-module(day3).

-export([solve/1]).

solve(InputData) ->
	Rucksacks = binary:split(InputData, <<$\n>>, [trim_all, global]),
	{lists:foldl(fun part1/2, 0, Rucksacks), part2(Rucksacks)}.

part2([]) -> 0;
part2([Ruck1, Ruck2, Ruck3 | Rest]) ->
	[Badge] = maps:keys(
		    sets:intersection(
			      sets:intersection(tally(Ruck1), tally(Ruck2)),
			      tally(Ruck3)
			     )),
	priority(Badge) + part2(Rest).

part1(Rucksack, TotalPriority) ->
	CompartmentSize = byte_size(Rucksack) div 2,
	<<Compartment1:CompartmentSize/binary, Compartment2:CompartmentSize/binary>> = Rucksack,
	[DupeLetter] = maps:keys(sets:intersection(tally(Compartment1), tally(Compartment2))),
	priority(DupeLetter) + TotalPriority.

tally(Compartment) -> tally(Compartment, #{}).
tally(<<>>, Set) -> Set;
tally(<<Item, Rest/binary>>, Set) when is_map_key(Item, Set) -> tally(Rest, Set);
tally(<<Item, Rest/binary>>, Set) -> tally(Rest, Set#{Item => []}).

priority(Item) when Item >= $a andalso Item =< $z -> Item - $a + 1;
priority(Item) when Item >= $A andalso Item =< $Z -> Item - $A + 27.