*L*- a collection (a la collection_to_list/2) of domain variables (Arg = 0) or a collection of terms (Arg > 0)
*Arg*- an integer, which is 0 if the list is a list of domain variables, or greater than 0 if the list consists of terms with arity at least Arg (the value Arg indicating the argument that contains the domain variables to be labeled)
*Select*- the name of a predefined selection method (input_order, first_fail, smallest, largest, occurrence, most_constrained, max_regret, anti_first_fail), or an atom or compound term specifying a user-defined selection method
*Choice*- the name of a predefined choice method (indomain, indomain_min, indomain_max, indomain_middle, indomain_reverse_min, indomain_reverse_max, indomain_median, indomain_split, indomain_reverse_split, indomain_random, indomain_interval), or an atom or compound term specifying a user-defined method
*Method*- one of the following search method specifications: complete, bbs(Steps:integer), lds(Disc:integer), credit(Credit:integer, Extra:integer or bbs(Steps:integer) or lds(Disc:integer)), dbs(Level:integer, Extra:integer or bbs(Steps:integer) or lds(Disc:integer)), sbds, gap_sbds, gap_sbdd
*Option*- a list of option terms. Currently recognized are backtrack(-N), node(++Call), nodes(++N)

All search methods use a *stable* selection method. If several entries
have the same heuristic value, then the first one is selected. The rest of
the collection (treated as a list) is equal to the original list with the
selected entry removed, the order of the non-selected entries does not
change.

The pre-defined **selection methods** are the following:

**input_order**the first entry in the list is selected**first_fail**the entry with the smallest domain size is selected**anti_first_fail**the entry with the largest domain size is selected**smallest**the entry with the smallest value in the domain is selected**largest**the entry with the largest value in the domain is selected**occurrence**the entry with the largest number of attached constraints is selected**most_constrained**the entry with the smallest domain size is selected. If several entries have the same domain size, the entry with the largest number of attached constraints is selected.**max_regret**the entry with the largest difference between the smallest and second smallest value in the domain is selected. This method is typically used if the variable represents a cost, and we are interested in the choice which could increase overall cost the most if the best possibility is not taken. Unfortunately, the implementation does not always work: If two decision variables incur the same minimal cost, the regret is not calculated as zero, but as the difference from this minimal value to the next greater value.

Any other atom will be taken as the specification of a user-defined selection predicate. This will be invoked with 2 arguments (X,Criterion) added and is expected to compute a selection criterion (typically a number) from a variable or value X. E.g. if Select is 'my_select', a predicate definition like the following has to be provided:

my_select(X,Criterion) :- ... % compute Criterion from variable XThe variable-selection will then select the variable with the lowest value (in standard term order) of Criterion. If several variables have the same value, the first one is selected.

The above selection methods use the predefined delete/5 predicate. If this is not general enough, you can replace it with your own: if Select is given as select(my_delete), then my_delete(-SelectedVar,+List,-Rest,+Arg) will be invoked for selecting a variable from List.

The pre-defined **choice methods** have the following meaning:

**indomain**uses the built-in indomain/1. Values are tried in increasing order. On failure, the previously tested value is not removed.**indomain_min**Values are tried in increasing order. On failure, the previously tested value is removed. The values are tested in the same order as for**indomain**, but backtracking may occur earlier.**indomain_max**Values are tried in decreasing order. On failure, the previously tested value is removed.**indomain_reverse_min**Like indomain_min, but the alternatives are tried in reverse order. I.e. the smallest value is first removed from the domain, and only if that fails, the value is assigned.**indomain_reverse_max**Like indomain_max, but the alternatives are tried in reverse order. I.e. the largest value is first removed from the domain, and only if that fails, the value is assigned.**indomain_middle**Values are tried beginning from the middle of the domain. On failure, the previously tested value is removed.**indomain_median**Values are tried beginning from the median value of the domain. On failure, the previously tested value is removed.**indomain_split**Values are tried by succesive domain splitting, trying the lower half of the domain first. On failure, the tried interval is removed. This enumerates values in the same order as indomain or indomain_min, but may fail earlier.**indomain_reverse_split**Values are tried by succesive domain splitting, trying the upper half of the domain first. On failure, the tried interval is removed. This enumerates values in the same order as indomain_max, but may fail earlier.**indomain_random**Values are tried in a random order. On backtracking, the previously tried value is removed. Using this routine may lead to non-reproducible results, as another call will create random numbers in a different sequence. This method uses the built-in**random/1**to create random numbers,**seed/1**can be used to force the same number generation sequence in another run.**indomain_interval**If the domain consists of several intervals, we first branch on the choice of the interval. For one interval, we use domain splitting.

Any other name is taken as the name of a user-defined predicate of arity 1, to which the variable to be labeled (or a whole element of list L, in the Arg>0 case) is passed, e.g.

my_choice(X) :- ... % make a choice on variable XAlternatively, a term with 2 arguments can be given as the choice-method, e.g. my_choice(FirstIn,LastOut). this will lead to the invocation of a choice predicate with arity 3, e.g.

my_choice(X,In,Out) :- ... % make a choice on variable X, using In-OutThis allows user-defined state to be transferred between the subsequent invocations of the choice-predicate (the Out argument of a call to my_choice/3 for one variable is unified with the In argument of the call to my_choice/3 for the next variable, and so on).

In addition, a fixed argument can be passed: my_choice(Param) leads to invocation of my_choice(X,Param), and my_choice(Param,FirstIn,LastOut) leads to invocation of my_choice(X,Param,In,Out).

The different **search methods** are

**complete**a complete search routine which explores all alternative choices.**bbs(Steps)**The*bounded backtracking search*allows**Steps**backtracking steps.**lds(Disc)**A form of the*limited discrepancy search*. This method iteratively tries 0, 1, 2 ..**Disc**changes against the heuristic (first) value. Typical values are between 1 and 3 (which already may create too many alternatives for large problems). The original LDS paper stated that the discrepancy to be tested first should be at the top of the tree. Our implementation tries the first discrepancy at the bottom of the tree. This means that solutions may be found in a different order compared to the original algorithm. This change is imposed by the evaluation strategy used and can not be easily modified.**credit(Credit, bbs(Steps))**The credit based search explores the top of the search tree completely. Initially, a given number of credits (**Credit**) are given. At each choice point, the first alternative gets half of the available credit, the second alternative half of the remaining credit, and so on. When the credit run out, the system switches to another search routine, here bbs. In each of these bounded backtracking searches**Steps**backtracking steps are allowed before returning to the top most part of the tree and choosing the next remaining candidate. A good value for**Steps**is 5, a value of 0 forces a deterministic search using the heuristic. Typical values for**Credit**are either N or N*N, where N is the number of entries in the collection.**credit(Credit, lds(Disc))**like the one above, but using*lds*when the credit runs out. Typically, only one (perhaps 2) discrepancies should be allowed.**dbs(Level, bbs(Steps))**The*depth bounded search*explores the first**Level**choices in the search tree completely, i.e. it tries all values for the first**Level**selected variables. After that, it switches to another search method, here bbs. In each of these searches,**Steps**backtracking steps are allowed.**dbs(Level, lds(Disc))**like the method above, but switches to lds after the first**Level**variables.**sbds**A complete search routine which uses the SBDS symmetry breaking library (lib(ic_sbds) or lib(fd_sbds)) to exclude symmetric parts of the search tree from consideration. The symmetry breaking must be initialised through a call to sbds_initialise/4,5 before calling search/6. Currently the only pre-defined choice methods supported by this search method are**indomain_min**,**indomain_max**,**indomain_middle**,**indomain_median**and**indomain_random**. Any user-defined choice method used in conjunction with this search method must use sbds_try/2 to assign/exclude values or the symmetry breaking will not work correctly.**gap_sbds**(Not available for FD.) A complete search routine which uses the GAP-based SBDS symmetry breaking library lib(ic_gap_sbds) to exclude symmetric parts of the search tree from consideration. The symmetry breaking must be initialised through a call to sbds_initialise/5 before calling search/6. Currently the only pre-defined choice methods supported by this search method are**indomain_min**,**indomain_max**,**indomain_middle**,**indomain_median**and**indomain_random**. Any user-defined choice method used in conjunction with this search method must use sbds_try/2 to assign/exclude values or the symmetry breaking will not work correctly.**gap_sbdd**(Not available for FD.) A complete search routine which uses the GAP-based SBDD symmetry breaking library lib(ic_gap_sbdd) to exclude symmetric parts of the search tree from consideration. The symmetry breaking must be initialised through a call to sbdd_initialise/5 before calling search/6. Currently the only pre-defined choice methods supported by this search method are**indomain_min**,**indomain_max**,**indomain_middle**,**indomain_median**and**indomain_random**. Any user-defined choice method used in conjunction with this search method must use sbdd_try/2 to assign/exclude values or the symmetry breaking will not work correctly.

The option list is used to pass additional parameters to and from the procedure. The currently recognized options are:

**backtrack(-N)**returns the number of backtracking steps used in the search routine**nodes(++N)**sets an upper limit on the number of nodes explored in the search. If the given limit is exceeded, the search routine stops the exploration of the search tree.**node(daVinci)**create a drawing of the search tree using the daVinci graph drawing tool. Each node of the search tree is shown as a node in the tree. The label of the node is the selected term of the collection.**node(daVinci(++Call))**as the previous option, it creates a drawing of the search tree using the daVinci graph drawing tool. But instead of using the complete selected term as the label, it call the predicate**Call/2**to choose which part of the selected term to display.

top:- length(L,8), L :: 1..8, search(L,0,input_order,indomain,complete,[]). top:- length(L,8), L :: 1..8, search(L,0,input_order,indomain,bbs(15),[]). top:- length(L,8), L :: 1..8, search(L,0,input_order,indomain,lds(2),[]). top:- length(L,8), L :: 1..8, search(L,0,input_order,indomain,credit(64,bbs(5)),[]). top:- length(L,8), L :: 1..8, search(L,0,input_order,indomain,dbs(2,lds(1)),[]). % a more complex example with different methods and heuristics % the list to be assigned is a list of terms queen/2 :- local struct(queen(place,var)). top:- member(Method,[complete,lds(2),credit(64,5),bbs(1000),dbs(5,10)]), member(Select,[first_fail,most_constrained,input_order]), member(Choice,[indomain, indomain_min, indomain_max, indomain_middle, indomain_median, indomain_split, indomain_random]), writeln(queen(Method,Select,Choice)), once(queen_credit(64,Select,Choice,Method,L,Back)), writeln(L), writeln(backtrack(Back)), fail. top:- nl. queen_credit(N,Select,Choice,Method,L,Back):- create_queens(1,N,Queens,L), setup(L), rearrange(Queens,Queens,[],[],Queens1), search(Queens1, var of queen, Select, Choice, Method, [backtrack(Back)]). rearrange([],Last,Last,Res,Res). rearrange([_],[Mid|Last],Last,Res,[Mid|Res]). rearrange([_,_|S],[H|T],A1,In,Res):- rearrange(S,T,[A|A1],[H,A|In],Res). create_queens(N,M,[],[]):- N > M, !. create_queens(N,M,[queen{place:N,var:X}|T],[X|L]):- X :: 1..M, N1 is N+1, create_queens(N1,M,T,L). setup([]). setup([H|T]):- setup1(H,T,1), setup(T). setup1(_,[],_). setup1(X,[Y|R],N):- X #\= Y, X #\= Y + N, Y #\= X + N, N1 is N+1, setup1(X,R,N1). % this example shows how to pass information from one assignment step % to the next % this uses a term of two arguments as the choice argument % The example also shows the use of the option argument: % the search tree generated is drawn with the daVinci graph drawing tool % and the search is limited to 1000 nodes. % The number of backtracking steps is returned in the variables Back. :-local struct(country(name,color)). top:- countries(C), create_countries(C,Countries,Vars), findall(n(A,B),n(A,B),L), setup(L,Countries), search(Countries, color of country, % select based on this variable most_constrained, assign([1,2,3,4],Out), % this calls assign/3 complete, [backtrack(Back),node(daVinci),nodes(1000)]), writeln(Vars), writeln(Back), writeln(Out). create_countries([],[],[]). create_countries([C|C1],[country{name:C, color:V}|R1],[V|V1]):- V :: 1..4, create_countries(C1,R1,V1). setup([],_L). setup([n(A,B)|N1],L):- member(country{name:A, color:Av},L), member(country{name:B, color:Bv},L), Av #\= Bv, setup(N1,L). % this is the choice predicate % the first argument is the complete selected term % the second is the input argument % the third is the output argument % here we pass a list of values and rotate this list from one step to the next assign(country{color:X},L,L1):- rotate(L,L1), member(X,L). rotate([A,B,C,D],[B,C,D,A]). % another example of argument passing % here each entry gets the same information % it is passed unchanged from one level to the next top:- ... length(L,N), L :: 1..10, ... search(L, 0, most_constrained, % pass two lists as the In argument of assign % try the odd numbers before the even numbers assign([1,3,5,7,9]-[2,4,6,8,10],_), complete,[]), ... % this is the assignment routine % the first argument is a % Pass the In argument as the Out argument % try values from list L1 before values from list L2 assign(X,L1-L2,L1-L2):- member(X,L1);member(X,L2). % and another example from square placement % alternatively try minimal and maximal values first :-local struct(square(x,y,size)). top:- data(L), create_squares(L,Squares), ... search(Squares, 0, % this value does not matter if input_order is chosen input_order, assign(min,_), complete, []), ... % the assignment routine % alternate between min and max for consecutive levels in the search assign(square{x:X,y:Y},Type,Type1):- swap(Type,Type1), indomain(X,Type), indomain(Y,Type). swap(max,min). swap(min,max). % this example shows that the choice routine may consist of several clauses % the idea comes from a graph coloring heuristic top:- length(L,N), L :: 1..100, ... search(L, 0, most_constrained, assign(0,K), The In argument is the highest color used so far complete,[]), ... % assign variable X either to one of the colors 1..K % which have already been used, or to the new color K+1 % we do not need to try other values K+2 etc, as this is a symmetry that % we can avoid assign(X,K,K):- X #=< K, indomain(X). assign(K1,K,K1):- K1 is K+1. % example showing use of the SBDS library with a user-defined choice method % which calls sbds_try/2. top:- dim(M, [8]), M[1..8] :: 1..8, ... sbds_initialise(M,SymPreds,#=,[]), M =.. [_|L], % get list of variables for search routine search(L,0,first_fail,sbds_indomain_max,sbds,[]). sbds_indomain_max(X):- nonvar(X). sbds_indomain_max(X):- var(X), get_max(X,Max), sbds_try(X,Max), sbds_indomain_max(X). % Example showing use of the GAP-based SBDS library with a user-defined % choice method which calls sbds_try/2. (For the GAP-based SBDD library, % just substitute "sbdd" for each occurrence of "sbds" below.) top:- dim(M, [8]), M[1..8] :: 1..8, sbds_initialise(M,[vars],values:1..8,[symmetry(s_n,[vars])],[]), M =.. [_|L], % get list of variables for search routine search(L,0,first_fail,gap_sbds_indomain_max,gap_sbds,[]). gap_sbds_indomain_max(X):- nonvar(X). gap_sbds_indomain_max(X):- var(X), get_max(X,Max), sbds_try(X,Max), gap_sbds_indomain_max(X).