day17 solution

day17/Makefile

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+all: day17.beam
+	erl -noshell -s day17 start -s init stop
+
+day17.beam:
+	erlc day17.erl
+
+clean:
+	rm -f day17.beam

day17/day17.erl

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+-module(day17).
+-export([start/0]).
+
+spin_the_lock(NextValue, SpinQueue, Index, _Steps)
+  when NextValue == 2018 -> lists:nth(Index + 1 rem 2018, SpinQueue);
+spin_the_lock(NextValue, SpinQueue, Index, Steps)
+  -> NextPosition = (Index + Steps) rem NextValue,
+     { First, Last } = lists:split(NextPosition, SpinQueue),
+     spin_the_lock(NextValue + 1, lists:append([First, [NextValue], Last]), NextPosition + 1, Steps).
+
+spin_the_lock2(NextValue, ValAtOne, _Index, _Steps)
+  when NextValue == 50000001 -> ValAtOne;
+spin_the_lock2(NextValue, ValAtOne, Index, Steps)
+  -> NextPosition = (Index + Steps) rem NextValue,
+     if NextPosition == 0 -> spin_the_lock2(NextValue + 1, NextValue, NextPosition + 1, Steps);
+        true -> spin_the_lock2(NextValue + 1, ValAtOne, NextPosition + 1, Steps)
+     end.
+
+start()
+  -> io:fwrite("~p~n", [spin_the_lock(1, [0], 0, 3)]),
+     io:fwrite("~p~n", [spin_the_lock(1, [0], 0, 303)]),
+     io:fwrite("~p~n", [spin_the_lock2(1, 0, 0, 303)]).