Add QuadraticModelBase::remove_interactions() method

Author: arcondello

dimod/include/dimod/abc.h

@@ -331,6 +331,12 @@ class QuadraticModelBase {
     /// Remove the interaction between variables `u` and `v`.
     bool remove_interaction(index_type u, index_type v);
 
+    /// Remove all interactions for which `filter` returns `true`.
+    /// Returns the number of interactions removed.
+    /// `filter` must be symmetric. That is `filter(u, v, bias)` must equal `filter(v, u, bias)`.
+    template<class Filter>
+    size_type remove_interactions(Filter filter);
+
     /**
      * Remove variable `v` from the model.
      *
@@ -889,6 +895,35 @@ bool QuadraticModelBase<bias_type, index_type>::remove_interaction(index_type u,
     return false;
 }
 
+template <class bias_type, class index_type>
+template <class Filter>
+std::size_t QuadraticModelBase<bias_type, index_type>::remove_interactions(Filter filter) {
+    if (!has_adj()) return 0;  // nothing to filter
+
+    std::size_t num_removed = 0;
+
+    index_type u = 0;
+    for (auto& n : *adj_ptr_) {
+        auto it = std::remove_if(n.begin(), n.end(),
+                                 [&u, &filter](const OneVarTerm<bias_type, index_type>& term) {
+                                     const index_type& v = term.v;
+                                     const bias_type& bias = term.bias;
+                                     assert(filter(u, v, bias) == filter(v, u, bias));
+                                     return filter(u, v, bias);
+                                 });
+
+        num_removed += n.end() - it;
+
+        n.erase(it, n.end());
+
+        u += 1;
+    }
+
+    assert(num_removed % 2 == 0);
+
+    return num_removed / 2;
+}
+
 template <class bias_type, class index_type>
 void QuadraticModelBase<bias_type, index_type>::remove_variable(index_type v) {
     assert(0 <= v && static_cast<size_type>(v) < num_variables());

releasenotes/notes/QuadraticModelBase.remove_interactions-1580cb5e00bc96f4.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Add C++ ``QuadraticModelBase::remove_interactions()`` method.

testscpp/tests/test_quadratic_model.cpp

@@ -546,4 +546,26 @@ SCENARIO("quadratic models can be swapped", "[qm]") {
         }
     }
 }
+
+SCENARIO("quadratic models can have their interactions filtered") {
+    GIVEN("a binary quadratic model") {
+        auto bqm = dimod::BinaryQuadraticModel<double>(5, dimod::Vartype::BINARY);
+        bqm.add_quadratic(0, 1, 1.5);
+        bqm.add_quadratic(1, 2, 2.5);
+        bqm.add_quadratic(2, 3, 3.5);
+
+        WHEN("We filter all interactions with variable 1") {
+            CHECK(bqm.remove_interactions([](int u, int v, double) { return u == 1 || v == 1; }) ==
+                  2);
+
+            CHECK(bqm.quadratic(0, 1) == 0);
+            CHECK(bqm.quadratic(1, 2) == 0);
+            CHECK(bqm.quadratic(2, 3) == 3.5);
+
+            CHECK(bqm.quadratic(1, 0) == 0);
+            CHECK(bqm.quadratic(2, 1) == 0);
+            CHECK(bqm.quadratic(3, 2) == 3.5);
+        }
+    }
+}
 }  // namespace dimod