aoc23/11/part1.pl

:- use_module(library(pio)).
:- initialization(main, main).

:- table galaxies/3.

main([FileName | _]) :-
    input(FileName, Map),
    findall(Dist,
            (   galaxies(Axis, Map, A-ACount), ACount =\= 0,
                galaxies(Axis, Map, B-BCount), BCount =\= 0,
                A < B,
                distance(Axis, Map, A, B, DistPair),
                Dist is DistPair * ACount * BCount),
            Dists),
    % Sum of manhattan distances is sum of distances on all axes.
    sum_list(Dists, Sum),
    writef('Answer=%t\n', [Sum]).

distance(Axis, Map, X1, X2, Dist) :-
    true_coord(Axis, Map, X1, TrueX1), true_coord(Axis, Map, X2, TrueX2),
    Dist is abs(TrueX1 - TrueX2).

true_coord(Axis, Map, X, TrueX) :-
    findall(E, (call(galaxies, Axis, Map, E-0), E < X), EmptyRows),
    length(EmptyRows, ToAdd),
    TrueX is X + ToAdd.

% galaxies(Axis, Map, X-Count): There are Count galaxies at coordinate X on Axis
galaxies(x, Map, X-Count) :- nth0(X, Map, Row), foldl(plus, Row, 0, Count).
galaxies(y, Map, Y-Count) :-
    foldl({Y}/[Row, V0, V]>>(nth0(Y, Row, C), V is V0 + C), Map, 0, Count).

input(FileName, Map) :- phrase_from_file(lines(Map), FileName).
lines([]) --> eos, !.
lines([Line|Lines]) --> line(Line), lines(Lines).
line([]) --> ( "\n" ; eos ), !.
line([L|Ls]) --> ([C], {C =:= 35 -> L = 1; L = 0}), line(Ls).
eos([], []).