aoc23/11/day11.pl

% usase: swipl day11.pl input.txt 1000000
:- use_module(library(pio)).
:- initialization(main, main).

main([FileName, ExpandFactorArg | _]) :-
    input(FileName, Map),
    atom_number(ExpandFactorArg, ExpandFactor),
    findall(Dist, distance(_, Map, _, ExpandFactor, Dist), Dists),
    sum_list(Dists, Sum),
    writef('Answer=%t\n', [Sum]).

% Dist is distance along Axis in Map between two row/col A & B (after expansion)
distance(Axis, Map, A-B, ExpandFactor, Dist) :-
    true_coords(Axis, Map, ExpandFactor, TrueNs),
    member(A-ACount, TrueNs), member(B-BCount, TrueNs), A < B,
    Dist is (B - A) * ACount * BCount.

% TrueXs is the list of all coordinates after expanion along Axis in Map
true_coords(x, Map, ExpandFactor, TrueXs) :-
    NextX = {ExpandFactor}/[Row, LastX-_, X-Count]>>(
        sum_list(Row, Count),
        (Count = 0 -> X is LastX + ExpandFactor; X is LastX + 1)),
    scanl(NextX, Map, -1-0, Xs), Xs = [_ | TrueXs].

true_coords(y, [FirstRow | Map], ExpandFactor, TrueYs) :-
    foldl(add_vectors, Map, FirstRow, CountY),
    NextY = {ExpandFactor}/[Count, LastY-_, Y-Count]>>(
        Count = 0 -> Y is LastY + ExpandFactor; Y is LastY + 1),
    scanl(NextY, CountY, -1-0, Ys), Ys = [_ | TrueYs].

add_vectors([], [], []).
add_vectors([A | V1], [B | V2], [C | V]) :- C is A + B, add_vectors(V1, V2, V).

% Read file into 2D array Map. 1 corresponding to a galaxy and 0 otherwise.
input(FileName, Map) :- phrase_from_file(lines(Map), FileName).
lines([]) --> eos.
lines([Line|Lines]) --> line(Line), lines(Lines).
line([]) --> "\n" ; eos.
line([1|Ls]) --> "#", line(Ls).
line([0|Ls]) --> ".", line(Ls).
eos([], []).