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Polysat: fixes in solver, forbidden intervals for eq_constraint (#5240)

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* Rename to neg_cond

* Add some logging utilities

* Implement case of forbidden interval covering the whole domain

* Implement diseq_narrow

* Do not activate constraint if we are in a conflict state

* comments

* Assert that lemma isn't undefined

* Update revert_decision to work in the case where narrowing causes propagation

* Fix case of non-disjunctive lemma from forbidden intervals

* Conflict should not leak outside user scope

* Add guard to decide(), some notes

* Add test case

* Add constraints to watchlist of unassigned variable during propagation

* Move common propagation functionality into base class

* Combine eq/diseq narrow

* Compute forbidden interval for equality constraints by considering them as p <=u 0 (or p >u 0 for disequalities)
This commit is contained in:
Jakob Rath 2021-05-03 18:30:17 +02:00 • committed by GitHub
parent 04876ba8b7
commit f7e476a4a0
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15 changed files with 350 additions and 130 deletions
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165
src/math/polysat/eq_constraint.cpp
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@ -22,26 +22,6 @@ namespace polysat {
return out << p() << (sign() == pos_t ? " == 0" : " != 0") << " [" << m_status << "]";
}
bool eq_constraint::propagate(solver& s, pvar v) {
LOG_H3("Propagate " << s.m_vars[v] << " in " << *this);
SASSERT(!vars().empty());
unsigned idx = 0;
if (vars()[idx] != v)
idx = 1;
SASSERT(v == vars()[idx]);
// find other watch variable.
for (unsigned i = vars().size(); i-- > 2; ) {
if (!s.is_assigned(vars()[i])) {
std::swap(vars()[idx], vars()[i]);
return true;
}
}
// at most one variable remains unassigned.
narrow(s);
return false;
}
constraint* eq_constraint::resolve(solver& s, pvar v) {
if (is_positive())
return eq_resolve(s, v);
@ -52,10 +32,50 @@ namespace polysat {
}
void eq_constraint::narrow(solver& s) {
if (is_positive())
eq_narrow(s);
if (is_negative())
diseq_narrow(s);
SASSERT(!is_undef());
LOG("Assignment: " << s.m_search);
auto q = p().subst_val(s.m_search);
LOG("Substituted: " << p() << " := " << q);
if (q.is_zero()) {
if (is_positive())
return;
if (is_negative()) {
LOG("Conflict (zero under current assignment)");
s.set_conflict(*this);
return;
}
}
if (q.is_never_zero()) {
if (is_positive()) {
LOG("Conflict (never zero under current assignment)");
s.set_conflict(*this);
return;
}
if (is_negative())
return;
}
if (q.is_unilinear()) {
// a*x + b == 0
pvar v = q.var();
rational a = q.hi().val();
rational b = q.lo().val();
bddv const& x = s.var2bits(v).var();
bddv lhs = a * x + b;
rational zero = rational::zero();
bdd xs = is_positive() ? (lhs == zero) : (lhs != zero);
s.push_cjust(v, this);
s.intersect_viable(v, xs);
rational val;
if (s.find_viable(v, val) == dd::find_t::singleton) {
s.propagate(v, val, *this);
}
return;
}
// TODO: what other constraints can be extracted cheaply?
}
bool eq_constraint::is_always_false() {
@ -87,6 +107,7 @@ namespace polysat {
return false;
}
/**
* Equality constraints
*/
@ -112,39 +133,6 @@ namespace polysat {
return nullptr;
}
void eq_constraint::eq_narrow(solver& s) {
LOG("Assignment: " << s.m_search);
auto q = p().subst_val(s.m_search);
LOG("Substituted: " << p() << " := " << q);
if (q.is_zero())
return;
if (q.is_never_zero()) {
LOG("Conflict (never zero under current assignment)");
s.set_conflict(*this);
return;
}
if (q.is_unilinear()) {
// a*x + b == 0
pvar v = q.var();
rational a = q.hi().val();
rational b = q.lo().val();
bddv const& x = s.var2bits(v).var();
bdd xs = (a * x + b == rational(0));
s.push_cjust(v, this);
s.intersect_viable(v, xs);
rational val;
if (s.find_viable(v, val) == dd::find_t::singleton) {
s.propagate(v, val, *this);
}
return;
}
// TODO: what other constraints can be extracted cheaply?
}
/**
* Disequality constraints
@ -155,8 +143,67 @@ namespace polysat {
return nullptr;
}
void eq_constraint::diseq_narrow(solver& s) {
NOT_IMPLEMENTED_YET();
/// Compute forbidden interval for equality constraint by considering it as p <=u 0 (or p >u 0 for disequality)
bool eq_constraint::forbidden_interval(solver& s, pvar v, eval_interval& i, constraint*& neg_condition)
{
SASSERT(!is_undef());
// Current only works when degree(v) is at most one
unsigned const deg = p().degree(v);
if (deg > 1)
return false;
if (deg == 0) {
UNREACHABLE(); // this case is not useful for conflict resolution (but it could be handled in principle)
// i is empty or full, condition would be this constraint itself?
return true;
}
unsigned const sz = s.size(v);
dd::pdd_manager& m = s.sz2pdd(sz);
pdd p1 = m.zero();
pdd e1 = m.zero();
p().factor(v, 1, p1, e1);
pdd e2 = m.zero();
// Currently only works if coefficient is a power of two
if (!p1.is_val())
return false;
rational a1 = p1.val();
// TODO: to express the interval for coefficient 2^i symbolically, we need right-shift/upper-bits-extract in the language.
// So currently we can only do it if the coefficient is 1.
if (!a1.is_zero() && !a1.is_one())
return false;
/*
unsigned j1 = 0;
if (!a1.is_zero() && !a1.is_power_of_two(j1))
return false;
*/
// Concrete values of evaluable terms
auto e1s = e1.subst_val(s.m_search);
auto e2s = m.zero();
SASSERT(e1s.is_val());
SASSERT(e2s.is_val());
// e1 + t <= 0, with t = 2^j1*y
// condition for empty/full: 0 == -1, never satisfied, so we always have a proper interval!
SASSERT(!a1.is_zero());
pdd lo = 1 - e1;
rational lo_val = (1 - e1s).val();
pdd hi = -e1;
rational hi_val = (-e1s).val();
if (is_negative()) {
swap(lo, hi);
lo_val.swap(hi_val);
}
i = eval_interval::proper(lo, lo_val, hi, hi_val);
neg_condition = nullptr;
return true;
}
}