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.

day3/main.pl

@@ -0,0 +1,52 @@
+:- use_module(library(lists)).
+
+main :-
+    File = "data.txt",
+    run(File, 2, Part1),
+    writeln(Part1),
+    run(File, 12, Part2),
+    writeln(Part2).
+
+file_bank(Path, Bank) :-
+    read_file_to_string(Path, S, []),
+    split_string(S, "\n", "\n", Lines),
+    member(Line, Lines),
+    number_string(Number, Line),
+    number_chars(Number, Chars),
+    maplist(atom_number, Chars, Bank).
+
+run(Path, DigitSize, Sum) :-
+    findall(N,
+        (file_bank(Path, Bank), bank_max(Bank, DigitSize, N)),
+        Ns),
+    sumlist(Ns, Sum).
+
+bank_max(Bank, DigitSize, Max) :-
+    bank_max_acc(Bank, DigitSize, [], Answer),
+    list_to_number(Answer, Max).
+
+bank_max_acc(_, 0, NumList, NumList).
+bank_max_acc(Bank, DigitsLeft, NumList, Answer) :-
+    length(Bank, Len),
+    SplitIndex is Len - DigitsLeft + 1,
+    split_list_at_index(Bank, SplitIndex, Left, _),
+    max_list(Left, Digit),
+    nth0(FoundIndex, Left, Digit),
+    DigitsLeft2 is DigitsLeft - 1,
+    append(NumList, [Digit], NumList2),
+    NewSplitIndex is FoundIndex + 1,
+    split_list_at_index(Bank, NewSplitIndex, _, Rest),
+    bank_max_acc(Rest, DigitsLeft2, NumList2, Answer),
+    !.
+
+% Helpers
+
+split_list_at_index(List, Index, Left, Right) :-
+    length(Left, Index),
+    append(Left, Right, List).
+
+list_to_number(Digits, Number) :- list_to_number_acc(Digits, 0, Number).
+list_to_number_acc([], Acc, Acc).
+list_to_number_acc([Digit|Rest], AccIn, Number) :-
+    AccNext is AccIn * 10 + Digit,
+    list_to_number_acc(Rest, AccNext, Number).