Day 2



This problem was fun because it involved some real parsing. Being able to treat strings directly as Prolog atoms with term_string/2 was a neat way to deconstruct the first column in the solution (e.g. 1-3) directly into a pair.

Before coming to use include/3, I tried out maplist/3 with some moderate success by adding an extra variable to verify. For some reason, the interpreter was unhappy with it. include/3 is a much nicer way to define the list anyhow. I was also surprised that I had to write my own predicate to count the number of times a particular item exists within a list. I might have been able to get away with this using include/3, but I’m not proficient enough yet to know.


count(_, [], 0).
count(C, [C|T], N):-
    count(C, T, Res),
    N is Res+1.
count(C, [X|T], N):-
    C \= X,
    count(C, T, N).
parse(String, Data):-
    split_string(String, '\s', '\s', [MinMax, CharColon, Password]),
    term_string(Min-Max, MinMax),
    % remove the colon in the data
    string_chars(CharColon, [Char|_]),
    Data = (Min, Max, Char, Password).

    parse(String, (Min, Max, Char, Password)),
    string_chars(Password, Chars),
    count(Char, Chars, N),
    N >= Min,
    N =< Max.

main(String, Result):-
    split_string(String, '\n', '\n', L),
    include(verify, L, Verified),
    length(Verified, Result).

    parse(String, (Min, Max, Char, Password)),
    string_chars(Password, Chars),
    nth1(Min, Chars, Pos0),
    nth1(Max, Chars, Pos1),
    Pos0 \= Pos1,
    (   Pos0 = Char, Pos1 \= Char;
       Pos0 \= Char, Pos1 = Char).

main2(String, Result):-
    split_string(String, '\n', '\n', L),
    include(verify2, L, Verified),
    length(Verified, Result).

    X = "1-3 a: abcde
1-3 b: cdefg
2-9 c: ccccccccc".