add support for SCIP cardinality constraints

Cargo.toml

@@ -21,7 +21,7 @@ coin_cbc = { version = "0.1", optional = true, default-features = false }
 minilp = { version = "0.2", optional = true }
 lpsolve = { version = "0.1", optional = true }
 highs = { version = "1.5.0", optional = true }
-russcip = { version = "0.2.4", optional = true }
+russcip = { version = "0.2.6", optional = true }
 lp-solvers = { version = "1.0.0", features = ["cplex"], optional = true }
 fnv = "1.0.5"
 

src/cardinality_constraint_solver_trait.rs

@@ -0,0 +1,7 @@
+use crate::{Variable, constraint::ConstraintReference};
+
+/// A trait for solvers that support cardinality constraints
+pub trait CardinalityConstraintSolver {
+    /// Add cardinality constraint. Constrains the number of non-zero variables from `vars` to at most `rhs`.
+    fn add_cardinality_constraint(&mut self, vars: &[Variable], rhs: usize) -> ConstraintReference;
+}

src/lib.rs

@@ -65,6 +65,7 @@
 //!
 
 pub use affine_expression_trait::IntoAffineExpression;
+pub use cardinality_constraint_solver_trait::CardinalityConstraintSolver;
 pub use constraint::Constraint;
 pub use expression::Expression;
 #[cfg_attr(docsrs, doc(cfg(feature = "minilp")))]
@@ -149,6 +150,7 @@ mod expression;
 #[macro_use]
 pub mod variable;
 mod affine_expression_trait;
+mod cardinality_constraint_solver_trait;
 pub mod constraint;
 pub mod solvers;
 mod variables_macro;

src/solvers/scip.rs

@@ -15,6 +15,7 @@ use russcip::WithSolutions;
 
 use crate::variable::{UnsolvedProblem, VariableDefinition};
 use crate::{
+    CardinalityConstraintSolver,
     constraint::ConstraintReference,
     solvers::{ObjectiveDirection, ResolutionError, Solution, SolverModel},
 };
@@ -83,6 +84,24 @@ impl SCIPProblem {
     }
 }
 
+impl CardinalityConstraintSolver for SCIPProblem {
+    /// Add cardinality constraint. Constrains the number of non-zero variables to at most `rhs`.
+    fn add_cardinality_constraint(&mut self, vars: &[Variable], rhs: usize) -> ConstraintReference {
+        let scip_vars = vars.iter()
+            .map(|v| Rc::clone(&self.id_for_var[v]))
+            .collect::<Vec<_>>();
+
+        let index = self.model.n_conss() + 1;
+        self.model.add_cons_cardinality(
+            scip_vars,
+            rhs,
+            format!("cardinality{}", index).as_str(),
+        );
+
+        ConstraintReference { index }
+    }
+}
+
 impl SolverModel for SCIPProblem {
     type Solution = SCIPSolved;
     type Error = ResolutionError;
@@ -162,7 +181,7 @@ impl Solution for SCIPSolved {
 
 #[cfg(test)]
 mod tests {
-    use crate::{constraint, variable, variables, Solution, SolverModel};
+    use crate::{constraint, variable, variables, Solution, SolverModel, CardinalityConstraintSolver};
 
     use super::scip;
 
@@ -194,4 +213,20 @@ mod tests {
             .unwrap();
         assert_eq!((solution.value(x), solution.value(y)), (1., 2.));
     }
+
+    #[test]
+    fn can_solve_cardinality_constraint() {
+        let mut vars = variables!();
+        let x = vars.add(variable().clamp(0, 2).integer());
+        let y = vars.add(variable().clamp(0, 3).integer());
+        let mut model = vars
+            .maximise(5.0 * x + 3.0 * y)
+            .using(scip);
+        model.add_cardinality_constraint(&[x, y], 1);
+        let solution = model
+            .solve()
+            .unwrap();
+        assert_eq!((solution.value(x), solution.value(y)), (2., 0.));
+    }
+
 }