Add hashmap_with_capacity (#313)

* add hash_map_with_capacity

* rm _

src/extract.rs

@@ -1,7 +1,7 @@
 use std::cmp::Ordering;
 use std::fmt::Debug;
 
-use crate::util::HashMap;
+use crate::util::{hashmap_with_capacity, HashMap};
 use crate::{Analysis, EClass, EGraph, Id, Language, RecExpr};
 
 /** Extracting a single [`RecExpr`] from an [`EGraph`].
@@ -134,8 +134,9 @@ pub trait CostFunction<L: Language> {
     /// down the [`RecExpr`].
     ///
     fn cost_rec(&mut self, expr: &RecExpr<L>) -> Self::Cost {
-        let mut costs: HashMap<Id, Self::Cost> = HashMap::default();
-        for (i, node) in expr.as_ref().iter().enumerate() {
+        let nodes = expr.as_ref();
+        let mut costs = hashmap_with_capacity::<Id, Self::Cost>(nodes.len());
+        for (i, node) in nodes.iter().enumerate() {
             let cost = self.cost(node, |i| costs[&i].clone());
             costs.insert(Id::from(i), cost);
         }

src/language.rs

@@ -430,7 +430,7 @@ impl<L: Language> RecExpr<L> {
     }
 
     pub(crate) fn compact(mut self) -> Self {
-        let mut ids = HashMap::<Id, Id>::default();
+        let mut ids = hashmap_with_capacity::<Id, Id>(self.nodes.len());
         let mut set = IndexSet::default();
         for (i, node) in self.nodes.drain(..).enumerate() {
             let node = node.map_children(|id| ids[&id]);

src/util.rs

@@ -61,6 +61,10 @@ mod hashmap {
     pub(crate) type HashSet<K> = hashbrown::HashSet<K, BuildHasher>;
 }
 
+pub(crate) fn hashmap_with_capacity<K, V>(cap: usize) -> hashmap::HashMap<K, V> {
+    hashmap::HashMap::with_capacity_and_hasher(cap, <_>::default())
+}
+
 pub(crate) type IndexMap<K, V> = indexmap::IndexMap<K, V, BuildHasher>;
 pub(crate) type IndexSet<K> = indexmap::IndexSet<K, BuildHasher>;