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%A sat solver, utilising delay declaration to implement
%watched literals
%
%Version using when for delay (SWI)
%
%Authors: Jacob Howe and Andy King
%Last modified: 4/4/11
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

:- module(sat_solver, [initialise/1, sat/2, search/4]).

initialise(_).

search(Clauses, Vars, Sat, _) :-
    sat(Clauses, Vars),
    !,
    Sat = true.
search(_Clauses, _Vars, false, _).

sat(Clauses, Vars) :-
    problem_setup(Clauses), elim_var(Vars).

elim_var([]). 
elim_var([Var | Vars]) :- 
    elim_var(Vars), (Var = true; Var = false). 

problem_setup([]). 
problem_setup([Clause | Clauses]) :- 
    clause_setup(Clause), 
    problem_setup(Clauses). 

clause_setup([Pol-Var | Pairs]) :- set_watch(Pairs, Var, Pol). 

set_watch([], Var, Pol) :- Var = Pol. 
set_watch([Pol2-Var2 | Pairs], Var1, Pol1) :- 
    when(;(nonvar(Var1),nonvar(Var2)),watch(Var1, Pol1, Var2, Pol2, Pairs)). 

watch(Var1, Pol1, Var2, Pol2, Pairs) :- 
    nonvar(Var1) -> 
      update_watch(Var1, Pol1, Var2, Pol2, Pairs); 
      update_watch(Var2, Pol2, Var1, Pol1, Pairs). 

update_watch(Var1, Pol1, Var2, Pol2, Pairs) :- 
    Var1 == Pol1 -> true; set_watch(Pairs, Var2, Pol2).