Use ranges to clarify bounds on cell assignment.

src/dev.rs

@@ -3,6 +3,7 @@
 use std::collections::HashMap;
 use std::fmt;
 use std::iter;
+use std::ops::RangeTo;
 
 use ff::Field;
 
@@ -244,9 +245,8 @@ pub struct MockProver<F: Group + Field> {
 
     permutation: permutation::keygen::Assembly,
 
-    // All rows including and above this one are off
-    // limits due to blinding factors.
-    upper_bound_cell_index: usize,
+    // A range of available rows for assignment and copies.
+    usable_rows: RangeTo<usize>,
 }
 
 impl<F: Field + Group> Assignment<F> for MockProver<F> {
@@ -278,7 +278,7 @@ impl<F: Field + Group> Assignment<F> for MockProver<F> {
         A: FnOnce() -> AR,
         AR: Into<String>,
     {
-        if row >= self.upper_bound_cell_index {
+        if !self.usable_rows.contains(&row) {
             return Err(Error::BoundsFailure);
         }
 
@@ -319,7 +319,7 @@ impl<F: Field + Group> Assignment<F> for MockProver<F> {
         A: FnOnce() -> AR,
         AR: Into<String>,
     {
-        if row >= self.upper_bound_cell_index {
+        if !self.usable_rows.contains(&row) {
             return Err(Error::BoundsFailure);
         }
 
@@ -350,7 +350,7 @@ impl<F: Field + Group> Assignment<F> for MockProver<F> {
         A: FnOnce() -> AR,
         AR: Into<String>,
     {
-        if row >= self.upper_bound_cell_index {
+        if !self.usable_rows.contains(&row) {
             return Err(Error::BoundsFailure);
         }
 
@@ -375,7 +375,7 @@ impl<F: Field + Group> Assignment<F> for MockProver<F> {
         right_column: Column<Any>,
         right_row: usize,
     ) -> Result<(), crate::plonk::Error> {
-        if left_row >= self.upper_bound_cell_index || right_row >= self.upper_bound_cell_index {
+        if !self.usable_rows.contains(&left_row) || !self.usable_rows.contains(&right_row) {
             return Err(Error::BoundsFailure);
         }
 
@@ -428,7 +428,7 @@ impl<F: FieldExt> MockProver<F> {
             advice,
             instance,
             permutation,
-            upper_bound_cell_index: n - (blinding_factors + 1),
+            usable_rows: ..n - (blinding_factors + 1),
         };
 
         ConcreteCircuit::FloorPlanner::synthesize(&mut prover, circuit, config)?;

src/plonk/keygen.rs

@@ -1,5 +1,7 @@
 #![allow(clippy::int_plus_one)]
 
+use std::ops::RangeTo;
+
 use ff::Field;
 use group::Curve;
 
@@ -42,9 +44,8 @@ where
 struct Assembly<F: Field> {
     fixed: Vec<Polynomial<Assigned<F>, LagrangeCoeff>>,
     permutation: permutation::keygen::Assembly,
-    // All rows including and above this one are off
-    // limits due to blinding factors.
-    upper_bound_cell_index: usize,
+    // A range of available rows for assignment and copies.
+    usable_rows: RangeTo<usize>,
     _marker: std::marker::PhantomData<F>,
 }
 
@@ -71,7 +72,7 @@ impl<F: Field> Assignment<F> for Assembly<F> {
         A: FnOnce() -> AR,
         AR: Into<String>,
     {
-        if row >= self.upper_bound_cell_index {
+        if !self.usable_rows.contains(&row) {
             return Err(Error::BoundsFailure);
         }
         // Selectors are just fixed columns.
@@ -117,7 +118,7 @@ impl<F: Field> Assignment<F> for Assembly<F> {
         A: FnOnce() -> AR,
         AR: Into<String>,
     {
-        if row >= self.upper_bound_cell_index {
+        if !self.usable_rows.contains(&row) {
             return Err(Error::BoundsFailure);
         }
 
@@ -137,8 +138,7 @@ impl<F: Field> Assignment<F> for Assembly<F> {
         right_column: Column<Any>,
         right_row: usize,
     ) -> Result<(), Error> {
-        // Check bounds first
-        if left_row >= self.upper_bound_cell_index || right_row >= self.upper_bound_cell_index {
+        if !self.usable_rows.contains(&left_row) || !self.usable_rows.contains(&right_row) {
             return Err(Error::BoundsFailure);
         }
 
@@ -177,7 +177,7 @@ where
     let mut assembly: Assembly<C::Scalar> = Assembly {
         fixed: vec![domain.empty_lagrange_assigned(); cs.num_fixed_columns],
         permutation: permutation::keygen::Assembly::new(params.n as usize, &cs.permutation),
-        upper_bound_cell_index: params.n as usize - (cs.blinding_factors() + 1),
+        usable_rows: ..params.n as usize - (cs.blinding_factors() + 1),
         _marker: std::marker::PhantomData,
     };
 
@@ -225,7 +225,7 @@ where
     let mut assembly: Assembly<C::Scalar> = Assembly {
         fixed: vec![vk.domain.empty_lagrange_assigned(); vk.cs.num_fixed_columns],
         permutation: permutation::keygen::Assembly::new(params.n as usize, &vk.cs.permutation),
-        upper_bound_cell_index: params.n as usize - (vk.cs.blinding_factors() + 1),
+        usable_rows: ..params.n as usize - (vk.cs.blinding_factors() + 1),
         _marker: std::marker::PhantomData,
     };
 

src/plonk/prover.rs

@@ -1,6 +1,7 @@
 use ff::Field;
 use group::Curve;
 use std::iter;
+use std::ops::RangeTo;
 
 use super::{
     circuit::{
@@ -128,7 +129,7 @@ pub fn create_proof<
             struct WitnessCollection<'a, F: Field> {
                 pub advice: Vec<Polynomial<Assigned<F>, LagrangeCoeff>>,
                 instances: &'a [&'a [F]],
-                upper_bound_cell_index: usize,
+                usable_rows: RangeTo<usize>,
                 _marker: std::marker::PhantomData<F>,
             }
 
@@ -185,7 +186,7 @@ pub fn create_proof<
                     A: FnOnce() -> AR,
                     AR: Into<String>,
                 {
-                    if row >= self.upper_bound_cell_index {
+                    if !self.usable_rows.contains(&row) {
                         return Err(Error::BoundsFailure);
                     }
 
@@ -241,6 +242,8 @@ pub fn create_proof<
                 }
             }
 
+            let unusable_rows_start = params.n as usize - (meta.blinding_factors() + 1);
+
             let mut witness = WitnessCollection {
                 advice: vec![domain.empty_lagrange_assigned(); meta.num_advice_columns],
                 instances,
@@ -248,7 +251,7 @@ pub fn create_proof<
                 // cells that exist within inactive rows, which include some
                 // number of blinding factors and an extra row for use in the
                 // permutation argument.
-                upper_bound_cell_index: params.n as usize - (meta.blinding_factors() + 1),
+                usable_rows: ..unusable_rows_start,
                 _marker: std::marker::PhantomData,
             };
 
@@ -259,7 +262,7 @@ pub fn create_proof<
 
             // Add blinding factors to advice columns
             for advice in &mut advice {
-                for cell in &mut advice[witness.upper_bound_cell_index..] {
+                for cell in &mut advice[unusable_rows_start..] {
                     *cell = C::Scalar::rand();
                 }
             }