From aed279191e981b270a84d243f5364c8ba5b0b5cf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fran=C3=A7ois=20Garillot?= <francois@garillot.net>
Date: Thu, 8 Feb 2024 14:00:51 -0500
Subject: [PATCH] fix: one forgotten instance of the poly_X refactor

---
 src/spartan/ppsnark.rs | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/src/spartan/ppsnark.rs b/src/spartan/ppsnark.rs
index 109a7b2c0..56edc70e6 100644
--- a/src/spartan/ppsnark.rs
+++ b/src/spartan/ppsnark.rs
@@ -927,13 +927,15 @@ impl<E: Engine, EE: EvaluationEngineTrait<E>> RelaxedR1CSSNARKTrait<E> for Relax
 
           let eval_X = {
             // constant term
-            let mut poly_X = vec![(0, U.u)];
-            //remaining inputs
-            poly_X.extend(
-              (0..U.X.len())
-                .map(|i| (i + 1, U.X[i]))
-                .collect::<Vec<(usize, E::Scalar)>>(),
-            );
+            let poly_X = std::iter::once((0, U.u))
+              .chain(
+                //remaining inputs
+                (0..U.X.len())
+                // filter_map uses the sparsity of the polynomial, if irrelevant
+                // we should replace by UniPoly
+                .filter_map(|i| (!U.X[i].is_zero_vartime()).then_some((i + 1, U.X[i]))),
+              )
+              .collect();
             SparsePolynomial::new(vk.num_vars.log_2(), poly_X).evaluate(&rand_sc_unpad[1..])
           };