bringing hashbrown back

Cargo.toml

@@ -30,7 +30,7 @@ serde_derive = { version = "1", default-features = false }
 thiserror = { version = "1", optional = true }
 merlin = { version = "2", default-features = false }
 clear_on_drop = { version = "0.2", default-features = false, features = ["nightly"] }
-# hashbrown = { version = "0.9.1", default-features = false }
+hashbrown = { version = "0.9.1", default-features = false }
 
 [dev-dependencies]
 hex = "0.3"

src/r1cs/linear_combination.rs

@@ -4,8 +4,8 @@ use alloc::vec::Vec;
 use core::iter::FromIterator;
 use core::ops::{Add, Mul, Neg, Sub};
 use curve25519_dalek::scalar::Scalar;
-// #[cfg(feature = "std")]
-// use hashbrown::hash_map::HashMap;
+#[cfg(feature = "std")]
+use hashbrown::hash_map::HashMap;
 
 /// Represents a variable in a constraint system.
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
@@ -119,22 +119,25 @@ impl LinearCombination {
         self.terms
     }
 
-    // #[cfg(feature = "std")]
-    // pub fn simplify(self) -> Self {
-    //     // Build hashmap to hold unique variables with their values.
-    //     let mut vars: HashMap<Variable, Scalar> = HashMap::new();
+    /// Simplify linear combination by taking Variables common across terms and adding their corresponding scalars.
+    /// Useful when linear combinations become large. Takes ownership of linear combination as this function is useful
+    /// when memory is limited and the obvious action after this function call will be to free the memory held by the old linear combination
+    #[cfg(feature = "std")]
+    pub fn simplify(self) -> Self {
+        // Build hashmap to hold unique variables with their values.
+        let mut vars: HashMap<Variable, Scalar> = HashMap::new();
 
-    //     let terms = self.get_terms();
-    //     for (var, val) in terms {
-    //         *vars.entry(var).or_insert(Scalar::zero()) += val;
-    //     }
+        let terms = self.get_terms();
+        for (var, val) in terms {
+            *vars.entry(var).or_insert(Scalar::zero()) += val;
+        }
 
-    //     let mut new_lc_terms = vec![];
-    //     for (var, val) in vars {
-    //         new_lc_terms.push((var, val));
-    //     }
-    //     new_lc_terms.iter().collect()
-    // }
+        let mut new_lc_terms = vec![];
+        for (var, val) in vars {
+            new_lc_terms.push((var, val));
+        }
+        new_lc_terms.iter().collect()
+    }
 }
 
 impl Default for LinearCombination {