feat: report mip gap

src/lib.rs

@@ -591,12 +591,24 @@ impl HighsPtr {
 }
 
 impl SolvedModel {
-    /// The status of the solution. Should be Optimal if everything went well
+    /// The status of the solution. Should be Optimal if everything went well.
     pub fn status(&self) -> HighsModelStatus {
         let model_status = unsafe { Highs_getModelStatus(self.highs.unsafe_mut_ptr()) };
         HighsModelStatus::try_from(model_status).unwrap()
     }
 
+    /// The mip gap of the solution. Should be 0.0 if an optimal solution was
+    /// found. Will be INFINITY if no variables have an integer constraint.
+    pub fn mip_gap(&self) -> f64 {
+        let name = CString::new("mip_gap").unwrap();
+        let gap: &mut f64 = &mut -1.0;
+        let status =
+            unsafe { Highs_getDoubleInfoValue(self.highs.unsafe_mut_ptr(), name.as_ptr(), gap) };
+        try_handle_status(status, "Highs_getDoubleInfoValue")
+            .map(|_| *gap)
+            .unwrap()
+    }
+
     /// Get the solution to the problem
     pub fn get_solution(&self) -> Solution {
         let cols = self.num_cols();
@@ -686,6 +698,8 @@ fn try_handle_status(status: c_int, msg: &str) -> Result<HighsStatus, HighsStatu
 
 #[cfg(test)]
 mod test {
+    use std::f64::INFINITY;
+
     use super::*;
 
     fn test_coefs(coefs: [f64; 2]) {
@@ -755,4 +769,36 @@ mod test {
         assert_eq!(solved.status(), Optimal);
         assert_eq!(solved.get_solution().columns(), &[50.0]);
     }
+
+    #[test]
+    fn test_mip_gap() {
+        use crate::status::HighsModelStatus::Optimal;
+        use crate::{Model, RowProblem, Sense};
+        let mut p = RowProblem::default();
+        p.add_integer_column(1., 0..50);
+        let mut m = Model::new(p);
+        m.make_quiet();
+        m.set_sense(Sense::Maximise);
+        let solved = m.solve();
+        println!("{:?}", solved.get_solution());
+        assert_eq!(solved.status(), Optimal);
+        assert_eq!(solved.mip_gap(), 0.0);
+        assert_eq!(solved.get_solution().columns(), &[50.0]);
+    }
+
+    #[test]
+    fn test_inf_mip_gap() {
+        use crate::status::HighsModelStatus::Optimal;
+        use crate::{Model, RowProblem, Sense};
+        let mut p = RowProblem::default();
+        p.add_column(1., 0..50);
+        let mut m = Model::new(p);
+        m.make_quiet();
+        m.set_sense(Sense::Maximise);
+        let solved = m.solve();
+        println!("{:?}", solved.get_solution());
+        assert_eq!(solved.status(), Optimal);
+        assert_eq!(solved.mip_gap(), INFINITY);
+        assert_eq!(solved.get_solution().columns(), &[50.0]);
+    }
 }