perf: remove redundant scanning (#175)

* make a test case

* do not add things to the buffer more than once

* remove useless test

* a cheaper check

* add a comment

src/internal/core.rs

@@ -131,7 +131,14 @@ impl<P: Package, VS: VersionSet, Priority: Ord + Clone> State<P, VS, Priority> {
                         break;
                     }
                     Relation::AlmostSatisfied(package_almost) => {
-                        self.unit_propagation_buffer.push(package_almost.clone());
+                        // Add `package_almost` to the `unit_propagation_buffer` set.
+                        // Putting items in `unit_propagation_buffer` more than once waste cycles,
+                        // but so does checking for duplicates.
+                        // In practice the most common pathology is adding the same package repeatedly.
+                        // So we only check if it is duplicated with the last item.
+                        if self.unit_propagation_buffer.last() != Some(&package_almost) {
+                            self.unit_propagation_buffer.push(package_almost.clone());
+                        }
                         // Add (not term) to the partial solution with incompat as cause.
                         self.partial_solution.add_derivation(
                             package_almost,