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.

day2/main.pl

@@ -0,0 +1,79 @@
+file_to_ranges(Path, Ranges) :-
+    read_file_to_string(Path, S, []),
+    split_string(S, ",", ",", Ranges).
+
+range_to_tuple(Range, Start-End) :-
+    split_string(Range, "-", "\n", [Start0|[End0]]),
+    number_string(Start, Start0),
+    number_string(End, End0).
+
+is_even(N) :- integer(N), mod(N, 2) =:= 0.
+
+main :-
+    file_to_ranges("data.txt", Ranges),
+    maplist(range_to_tuple, Ranges, Tuples),
+
+    % Part 1
+    maplist(find_invalids_part1, Tuples, InvalidsPart1Unflattened),
+    flatten(InvalidsPart1Unflattened, InvalidsPart1),
+    sumlist(InvalidsPart1, Part1Sum),
+    write("Part 1: "),
+    writeln(Part1Sum),
+
+    % Part 2
+    maplist(find_invalids_part2, Tuples, InvalidsPart2Unflattened),
+    flatten(InvalidsPart2Unflattened, InvalidsPart2),
+    sumlist(InvalidsPart2, Part2Sum),
+    write("Part 2: "),
+    writeln(Part2Sum).
+
+% Part 1
+
+find_invalids_part1(Start-End, Invalids) :-
+    numlist(Start, End, Numbers),
+    include(is_invalid_part1, Numbers, Invalids).
+
+is_invalid_part1(Number) :-
+    number_string(Number, String),
+    string_length(String, Len),
+    (is_even(Len) ->
+        Len2 is Len / 2,
+        sub_string(String, 0, Len2, _, Start),
+        sub_string(String, Len2, Len2, _, End),
+        Start = End
+    ; false).
+
+% Part 2
+
+find_invalids_part2(Start-End, Invalids) :-
+    numlist(Start, End, Numbers),
+    include(is_invalid_part2, Numbers, Invalids).
+
+is_invalid_part2(Number) :-
+    number_string(Number, String),
+    string_length(String, FullLength),
+    SubLenMax is floor(FullLength / 2),
+    numlist(1, SubLenMax, Lengths),
+    maplist(generate_substrings(String), Lengths, Subs),
+    include(is_made_of_subs(String), Subs, Results),
+    length(Results, L),
+    L > 0.
+
+generate_substrings(S, Len, Sub) :- sub_string(S, 0, Len, _, Sub).
+
+% don't want to try to do this smartly, let's just check a bunch
+is_made_of_subs(String, Sub) :-
+    string_concat(Sub, Sub, Twice),
+    string_concat(Twice, Sub, Thrice),
+    string_concat(Thrice, Sub, FourTimes),
+    string_concat(FourTimes, Sub, FiveTimes),
+    string_concat(FiveTimes, Sub, SixTimes),
+    string_concat(SixTimes, Sub, SevenTimes),
+    (
+    String = Twice;
+    String = Thrice;
+    String = FourTimes;
+    String = FiveTimes;
+    String = SixTimes;
+    String = SevenTimes
+    ).