diff --git a/16/part2.pl b/16/part2.pl
new file mode 100644
index 0000000..41f47c0
--- /dev/null
+++ b/16/part2.pl
@@ -0,0 +1,75 @@
+:- use_module(library(pio)).
+:- initialization(main, main).
+
+main([FileName|_]) :-
+    input(FileName, Map),
+    length(Map, Height), MaxX is Height - 1,
+    Map = [Row|_], length(Row, Width), MaxY is Width - 1,
+    findall(N, (between(0, MaxX, X), propagate(Map, [X-0-3], End), count(End, N)), N1), nl,
+    findall(N, (between(0, MaxX, X), propagate(Map, [X-MaxY-1], End), count(End, N)), N2), nl,
+    findall(N, (between(0, MaxY, Y), propagate(Map, [0-Y-0], End), count(End, N)), N3), nl,
+    findall(N, (between(0, MaxY, Y), propagate(Map, [MaxX-Y-2], End), count(End, N)), N4), nl,
+    append([N1, N2, N3, N4], Ns), max_list(Ns, Answer),
+    write(Answer), nl.
+
+count(Map, X) :-
+    findall(1, (nth0(_, Map, Row), nth0(_, Row, _-D), \+ D = [0,0,0,0]), Ls),
+    length(Ls, X).
+    
+% propagate(Map, Queue, NewMap). 'Dir' is 0/1/2/3 == light coming from N/E/S/W.
+propagate(Map, [], Map) :- write("."), flush_output.
+propagate(Map, [X-Y-Dir|Queue], FinalMap) :-
+    (   (nth0(X, Map, Row), nth0(Y, Row, Cell-Done), nth0(Dir, Done, 0))
+    ->  replace(Dir, Done, 1, NewDone),
+        replace(Y, Row, Cell-NewDone, NewRow),
+        replace(X, Map, NewRow, NewMap),
+        call(Cell, X, Y, Dir, NewCells),
+        append(Queue, NewCells, NewQueue)
+    ;   NewQueue = Queue,
+        NewMap = Map
+    ),
+    propagate(NewMap, NewQueue, FinalMap).
+
+'.'(X, Y, 0, [NewX-Y-0]) :- NewX is X + 1.
+'.'(X, Y, 1, [X-NewY-1]) :- NewY is Y - 1.
+'.'(X, Y, 2, [NewX-Y-2]) :- NewX is X - 1.
+'.'(X, Y, 3, [X-NewY-3]) :- NewY is Y + 1.
+
+'/'(X, Y, 0, [X-NewY-1]) :- NewY is Y - 1.
+'/'(X, Y, 1, [NewX-Y-0]) :- NewX is X + 1.
+'/'(X, Y, 2, [X-NewY-3]) :- NewY is Y + 1.
+'/'(X, Y, 3, [NewX-Y-2]) :- NewX is X - 1.
+
+'\\'(X, Y, 0, [X-NewY-3]) :- NewY is Y + 1.
+'\\'(X, Y, 1, [NewX-Y-2]) :- NewX is X - 1.
+'\\'(X, Y, 2, [X-NewY-1]) :- NewY is Y - 1.
+'\\'(X, Y, 3, [NewX-Y-0]) :- NewX is X + 1.
+
+'|'(X, Y, 0, NewCells) :- '.'(X, Y, 0, NewCells).
+'|'(X, Y, 2, NewCells) :- '.'(X, Y, 2, NewCells).
+'|'(X, Y, 3, NewCells) :- '|'(X, Y, 1, NewCells).
+'|'(X, Y, 1, [X1-Y-0, X2-Y-2]) :- X1 is X + 1, X2 is X - 1.
+
+'-'(X, Y, 1, NewCells) :- '.'(X, Y, 1, NewCells).
+'-'(X, Y, 3, NewCells) :- '.'(X, Y, 3, NewCells).
+'-'(X, Y, 2, NewCells) :- '-'(X, Y, 0, NewCells).
+'-'(X, Y, 0, [X-Y1-1, X-Y2-3]) :- Y1 is Y - 1, Y2 is Y + 1.
+
+replace(I, List, Elem, NewList) :-
+    nth0(I, List, _, Rest),
+    nth0(I, NewList, Elem, Rest).
+
+% Input stuff. Cell-(N,E,S,W) indicating which direction was already handled
+input(Name, Map) :- phrase_from_file(lines(Map), Name).
+
+lines([]) --> eos, !.
+lines([Line|Lines]) --> line(Line), lines(Lines).
+
+line([]) --> ("\n"; eos), !.
+line([Char-[0,0,0,0]|Chars]) --> [C], line(Chars), {char_code(Char, C)}.
+
+eos([], []).
+
+% Debug stuff
+print(Map) :- maplist([X]>>(write(X), nl), Map), nl.
+