add trait for solvers that support cardinality constraints

this addressess @lovasoa 's comment rust-or#39 (comment)

also upgrades russcip dependency to one that supports 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},
 };
@@ -81,9 +82,11 @@ impl SCIPProblem {
     pub fn as_inner_mut(&mut self) -> &mut Model<ProblemCreated> {
         &mut self.model
     }
+}
 
+impl CardinalityConstraintSolver for SCIPProblem {
     /// Add cardinality constraint. Constrains the number of non-zero variables to at most `rhs`.
-    pub fn add_cardinality_constraint(&mut self, vars: &[Variable], rhs: usize) -> ConstraintReference {
+    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<_>>();
@@ -97,7 +100,6 @@ impl SCIPProblem {
 
         ConstraintReference { index }
     }
-
 }
 
 impl SolverModel for SCIPProblem {
@@ -175,7 +177,7 @@ 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;