Correctly evaluate over domain when self is zero polynomial (#575)

Author: Pratyush

poly/src/polynomial/univariate/dense.rs

@@ -608,9 +608,9 @@ impl<F: Field> Zero for DensePolynomial<F> {
 
 #[cfg(test)]
 mod tests {
-    use crate::{polynomial::univariate::*, EvaluationDomain, GeneralEvaluationDomain};
+    use crate::{polynomial::univariate::*, EvaluationDomain, GeneralEvaluationDomain, Polynomial};
     use ark_ff::{Field, One, UniformRand, Zero};
-    use ark_std::{rand::Rng, test_rng};
+    use ark_std::{rand::Rng, test_rng, vec::Vec};
     use ark_test_curves::bls12_381::Fr;
 
     fn rand_sparse_poly<R: Rng>(degree: usize, rng: &mut R) -> SparsePolynomial<Fr> {
@@ -889,4 +889,24 @@ mod tests {
         assert_eq!(y.degree(), n - 1);
         assert!(!y.coeffs.last().unwrap().is_zero());
     }
+
+    #[test]
+    fn evaluate_over_domain_test() {
+        let rng = &mut ark_std::test_rng();
+        let domain = crate::domain::Radix2EvaluationDomain::<Fr>::new(1 << 10).unwrap();
+        for _ in 0..100 {
+            let poly = DensePolynomial::<Fr>::rand(1 << 11, rng);
+            let evaluations = domain
+                .elements()
+                .map(|e| poly.evaluate(&e))
+                .collect::<Vec<_>>();
+            assert_eq!(evaluations, poly.evaluate_over_domain(domain).evals);
+        }
+        let zero = DensePolynomial::zero();
+        let evaluations = domain
+            .elements()
+            .map(|e| zero.evaluate(&e))
+            .collect::<Vec<_>>();
+        assert_eq!(evaluations, zero.evaluate_over_domain(domain).evals);
+    }
 }

poly/src/polynomial/univariate/mod.rs

@@ -156,27 +156,35 @@ impl<'a, F: 'a + FftField> DenseOrSparsePolynomial<'a, F> {
                 Evaluations::from_vec_and_domain(evals, domain)
             },
             DPolynomial(Cow::Borrowed(d)) => {
-                // Reduce the coefficients of the polynomial mod X^domain.size()
-                let mut chunks = d.coeffs.chunks(domain.size());
-                let mut reduced = chunks.next().unwrap().to_vec();
-                for chunk in chunks {
-                    cfg_iter_mut!(reduced).zip(chunk).for_each(|(x, y)| {
-                        *x += y;
-                    });
+                if d.is_zero() {
+                    Evaluations::from_vec_and_domain(vec![F::zero(); domain.size()], domain)
+                } else {
+                    // Reduce the coefficients of the polynomial mod X^domain.size()
+                    let mut chunks = d.coeffs.chunks(domain.size());
+                    let mut reduced = chunks.next().unwrap().to_vec();
+                    for chunk in chunks {
+                        cfg_iter_mut!(reduced).zip(chunk).for_each(|(x, y)| {
+                            *x += y;
+                        });
+                    }
+                    Evaluations::from_vec_and_domain(domain.fft(&reduced), domain)
                 }
-                Evaluations::from_vec_and_domain(domain.fft(&reduced), domain)
             },
             DPolynomial(Cow::Owned(mut d)) => {
                 // Reduce the coefficients of the polynomial mod X^domain.size()
-                let mut chunks = d.coeffs.chunks_mut(domain.size());
-                let coeffs = chunks.next().unwrap();
-                for chunk in chunks {
-                    cfg_iter_mut!(coeffs).zip(chunk).for_each(|(x, y)| {
-                        *x += y;
-                    });
+                if d.is_zero() {
+                    Evaluations::from_vec_and_domain(vec![F::zero(); domain.size()], domain)
+                } else {
+                    let mut chunks = d.coeffs.chunks_mut(domain.size());
+                    let coeffs = chunks.next().unwrap();
+                    for chunk in chunks {
+                        cfg_iter_mut!(coeffs).zip(chunk).for_each(|(x, y)| {
+                            *x += y;
+                        });
+                    }
+                    domain.fft_in_place(&mut d.coeffs);
+                    Evaluations::from_vec_and_domain(d.coeffs, domain)
                 }
-                domain.fft_in_place(&mut d.coeffs);
-                Evaluations::from_vec_and_domain(d.coeffs, domain)
             },
         }
     }