best structure Graph to implement Dijkstra in prolog

Question

The question is simple. How can I struct my Graph in SWI prolog to implement the Dijkstra's algorithm?

I have found this but it's too slow for my job.

Answer 1

That implementation isn't so bad:

?- time(dijkstra(penzance, Ss)).
% 3,778 inferences, 0,003 CPU in 0,003 seconds (99% CPU, 1102647 Lips)
Ss = [s(aberdeen, 682, [penzance, exeter, bristol, birmingham, manchester, carlisle, edinburgh|...]), s(aberystwyth, 352, [penzance, exeter, bristol, swansea, aberystwyth]), s(birmingham, 274, [penzance, exeter, bristol, birmingham]), s(brighton, 287, [penzance, exeter, portsmouth, brighton]), s(bristol, 188, [penzance, exeter, bristol]), s(cambridge, 339, [penzance, exeter|...]), s(cardiff, 322, [penzance|...]), s(carlisle, 474, [...|...]), s(..., ..., ...)|...].

SWI-Prolog offers attributed variables, then this answer could be relevant to you. I hope I will post later today an implementation of dijkstra/2 using attribute variables.

edit well, I must say that first time programming with attribute variables is not too much easy.

I'm using the suggestion from the answer by @Mat I linked above, abusing of attribute variables to get constant time access to properties attached to data as required of algorithm. I've (blindly) implemented the wikipedia algorithm, here my effort:

/*  File:    dijkstra_av.pl
    Author:  Carlo,,,
    Created: Aug  3 2012
    Purpose: learn graph programming with attribute variables
*/

:- module(dijkstra_av, [dijkstra_av/3]).

dijkstra_av(Graph, Start, Solution) :-
    setof(X, Y^D^(member(d(X,Y,D), Graph)
             ;member(d(Y,X,D), Graph)), Xs),
    length(Xs, L),
    length(Vs, L),
    aggregate_all(sum(D), member(d(_, _, D), Graph), Infinity),
    catch((algo(Graph, Infinity, Xs, Vs, Start, Solution),
           throw(sol(Solution))
          ), sol(Solution), true).

algo(Graph, Infinity, Xs, Vs, Start, Solution) :-
    pairs_keys_values(Ps, Xs, Vs),
    maplist(init_adjs(Ps), Graph),
    maplist(init_dist(Infinity), Ps),
    ord_memberchk(Start-Sv, Ps),
    put_attr(Sv, dist, 0),
    time(main_loop(Vs)),
    maplist(solution(Start), Vs, Solution).

solution(Start, V, s(N, D, [Start|P])) :-
    get_attr(V, name, N),
    get_attr(V, dist, D),
    rpath(V, [], P).

rpath(V, X, P) :-
    get_attr(V, name, N),
    (   get_attr(V, previous, Q)
    ->  rpath(Q, [N|X], P)
    ;   P = X
    ).

init_dist(Infinity, N-V) :-
    put_attr(V, name, N),
    put_attr(V, dist, Infinity).

init_adjs(Ps, d(X, Y, D)) :-
    ord_memberchk(X-Xv, Ps),
    ord_memberchk(Y-Yv, Ps),
    adj_add(Xv, Yv, D),
    adj_add(Yv, Xv, D).

adj_add(X, Y, D) :-
    (   get_attr(X, adjs, L)
    ->  put_attr(X, adjs, [Y-D|L])
    ;   put_attr(X, adjs, [Y-D])
    ).

main_loop([]).
main_loop([Q|Qs]) :-
    smallest_distance(Qs, Q, U, Qn),
    put_attr(U, assigned, true),
    get_attr(U, adjs, As),
    update_neighbours(As, U),
    main_loop(Qn).

smallest_distance([A|Qs], C, M, [T|Qn]) :-
    get_attr(A, dist, Av),
    get_attr(C, dist, Cv),
    (   Av < Cv
    ->  (N,T) = (A,C)
    ;   (N,T) = (C,A)
    ),
    !, smallest_distance(Qs, N, M, Qn).
smallest_distance([], U, U, []).

update_neighbours([V-Duv|Vs], U) :-
    (   get_attr(V, assigned, true)
    ->  true
    ;   get_attr(U, dist, Du),
        get_attr(V, dist, Dv),
        Alt is Du + Duv,
        (   Alt < Dv
        ->  put_attr(V, dist, Alt),
        put_attr(V, previous, U)
        ;   true
        )
    ),
    update_neighbours(Vs, U).
update_neighbours([], _).

:- begin_tests(dijkstra_av).

test(1) :-
    nl,
    time(dijkstra_av([d(a,b,1),d(b,c,1),d(c,d,1),d(a,d,2)], a, L)),
    maplist(writeln, L).

test(2) :-
    open('salesman.pl', read, F),
    readf(F, L),
    close(F),
    nl,
    dijkstra_av(L, penzance, R),
    maplist(writeln, R).

readf(F, [d(X,Y,D)|R]) :-
    read(F, dist(X,Y,D)), !, readf(F, R).
readf(_, []).

:- end_tests(dijkstra_av).

To be true, I prefer the code you linked in the question. There is an obvious point to optimize, smallest_distance/4 now use a dumb linear scan, using an rbtree the runtime should be better. But attributed variables must be handled with care.

time/1 apparently show an improvement

% 2,278 inferences, 0,003 CPU in 0,003 seconds (97% CPU, 747050 Lips)
s(aberdeen,682,[penzance,exeter,bristol,birmingham,manchester,carlisle,edinburgh,aberdeen])
....

but the graph is too small for any definitive assertion. Let we know if this snippet reduce the time required for your program.

File salesman.pl contains dist/3 facts, it's taken verbatim from the link in the question.