Fix padding for LookupTableGate (#1661)

Author: LindaGuiga

plonky2/src/plonk/prover.rs

@@ -94,7 +94,6 @@ pub fn set_lookup_wires<
             multiplicities[0] += 1;
         }
 
-        // We don't need to pad the last `LookupTableGate`; extra wires are set to 0 by default, which satisfies the constraints.
         for lut_entry in 0..lut_len {
             let row = first_lut_gate - lut_entry / num_lut_entries;
             let col = lut_entry % num_lut_entries;

plonky2/src/plonk/vanishing_poly.rs

@@ -36,8 +36,8 @@ pub(crate) fn get_lut_poly<F: RichField + Extendable<D>, const D: usize>(
     let b = deltas[LookupChallenges::ChallengeB as usize];
     let mut coeffs = Vec::with_capacity(common_data.luts[lut_index].len());
     let n = common_data.luts[lut_index].len();
-    let nb_padded_elts = LookupTableGate::num_slots(&common_data.config)
-        - n % LookupTableGate::num_slots(&common_data.config);
+    let nb_slots = LookupTableGate::num_slots(&common_data.config);
+    let nb_padded_elts = (nb_slots - n % nb_slots) % nb_slots;
     let (padding_inp, padding_out) = common_data.luts[lut_index][0];
     for (input, output) in common_data.luts[lut_index].iter() {
         coeffs.push(F::from_canonical_u16(*input) + b * F::from_canonical_u16(*output));
@@ -763,8 +763,8 @@ pub(crate) fn get_lut_poly_circuit<F: RichField + Extendable<D>, const D: usize>
     let b = deltas[LookupChallenges::ChallengeB as usize];
     let delta = deltas[LookupChallenges::ChallengeDelta as usize];
     let n = common_data.luts[lut_index].len();
-    let nb_padded_elts = LookupTableGate::num_slots(&common_data.config)
-        - n % LookupTableGate::num_slots(&common_data.config);
+    let nb_slots = LookupTableGate::num_slots(&common_data.config);
+    let nb_padded_elts = (nb_slots - n % nb_slots) % nb_slots;
     let (padding_inp, padding_out) = common_data.luts[lut_index][0];
     let mut coeffs: Vec<Target> = common_data.luts[lut_index]
         .iter()