day16 solution

day16/Makefile

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+all: day16.beam
+	erl -noshell -s day16 start -s init stop
+
+day16.beam:
+	erlc day16.erl
+
+clean:
+	rm -f day16.beam

day16/day16.erl

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+-module(day16).
+-export([start/0]).
+
+dance([], Registers)
+  -> Registers;
+dance([Head|Tail], Registers)
+  -> Instr = string:slice(Head, 0, 1),
+     Args = string:slice(Head, 1),
+     NewRegisters = process_instruction(Instr, Args, Registers),
+     dance(Tail, NewRegisters).
+
+process_instruction(Instr, Args, Registers)
+  when Instr == <<"s">> -> Len = string:length(Registers),
+                           { Count, _ } = string:to_integer(Args),
+                           Index = Len - Count,
+                           First = string:slice(Registers, Index),
+                           Second = string:slice(Registers, 0, Index),
+                           string:concat(First, Second);
+process_instruction(Instr, Args, Registers)
+  when Instr == <<"x">> -> [FirstRegister|Rest] = re:split(Args, "/"),
+                           [SecondRegister|_] = Rest,
+                           { FirstIndex, _ } = string:to_integer(FirstRegister),
+                           { SecondIndex, _ } = string:to_integer(SecondRegister),
+                           swap_registers(FirstIndex, SecondIndex, Registers);
+process_instruction(Instr, Args, Registers)
+  when Instr == <<"p">> -> [FirstRegister|Rest] = re:split(Args, "/"),
+                           [SecondRegister|_] = Rest,
+                           IndexA = char_index(FirstRegister, Registers),
+                           IndexB = char_index(SecondRegister, Registers),
+                           swap_registers(IndexA, IndexB, Registers).
+
+char_index(Char, String)
+  -> StrLen = string:length(String),
+     StrLen - string:length(string:find(String, Char)).
+
+find_char_index(Index)
+  when Index == nomatch -> 0;
+find_char_index(Index)
+  -> Index.
+
+swap_registers(A, B, Registers)
+  when B < A -> swap_registers(B, A, Registers);
+swap_registers(A, B, Registers)
+  -> RegisterList = list_to_tuple(Registers),
+     ValueA = element(A + 1, RegisterList),
+     ValueB = element(B + 1, RegisterList),
+     tuple_to_list(setelement(B + 1, setelement(A + 1, RegisterList, ValueB), ValueA)).
+
+dance_countdown(Steps, Instructions, Registers)
+  -> dance_countdown(Steps, Instructions, Registers, Steps, []).
+
+dance_countdown(Steps, _Instructions, Registers, _OriginalSteps, _PreviousSteps)
+  when Steps == 0 -> Registers;
+dance_countdown(Steps, Instructions, Registers, OriginalSteps, PreviousSteps)
+  -> CurrentRegisters = dance(Instructions, Registers),
+     Found = lists:any(fun(X) -> string:equal(X, CurrentRegisters) end, PreviousSteps),
+     if Found -> Index = OriginalSteps rem length(PreviousSteps),
+                 lists:nth(Index, PreviousSteps);
+        true -> dance_countdown(Steps - 1, Instructions, CurrentRegisters, OriginalSteps, lists:append(PreviousSteps, [CurrentRegisters]))
+     end.
+
+start()
+  -> SampleInstructions = re:split("s1,x3/4,pe/b", ","),
+     io:fwrite("~p~n", [dance(SampleInstructions, "abcde")]),
+     { ok, File } = file:read_file("input"),
+     InputInstructions = re:split(unicode:characters_to_list(string:chomp(File)), ","),
+     io:fwrite("~p~n", [dance(InputInstructions, "abcdefghijklmnop")]),
+     io:fwrite("~p~n", [dance_countdown(1000000000, InputInstructions, "abcdefghijklmnop")]).