fix: stick to num_chunk of quotient poly calculated by constraint s…

…ystem (#29)

snark-verifier/src/system/halo2.rs

@@ -189,6 +189,7 @@ struct Polynomials<'a, F: FieldExt> {
     zk: bool,
     query_instance: bool,
     num_proof: usize,
+    degree: usize,
     num_fixed: usize,
     num_permutation_fixed: usize,
     num_instance: Vec<usize>,
@@ -245,6 +246,7 @@ impl<'a, F: FieldExt> Polynomials<'a, F> {
             zk,
             query_instance,
             num_proof,
+            degree,
             num_fixed: cs.num_fixed_columns(),
             num_permutation_fixed: cs.permutation().get_columns().len(),
             num_instance,
@@ -733,6 +735,7 @@ impl<'a, F: FieldExt> Polynomials<'a, F> {
             .collect_vec();
         let numerator = Expression::DistributePowers(constraints, self.alpha().into());
         QuotientPolynomial {
+            num_chunk: self.degree - 1,
             chunk_degree: 1,
             numerator,
         }

snark-verifier/src/verifier/plonk/protocol.rs

@@ -5,7 +5,6 @@ use crate::{
         Itertools,
     },
 };
-use num_integer::Integer;
 use num_traits::One;
 use std::{
     cmp::max,
@@ -264,15 +263,15 @@ where
 #[derive(Clone, Debug)]
 pub struct QuotientPolynomial<F: Clone> {
     pub chunk_degree: usize,
+    // Note that `num_chunk` might be larger than necessary, due to the degree
+    // calculation of the constraint system (e.g. halo2 has minimum degree 3).
+    pub num_chunk: usize,
     pub numerator: Expression<F>,
 }
 
 impl<F: Clone> QuotientPolynomial<F> {
     pub fn num_chunk(&self) -> usize {
-        Integer::div_ceil(
-            &(self.numerator.degree().checked_sub(1).unwrap_or_default()),
-            &self.chunk_degree,
-        )
+        self.num_chunk
     }
 }