From b02e74d88fab168f883d784890b32a204953b5e8 Mon Sep 17 00:00:00 2001
From: Alex Ozdemir <aozdemir@hmc.edu>
Date: Sun, 9 Feb 2020 11:31:01 -0800
Subject: [PATCH] Better implementation of Num::add_assign.

The original implementation of Num::add_assign had a few inefficiencies:

   * It instantiated the standard hashmap with small keys.
   * It put a bunch of items into an empty hashmap, without setting the
     intial hashmap capacity.
   * It used multiple hash lookups in an insert-or-modify pattern.

This commit:

   * Uses the Fnv hash function instead of the default: SipHash.
   * Initializes the hashmap with appropriate capacity.
   * Uses the entry API.

In my microbenchmarks, this sped up Poseidon evaluation by 1.7x.
---
 Cargo.toml         |  1 +
 src/circuit/num.rs | 32 ++++++++++++++++----------------
 src/lib.rs         |  1 +
 3 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index bc5581e3..976810f7 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,6 +21,7 @@ wasm = ["bellman/wasm"]
 [dependencies]
 rand = "0.4"
 digest = "0.8"
+fnv = "1.0.3"
 byteorder = "1"
 tiny-keccak = {version = "2.0", features = ["keccak"] }
 serde = "1.0"
diff --git a/src/circuit/num.rs b/src/circuit/num.rs
index 2c1a3a64..b4596907 100644
--- a/src/circuit/num.rs
+++ b/src/circuit/num.rs
@@ -1,3 +1,6 @@
+use fnv::FnvHashMap;
+use std::default::Default;
+
 use bellman::pairing::{
     Engine,
 };
@@ -779,27 +782,24 @@ impl<E: Engine> Num<E> {
         self.value = newval;
         let mut lc = LinearCombination::zero();
         // std::mem::swap(&mut self.lc, &mut lc);
-        use std::collections::HashMap;
-        let mut final_coeffs: HashMap<bellman::Variable, E::Fr> = HashMap::new();
+        let mut final_coeffs = FnvHashMap::<Variable, E::Fr>::with_capacity_and_hasher(
+            self.lc.as_ref().len() + other.lc.as_ref().len(),
+            Default::default(),
+        );
         for (var, coeff) in self.lc.as_ref() {
-            if final_coeffs.get(var).is_some() {
-                if let Some(existing_coeff) = final_coeffs.get_mut(var) {
-                    existing_coeff.add_assign(&coeff);
-                } 
-            } else {
-                final_coeffs.insert(*var, *coeff);
-            }
+            final_coeffs
+                .entry(*var)
+                .and_modify(|c| c.add_assign(coeff))
+                .or_insert_with(|| *coeff);
         }
 
         for (var, coeff) in other.lc.as_ref() {
-            if final_coeffs.get(var).is_some() {
-                if let Some(existing_coeff) = final_coeffs.get_mut(var) {
-                    existing_coeff.add_assign(&coeff);
-                }
-            } else {
-                final_coeffs.insert(*var, *coeff);
-            }
+            final_coeffs
+                .entry(*var)
+                .and_modify(|c| c.add_assign(coeff))
+                .or_insert_with(|| *coeff);
         }
+
         for (var, coeff) in final_coeffs.into_iter() {
             lc = lc + (coeff, var);
         }
diff --git a/src/lib.rs b/src/lib.rs
index 9698f78a..f5d7b56d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,6 +5,7 @@
 pub extern crate bellman;
 extern crate blake2_rfc_bellman_edition as blake2_rfc;
 extern crate digest;
+extern crate fnv;
 extern crate rand;
 extern crate byteorder;
 extern crate tiny_keccak;