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hypothesis_reducer: worked around propositional literals

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propositional formulas (disjunctions) can appear as literals.  This
makes it tricky to recognize whether a formula is a unit clause when
re-building unit resolution.

Added work-around that identifies whether a formula is a literal based
on its appearance in previous unit resolution step.
This commit is contained in:
Arie Gurfinkel 2018-05-18 14:12:15 -07:00
parent b39c532f19
commit 5a6bd5e782
2 changed files with 43 additions and 31 deletions
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72
src/muz/spacer/spacer_proof_utils.cpp
View file

@ -363,7 +363,7 @@ proof* hypothesis_reducer::reduce_core(proof* pf) {
else if (m.is_unit_resolution(p)) {
// unit: reduce untis; reduce the first premise; rebuild
// unit resolution
res = mk_unit_resolution_core(args);
res = mk_unit_resolution_core(p, args);
// -- re-compute hypsets
compute_hypsets(res);
}
@ -420,12 +420,13 @@ proof* hypothesis_reducer::mk_lemma_core(proof* premise, expr *fact) {
return res;
}
proof* hypothesis_reducer::mk_unit_resolution_core(ptr_buffer<proof>& args) {
proof* hypothesis_reducer::mk_unit_resolution_core(proof *ures,
ptr_buffer<proof>& args) {
// if any literal is false, we don't need a unit resolution step
// This can be the case due to some previous transformations
for (unsigned i = 1, sz = args.size(); i < sz; ++i) {
if (m.is_false(m.get_fact(args[i]))) {
// XXX just in case
// XXX pin just in case
m_pinned.push_back(args[i]);
return args[i];
}
@ -434,48 +435,58 @@ proof* hypothesis_reducer::mk_unit_resolution_core(ptr_buffer<proof>& args) {
proof* arg0 = args[0];
app *fact0 = to_app(m.get_fact(arg0));
ptr_buffer<proof> pf_args;
ptr_buffer<expr> pf_fact;
pf_args.push_back(arg0);
// check if fact0 can be resolved as a unit
// this is required for handling literals with OR
for (unsigned j = 1; j < args.size(); ++j) {
if (m.is_complement(fact0, m.get_fact(args[j]))) {
pf_args.push_back(args[j]);
break;
// compute literals to be resolved
ptr_buffer<expr> lits;
// fact0 is a literal whenever the original resolution was a
// binary resolution to an empty clause
if (m.get_num_parents(ures) == 2 && m.is_false(m.get_fact(ures))) {
lits.push_back(fact0);
}
// fact0 is a literal unless it is a dijsunction
else if (!m.is_or(fact0)) {
lits.push_back(fact0);
}
// fact0 is a literal only if it appears as a literal in the
// original resolution
else {
lits.reset();
app* ures_fact = to_app(m.get_fact(m.get_parent(ures, 0)));
for (unsigned i = 0, sz = ures_fact->get_num_args(); i < sz; ++i) {
if (ures_fact->get_arg(i) == fact0) {
lits.push_back(fact0);
break;
}
}
if (lits.empty()) {
lits.append(fact0->get_num_args(), fact0->get_args());
}
}
// if failed to find a resolvent, and the fact is a disjunction,
// attempt to resolve each disjunct
if (pf_args.size() == 1 && m.is_or(fact0)) {
ptr_buffer<expr> cls;
for (unsigned i = 0, sz = fact0->get_num_args(); i < sz; ++i)
cls.push_back(fact0->get_arg(i));
// -- find all literals that are resolved on
// XXX quadratic implementation
for (unsigned i = 0, sz = cls.size(); i < sz; ++i) {
bool found = false;
for (unsigned j = 1; j < args.size(); ++j) {
if (m.is_complement(cls.get(i), m.get_fact(args[j]))) {
found = true;
pf_args.push_back(args[j]);
break;
}
// -- find all literals that are resolved on
for (unsigned i = 0, sz = lits.size(); i < sz; ++i) {
bool found = false;
for (unsigned j = 1; j < args.size(); ++j) {
if (m.is_complement(lits.get(i), m.get_fact(args[j]))) {
found = true;
pf_args.push_back(args[j]);
break;
}
if (!found) pf_fact.push_back(cls.get(i));
}
SASSERT(pf_fact.size() + pf_args.size() - 1 == cls.size());
if (!found) {pf_fact.push_back(lits.get(i));}
}
// unit resolution got reduced to noop
if (pf_args.size() == 1) {
// XXX pin just in case
m_pinned.push_back(arg0);
return arg0;
}
@ -484,6 +495,7 @@ proof* hypothesis_reducer::mk_unit_resolution_core(ptr_buffer<proof>& args) {
tmp = mk_or(m, pf_fact.size(), pf_fact.c_ptr());
proof* res = m.mk_unit_resolution(pf_args.size(), pf_args.c_ptr(), tmp);
m_pinned.push_back(res);
return res;
}

2
src/muz/spacer/spacer_proof_utils.h
View file

@ -96,7 +96,7 @@ private:
proof* reduce_core(proof* pf);
proof* mk_lemma_core(proof *pf, expr *fact);
proof* mk_unit_resolution_core(ptr_buffer<proof>& args);
proof* mk_unit_resolution_core(proof* ures, ptr_buffer<proof>& args);
proof* mk_proof_core(proof* old, ptr_buffer<proof>& args);
};
}