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fix bug in bound simplification in Gomory for mixed integer linear cuts, enable fixed variable redution after bugfix, add notes that rewriting bounds does not work

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Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>
This commit is contained in:
Nikolaj Bjorner 2023-11-10 16:38:55 +01:00
parent aa9c7912dc
commit 3de5af3cb0
5 changed files with 121 additions and 261 deletions
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116
src/math/lp/int_solver.cpp
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@ -197,11 +197,14 @@ namespace lp {
if (r == lia_move::undef && should_find_cube()) r = int_cube(*this)();
if (r == lia_move::undef) lra.move_non_basic_columns_to_bounds();
if (r == lia_move::undef && should_hnf_cut()) r = hnf_cut();
std::function<lia_move(void)> gomory_fn = [&]() { return gomory(*this)(); };
m_cut_vars.reset();
#if 0
if (r == lia_move::undef && should_gomory_cut()) r = gomory(*this)();
#else
if (r == lia_move::undef && should_gomory_cut()) r = local_gomory(2);
if (r == lia_move::undef && should_gomory_cut()) r = local_cut(2, gomory_fn);
#endif
m_cut_vars.reset();
if (r == lia_move::undef) r = int_branch(*this)();
@ -711,6 +714,8 @@ namespace lp {
return;
#if 0
// in-processing simplification can go here, such as bounds improvements.
if (!lra.is_feasible()) {
@ -728,113 +733,9 @@ namespace lp {
if (has_inf_int())
local_gomory(5);
#if 0
stopwatch sw;
explanation exp1, exp2;
//
// tighten integer bounds
// It is a weak method as it ownly strengthens bounds if
// variables are already at one of the to-be discovered bounds.
//
sw.start();
unsigned changes = 0;
auto const& constraints = lra.constraints();
auto print_var = [this](lpvar j) {
if (lra.column_corresponds_to_term(j)) {
std::stringstream strm;
lra.print_column_info(j, strm);
return strm.str();
}
else
return std::string("j") + std::to_string(j);
};
unsigned start = random();
unsigned num_checks = 0;
for (lpvar j0 = 0; j0 < lra.column_count(); ++j0) {
lpvar j = (j0 + start) % lra.column_count();
if (num_checks > 1000)
break;
if (is_fixed(j))
continue;
if (!lra.column_is_int(j))
continue;
rational value = get_value(j).x;
bool tight_lower = false, tight_upper = false;
u_dependency* dep;
if (!value.is_int())
continue;
bool at_up = at_upper(j);
if (!at_lower(j)) {
++num_checks;
lra.push();
auto k = lp::lconstraint_kind::LE;
lra.update_column_type_and_bound(j, k, (value - 1).to_mpq(), nullptr);
lra.find_feasible_solution();
if (!lra.is_feasible()) {
tight_upper = true;
++changes;
lra.get_infeasibility_explanation(exp1);
#if 0
display_column(std::cout, j);
std::cout << print_var(j) << " >= " << value << "\n";
unsigned i = 0;
for (auto p : exp1) {
std::cout << "(" << p.ci() << ")";
constraints.display(std::cout, print_var, p.ci());
if (++i < exp1.size())
std::cout << " ";
}
#endif
}
lra.pop(1);
if (tight_upper) {
dep = nullptr;
for (auto& cc : exp1)
dep = lra.join_deps(dep, constraints[cc.ci()].dep());
lra.update_column_type_and_bound(j, lp::lconstraint_kind::GE, value.to_mpq(), dep);
}
}
if (!at_up) {
++num_checks;
lra.push();
auto k = lp::lconstraint_kind::GE;
lra.update_column_type_and_bound(j, k, (value + 1).to_mpq(), nullptr);
lra.find_feasible_solution();
if (!lra.is_feasible()) {
tight_lower = true;
++changes;
lra.get_infeasibility_explanation(exp1);
#if 0
display_column(std::cout, j);
std::cout << print_var(j) << " <= " << value << "\n";
unsigned i = 0;
for (auto p : exp1) {
std::cout << "(" << p.ci() << ")";
constraints.display(std::cout, print_var, p.ci());
if (++i < exp1.size())
std::cout << " ";
}
#endif
}
lra.pop(1);
if (tight_lower) {
dep = nullptr;
for (auto& cc : exp1)
dep = lra.join_deps(dep, constraints[cc.ci()].dep());
lra.update_column_type_and_bound(j, lp::lconstraint_kind::LE, value.to_mpq(), dep);
}
}
}
sw.stop();
std::cout << "changes " << changes << " columns " << lra.column_count() << " time: " << sw.get_seconds() << "\n";
std::cout.flush();
//
// identify equalities
//
@ -948,7 +849,8 @@ namespace lp {
#endif
}
lia_move int_solver::local_gomory(unsigned num_cuts) {
lia_move int_solver::local_cut(unsigned num_cuts, std::function<lia_move(void)>& cut_fn) {
struct ex { explanation m_ex; lar_term m_term; mpq m_k; bool m_is_upper; };
vector<ex> cuts;
@ -956,7 +858,7 @@ namespace lp {
m_ex->clear();
m_t.clear();
m_k.reset();
auto r = gomory(*this)();
auto r = cut_fn();
if (r != lia_move::cut)
break;
cuts.push_back({ *m_ex, m_t, m_k, is_upper() });