/**
* var unification.
*
* <p>First, verify the Term eventually already unified with the same Var if the Term exist, unify
* var with that term, in order to handle situation as (A = p(X) , A = p(1)) which must produce
* X/1.
*
* <p>If instead the var is not already unified, then:
*
* <p>if the Term is a var bound to X, then try unification with X so for example if A=1, B=A then
* B is unified to 1 and not to A (note that it's coherent with chronological backtracking: the
* eventually backtracked A unification is always after backtracking of B unification.
*
* <p>if are the same Var, unification must succeed, but without any new bindings (to avoid cycles
* for extends in A = B, B = A)
*
* <p>if the term is a number, then it's a success and new link is created (retractable by means
* of a code)
*
* <p>if the term is a compound, then occur check test is executed: the var must not appear in the
* compound ( avoid X=p(X), or p(X,X)=p(Y,f(Y)) ); if occur check is ok then it's success and a
* new link is created (retractable by a code)
*/
boolean unify(List vl1, List vl2, Term t) {
Term tt = getTerm();
if (tt == this) {
t = t.getTerm();
if (t instanceof Var) {
if (this == t) {
try {
vl1.add(this);
} catch (NullPointerException e) {
/* vl1==null mean nothing intresting for the caller */
}
return true;
}
} else if (t instanceof Struct) {
// occur-check
if (occurCheck(vl2, (Struct) t)) {
return false;
}
} else if (!(t instanceof Number)) {
return false;
}
link = t;
try {
vl1.add(this);
} catch (NullPointerException e) {
/* vl1==null mean nothing intresting for the caller */
}
// System.out.println("VAR "+name+" BOUND to "+link+" - time: "+time+" - mark: "+mark);
return true;
} else {
return (tt.unify(vl1, vl2, t));
}
}
@Override
public boolean unify(Term t1) {
// used to be implemented using recursion but caused stack overflow problems with long lists
Term t2 = this;
do {
TermType tType = t1.getType();
if (tType == TermType.LIST) {
if (t2.getArgument(0).unify(t1.getArgument(0)) == false) {
return false;
}
t1 = t1.getArgument(1);
t2 = t2.getArgument(1);
} else if (tType.isVariable()) {
return t1.unify(t2);
} else {
return false;
}
} while (t2.getType() == TermType.LIST);
return t2.unify(t1);
}