decompose_running_sum: Move range_check gate outside the helper.

Author: therealyingtong

halo2_gadgets/src/ecc/chip/mul_fixed.rs

@@ -2,7 +2,7 @@ use super::{
     add, add_incomplete, EccBaseFieldElemFixed, EccScalarFixed, EccScalarFixedShort, FixedPoint,
     NonIdentityEccPoint, FIXED_BASE_WINDOW_SIZE, H,
 };
-use crate::utilities::decompose_running_sum::RunningSumConfig;
+use crate::utilities::{decompose_running_sum::RunningSumConfig, range_check};
 
 use std::marker::PhantomData;
 
@@ -67,6 +67,17 @@ impl<FixedPoints: super::FixedPoints<pallas::Affine>> Config<FixedPoints> {
         let q_running_sum = meta.selector();
         let running_sum_config = RunningSumConfig::configure(meta, q_running_sum, window);
 
+        // Range-check each window in the running sum decomposition.
+        meta.create_gate("range check", |meta| {
+            let q_range_check = meta.query_selector(running_sum_config.q_range_check());
+            let word = running_sum_config.window_expr(meta);
+
+            Constraints::with_selector(
+                q_range_check,
+                Some(range_check(word, 1 << FIXED_BASE_WINDOW_SIZE)),
+            )
+        });
+
         let config = Self {
             running_sum_config,
             lagrange_coeffs,

halo2_gadgets/src/utilities/decompose_running_sum.rs

@@ -25,14 +25,10 @@
 use ff::PrimeFieldBits;
 use halo2_proofs::{
     circuit::{AssignedCell, Region, Value},
-    plonk::{
-        Advice, Column, ConstraintSystem, Constraints, Error, Expression, Selector, VirtualCells,
-    },
+    plonk::{Advice, Column, ConstraintSystem, Error, Expression, Selector, VirtualCells},
     poly::Rotation,
 };
 
-use super::range_check;
-
 use std::marker::PhantomData;
 
 /// The running sum $[z_0, ..., z_W]$. If created in strict mode, $z_W = 0$.
@@ -86,10 +82,6 @@ impl<F: PrimeFieldBits, const WINDOW_NUM_BITS: usize> RunningSumConfig<F, WINDOW
 
     /// `perm` MUST include the advice column `z`.
     ///
-    /// # Panics
-    ///
-    /// Panics if WINDOW_NUM_BITS > 3.
-    ///
     /// # Side-effects
     ///
     /// `z` will be equality-enabled.
@@ -98,25 +90,19 @@ impl<F: PrimeFieldBits, const WINDOW_NUM_BITS: usize> RunningSumConfig<F, WINDOW
         q_range_check: Selector,
         z: Column<Advice>,
     ) -> Self {
-        assert!(WINDOW_NUM_BITS <= 3);
-
         meta.enable_equality(z);
 
-        let config = Self {
+        // It is the caller's responsibility to enforce the range-check using q_range_check.
+        // The selector q_range_check will be enabled on every row of the decomposition,
+        // but is not tied to a gate or expression within this helper.
+        //
+        // This is to support different range check methods (e.g. expression, lookup).
+
+        Self {
             q_range_check,
             z,
             _marker: PhantomData,
-        };
-
-        // https://p.z.cash/halo2-0.1:decompose-short-range
-        meta.create_gate("range check", |meta| {
-            let q_range_check = meta.query_selector(config.q_range_check);
-            let word = config.window_expr(meta);
-
-            Constraints::with_selector(q_range_check, Some(range_check(word, 1 << WINDOW_NUM_BITS)))
-        });
-
-        config
+        }
     }
 
     /// Expression for a window