest_bridge(ProblemInput)]).

day24/problem

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+14/42
+2/3
+6/44
+4/10
+23/49
+35/39
+46/46
+5/29
+13/20
+33/9
+24/50
+0/30
+9/10
+41/44
+35/50
+44/50
+5/11
+21/24
+7/39
+46/31
+38/38
+22/26
+8/9
+16/4
+23/39
+26/5
+40/40
+29/29
+5/20
+3/32
+42/11
+16/14
+27/49
+36/20
+18/39
+49/41
+16/6
+24/46
+44/48
+36/4
+6/6
+13/6
+42/12
+29/41
+39/39
+9/3
+30/2
+25/20
+15/6
+15/23
+28/40
+8/7
+26/23
+48/10
+28/28
+2/13
+48/14

day24/sample

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+0/2
+2/2
+2/3
+3/4
+3/5
+0/1
+10/1
+9/10