day3 solution

day3/Makefile

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+all: day3.beam
+	erl -noshell -s day3 start -s init stop
+
+day3.beam:
+	erlc day3.erl
+
+clean:
+	rm -f day3.beam

day3/day3.erl

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+-module(day3).
+-export([start/0]).
+
+next_largest_square(Target)
+  -> recurse_next_largest_square(1, Target).
+
+recurse_next_largest_square(X, Target)
+  when (X rem 2) =/= 0, (X*X) >= Target -> X;
+recurse_next_largest_square(X, Target)
+  -> recurse_next_largest_square(X+1, Target).
+
+position_in_outer_shell(Square, Target)
+  -> PreviousSquare = (Square - 2) * (Square - 2),
+     Target - PreviousSquare.
+
+position_on_side(Position, Square)
+  when Position == 0 -> Square - 1;
+position_on_side(_Position, Square)
+  when Square == 1 -> 1;
+position_on_side(Position, Square)
+  -> Offset = Position rem (Square - 1),
+     if Offset == 0 -> Square - 1;
+               true -> Offset end.
+
+distance_to_center(Position, Square)
+  -> Center = Square div 2,
+     VertOffset = abs(Center - Position),
+     Center + VertOffset.
+
+find_distance(Target)
+  -> Square = next_largest_square(Target),
+     Position = position_in_outer_shell(Square, Target),
+     Index = position_on_side(Position, Square),
+     distance_to_center(Index, Square).
+
+start()
+  -> io:fwrite("~B ~B~n", [1, find_distance(1)]),
+     io:fwrite("~B ~B~n", [12, find_distance(12)]),
+     io:fwrite("~B ~B~n", [23, find_distance(23)]),
+     io:fwrite("~B ~B~n", [1024, find_distance(1024)]),
+     io:fwrite("~B ~B~n", [368078, find_distance(368078)]),
+     io:fwrite("https://oeis.org/A141481/b141481.txt~n").