day 6 solution

day6/Makefile

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+all: day6.beam
+	erl -noshell -s day6 start -s init stop
+
+day6.beam:
+	erlc day6.erl
+
+clean:
+	rm -f day6.beam

day6/day6.erl

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+-module(day6).
+-export([start/0]).
+
+index_of(Item, List) -> index_of(Item, List, 1).
+
+index_of(_, [], _)  -> not_found;
+index_of(Item, [Item|_], Index) -> Index;
+index_of(Item, [_|Tl], Index) -> index_of(Item, Tl, Index+1).
+
+redistribute_banks(BankConfiguration)
+  -> Max = lists:max(BankConfiguration),
+     Index = index_of(Max, BankConfiguration) - 1,
+     %io:fwrite("max ~B index ~B~n", [Max, Index]),
+     RawArray = array:from_list(BankConfiguration),
+     redistribute_banks(array:set(Index, 0, RawArray), Index, Max, length(BankConfiguration)).
+
+redistribute_banks(Array, _Index, RemainingBanks, _MaxSize)
+  when RemainingBanks == 0 -> array:to_list(Array);
+redistribute_banks(Array, Index, RemainingBanks, MaxSize)
+  -> Target = (Index + 1) rem MaxSize,
+     OldBankValue = array:get(Target, Array),
+     %io:fwrite("target ~B oldbankvalue ~B~n", [Target, OldBankValue]),
+     redistribute_banks(array:set(Target, OldBankValue + 1, Array), Target, RemainingBanks - 1, MaxSize).
+
+steps_until_repeated(Input)
+  -> steps_until_repeated(Input, [], []).
+steps_until_repeated(Input, Visited, UniqList)
+  when length(Visited) > 0, length(UniqList) /= length(Visited) -> [_|PriorVisits] = Visited,
+                                                                   InputIndex = index_of(Input, PriorVisits),
+                                                                   VisitedLength = length(Visited),
+                                                                   {VisitedLength,Input,InputIndex};
+steps_until_repeated(Input, Visited, _UniqList)
+  -> NewConfiguration = redistribute_banks(Input),
+     NewVisited = [NewConfiguration|Visited],
+     steps_until_repeated(NewConfiguration, NewVisited, lists:usort(NewVisited)).
+
+start()
+  -> io:fwrite("~p~n", [steps_until_repeated([0,2,7,0])]),
+     io:fwrite("~p~n", [steps_until_repeated([4,1,15,12,0,9,9,5,5,8,7,3,14,5,12,3])]).