Improve the performance of removing variables from expressions

dimod/include/dimod/abc.h

@@ -707,7 +707,7 @@ void QuadraticModelBase<bias_type, index_type>::fix_variable(index_type v, T ass
  add_offset(assignment * linear(v));
 
  // finally remove v
- remove_variable(v);
+ QuadraticModelBase<bias_type, index_type>::remove_variable(v);
 }
 
 template <class bias_type, class index_type>

dimod/include/dimod/expression.h

@@ -419,7 +419,20 @@ void Expression<bias_type, index_type>::clear() {
 template <class bias_type, class index_type>
 template <class T>
 void Expression<bias_type, index_type>::fix_variable(index_type v, T assignment) {
- throw std::logic_error("not implemented - fix_variable");
+ assert(v >= 0 && static_cast<size_type>(v) < parent_->num_variables());
+
+ auto vit = indices_.find(v);
+ if (vit == indices_.end()) return; // nothing to remove
+
+ // remove the biases
+ base_type::fix_variable(vit->second, assignment);
+
+ // update the indices
+ auto it = variables_.erase(variables_.begin() + vit->second);
+ indices_.erase(vit);
+ for (; it != variables_.end(); ++it) {
+ indices_[*it] -= 1;
+ }
 }
 
 template <class bias_type, class index_type>
@@ -592,19 +605,19 @@ bool Expression<bias_type, index_type>::remove_interaction(index_type u, index_t
 
 template <class bias_type, class index_type>
 void Expression<bias_type, index_type>::remove_variable(index_type v) {
+ assert(v >= 0 && static_cast<size_type>(v) < parent_->num_variables());
+
  auto vit = indices_.find(v);
  if (vit == indices_.end()) return; // nothing to remove
 
  // remove the biases
  base_type::remove_variable(vit->second);
 
  // update the indices
- variables_.erase(variables_.begin() + vit->second);
-
- // indices is no longer valid, so remake
- indices_.clear();
- for (size_type ui = 0; ui < variables_.size(); ++ui) {
- indices_[variables_[ui]] = ui;
+ auto it = variables_.erase(variables_.begin() + vit->second);
+ indices_.erase(vit);
+ for (; it != variables_.end(); ++it) {
+ indices_[*it] -= 1;
  }
 }
 

dimod/include/dimod/quadratic_model.h

@@ -64,6 +64,15 @@ class QuadraticModel : public abc::QuadraticModelBase<Bias, Index> {
  /// Change the vartype of `v`, updating the biases appropriately.
  void change_vartype(Vartype vartype, index_type v);
 
+ /**
+ * Remove variable `v` from the model by fixing its value.
+ *
+ * Note that this causes a reindexing, where all variables above `v` have
+ * their index reduced by one.
+ */
+ template <class T>
+ void fix_variable(index_type v, T assignment);
+
  /// Return the lower bound on variable ``v``.
  bias_type lower_bound(index_type v) const;
 
@@ -229,6 +238,13 @@ void QuadraticModel<bias_type, index_type>::change_vartype(Vartype vartype, inde
  }
 }
 
+template <class bias_type, class index_type>
+template <class T>
+void QuadraticModel<bias_type, index_type>::fix_variable(index_type v, T assignment) {
+ base_type::fix_variable(v, assignment);
+ varinfo_.erase(varinfo_.begin() + v);
+}
+
 template <class bias_type, class index_type>
 bias_type QuadraticModel<bias_type, index_type>::lower_bound(index_type v) const {
  // even though v is unused, we need this to conform the the QuadraticModelBase API

releasenotes/notes/expression-fix-variables-performance-f13a65b6a16fa484.yaml

@@ -0,0 +1,12 @@
+---
+features:
+ - |
+ Improve the performance of fixing and removing variables from constrained
+ quadratic model expressions.
+ - |
+ Implement the ``Expression::fix_variable()`` C++ method. Previously it would
+ throw ``std::logic_error("not implemented - fix_variable")``.
+upgrade:
+ - |
+ Add an overload to the C++ ``QuadraticModel::remove_variable()`` method.
+ This is binary compatible, but it makes ``&remove_variable`` ambiguous.

testscpp/tests/test_constrained_quadratic_model.cpp

@@ -410,16 +410,36 @@ SCENARIO("ConstrainedQuadraticModel tests") {
  }
  }
 
- // WHEN("we fix a variable") {
- //  cqm.fix_variable(x, 0);
+ WHEN("we fix a variable that is not used in the expression") {
+ const0.fix_variable(y, 2);
 
- // THEN("everything is updated correctly") {
- // REQUIRE(cqm.num_variables() == 3);
+ THEN("nothing changes") {
+ REQUIRE(const0.num_variables() == 3);
+ CHECK(const0.linear(x) == 0);
+ CHECK(const0.linear(y) == 0);
+ CHECK(const0.linear(i) == 3);
+ CHECK(const0.linear(j) == 0);
+
+ CHECK(const0.num_interactions() == 2);
+ CHECK(const0.quadratic(x, j) == 2);
+ CHECK(const0.quadratic(i, j) == 5);
+ }
+ }
 
- // REQUIRE(const0.num_variables() == 2);
- // REQUIRE(const0.linear(i-1) == 3);
- // }
- // }
+ WHEN("we fix a variable that is used in the expression") {
+ const0.fix_variable(x, 2);
+
+ THEN("the biases are updated") {
+ REQUIRE(const0.num_variables() == 2);
+ CHECK(const0.linear(x) == 0);
+ CHECK(const0.linear(y) == 0);
+ CHECK(const0.linear(i) == 3);
+ CHECK(const0.linear(j) == 4);
+
+ CHECK(const0.num_interactions() == 1);
+ CHECK(const0.quadratic(i, j) == 5);
+ }
+ }
  }
 
  GIVEN("A constraint with one-hot constraints") {