48 lines
1.5 KiB
Prolog
48 lines
1.5 KiB
Prolog
:- use_module(library(pio)).
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:- use_module(library(dcg/basics)).
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:- initialization(main, main).
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main([FileName|_]) :-
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input(FileName, Map, Starting),
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nsteps(Map, 64, [Starting], Ends), length(Ends, Answer),
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write(Answer), nl.
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nsteps(Map, N, Starts, Reachables) :-
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length(Range, N),
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foldl({Map}/[_, SIn, SOut]>>(step(Map, SIn, SOut)), Range, Starts, Reachables).
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step(Map, CurrentCells, NextCells) :-
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maplist(neighbors(Map), CurrentCells, NeighborsOfCells),
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foldl(
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[Neighbors, SetIn, SetOut]>>(
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list_to_ord_set(Neighbors, NeighborsSet),
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ord_union(SetIn, NeighborsSet, SetOut)),
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NeighborsOfCells, [], NextCells).
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neighbors(Map, X-Y, Neighbors) :-
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findall(X1-Y1, neighbor(Map, X-Y, X1-Y1), Neighbors).
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neighbor(Map, X-Y, X1-Y1) :-
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( X1 is X, Y1 is Y+1; X1 is X, Y1 is Y-1;
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X1 is X+1, Y1 is Y; X1 is X-1, Y1 is Y ),
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nth0(X1, Map, Row), nth0(Y1, Row, '.').
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replace(I, List, E, NewList) :- nth0(I, List, _, R), nth0(I, NewList, E, R).
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% input parsing stuff below
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input(FileName, Map, StartX-StartY) :-
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phrase_from_file(lines(MapS), FileName),
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nth0(StartX, MapS, Row), nth0(StartY, Row, s),
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select(s, Row, '.', NewRow), replace(StartX, MapS, NewRow, Map).
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lines([]) --> eos, !.
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lines([Line|Lines]) --> line(Line), lines(Lines).
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line([]) --> "\n"; eos.
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line([s|Chars]) --> "S", line(Chars).
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line(['.'|Chars]) --> ".", line(Chars).
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line(['#'|Chars]) --> "#", line(Chars).
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% Debug stuff
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print(Map) :- maplist([X]>>(atomics_to_string(X, XStr), write(XStr), nl), Map).
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