Gah. Prolog is hard. I'm not able to find logic programming solutions to these, so I'm just doing it like I would in any other language, which is sort of defeating the point. I may go back and redo days 1 and 2, now that I understand Prolog a bit more, I'm sure I can do those better.

day1/main.pl

@@ -0,0 +1,66 @@
+file_to_lines(Path, Lines) :-
+    read_file_to_string(Path, S, []),
+    split_string(S, "\n", "\n", Lines).
+
+% Convert "L68" into the tuple l-68
+line_to_turn(Turn, Dir-Dist) :-
+    sub_atom(Turn, 0, 1, Len, Dir0),
+    sub_string(Turn, 1, Len, _, Dist0),
+    number_string(Dist, Dist0),
+    downcase_atom(Dir0, Dir).
+
+main :-
+    file_to_lines("data.txt", Lines),
+    maplist(line_to_turn, Lines, Turns),
+    part1(50, 0, Turns, Res),
+    writeln(Res),
+    part2(50, 0, Turns, Res2),
+    writeln(Res2).
+
+% PART 1
+
+turn_knob(Start, Dir-Dist, Result) :-
+    Dir = l
+        -> Result is (Start - Dist) mod 100
+        ; Result is (Start + Dist) mod 100.
+
+% recursion base case, if list is empty, unify the NumZeros with the Result
+part1(_, Res, [], Res).
+
+% recurse through the list, turning the knob, counting zeros
+part1(Start, NumZeros, [Turn|Rest], Res) :-
+    turn_knob(Start, Turn, NewStart),
+    (NewStart = 0
+        -> NewNumZeros is NumZeros + 1,
+        part1(NewStart, NewNumZeros, Rest, Res)
+        ; part1(NewStart, NumZeros, Rest, Res)).
+
+% PART 2
+
+% 6225 too low
+% 6323 too high
+% 6238 wrong
+
+turn_knob_count_zeros(Start, Dir-Dist, Result, Zeros) :-
+    ( Dir = l -> Diff is -Dist ; Diff is Dist ),
+    Next is Start + Diff,
+    divmod(Next, 100, Zeros0, Result0),
+    Result is ((Result0 mod 100) + 100) mod 100,  % normalize result
+    Zeros1 is abs(Zeros0),
+    (Start = 0, Dir = l, Dist < 100 -> Zeros2 is Zeros1 - 1 ; Zeros2 is Zeros1),
+    (Next = 0 -> Zeros is Zeros2 + 1 ; Zeros = Zeros2),
+    format('Start: ~w Dir: ~w Dist: ~w Next: ~w Result: ~w Zeros: ~w~n',
+           [Start, Dir, Dist, Next, Result, Zeros]).
+
+part2(_, Res, [], Res).
+part2(Start, NumZeros, [Turn|Rest], Res) :-
+    turn_knob_count_zeros(Start, Turn, NewStart, Zeros),
+    NewNumZeros is Zeros + NumZeros,
+    format('New Zeros ~w~n', NewNumZeros),
+    part2(NewStart, NewNumZeros, Rest, Res).
+
+
+/*
+Start: 0 Dir: l Dist: 223 Next: -223 Result: 77 Zeros: 3
+Start: 52 Dir: r Dist: 192 Next: 244 Result: 44 Zeros: 2
+*/