Optimize BddVariableSet::new, 2x speedup

src/_impl_bdd_variable_set.rs

@@ -26,9 +26,39 @@
     ///
     /// *Panics:* `vars` must contain unique names which are allowed as variable names.
     pub fn new(vars: &[&str]) -> BddVariableSet {
-        let mut builder = BddVariableSetBuilder::new();
-        builder.make_variables(vars);
-        builder.build()
+        let num_vars = vars.len();
+        if num_vars >= ((u16::MAX - 1) as usize) {
+            panic!(
+                "Too many BDD variables. There can be at most {} variables.",
+                u16::MAX - 1
+            )
+        }
+        let var_names: Vec<String> = vars
+            .iter()
+            .map(|name| {
+                if name.chars().any(|c| NOT_IN_VAR_NAME.contains(&c)) {
+                    panic!(
+                        "Variable name {} is invalid. Cannot use {:?}",
+                        name, NOT_IN_VAR_NAME
+                    );
+                }
+                name.to_string()
+            })
+            .collect();
+        let var_index_mapping: HashMap<String, u16> = vars
+            .iter()
+            .enumerate()
+            .map(|(id, name)| (name.to_string(), id as u16))
+            .collect();
+
+        if var_index_mapping.len() != var_names.len() {
+            panic!("Existing duplicated BDD variable.");
+        }
+        BddVariableSet {
+            num_vars: num_vars as u16,
+            var_names,
+            var_index_mapping,
+        }
     }
 
     /// Return the number of variables in this set.
@@ -591,4 +621,35 @@
         let ctx = BddVariableSet::new(&[]);
         assert_eq!("[]", ctx.to_string());
     }
+
+    #[test]
+    fn bdd_new_performance() {
+        fn old_version(vars: &[&str]) -> BddVariableSet {
+            let mut builder = BddVariableSetBuilder::new();
+            builder.make_variables(vars);
+            builder.build()
+        }
+        // validate correctness
+        let ctx = BddVariableSet::new(&["a", "b", "x", "c", "y"]);
+        let ctx_old = old_version(&["a", "b", "x", "c", "y"]);
+        assert_eq!(ctx.num_vars, ctx_old.num_vars);
+        assert_eq!(ctx.var_names, ctx_old.var_names);
+        assert_eq!(ctx.var_index_mapping, ctx_old.var_index_mapping);
+        // validate performance
+        use std::time::SystemTime;
+        let n = 1000;
+        let t_bgn1 = SystemTime::now();
+        for _ in 0..n {
+            let _ctx = BddVariableSet::new(&["a", "b", "x", "c", "y"]);
+        }
+        let t1 = t_bgn1.elapsed().unwrap();
+        println!("New BddVariableSet::new runtime: {t1:?}");
+        let t_bgn2 = SystemTime::now();
+        for _ in 0..n {
+            let _ctx = old_version(&["a", "b", "x", "c", "y"]);
+        }
+        let t2 = t_bgn2.elapsed().unwrap();
+        println!("Old BddVariableSet::new runtime: {t2:?}");
+        assert!(t1 < t2);
+    }
 }