mul::overflow: Refactor overflow::Config.

This is only used in chip::mul::Config. In a subsequent commit,
this will be configured from mul::Config instead of from
ecc::chip::Config.

This commit does not result in circuit changes.

src/circuit/gadget/ecc/chip.rs

@@ -16,6 +16,8 @@ use halo2::{
 };
 use pasta_curves::{arithmetic::CurveAffine, pallas};
 
+use std::convert::TryInto;
+
 pub(super) mod add;
 pub(super) mod add_incomplete;
 pub(super) mod mul;
@@ -153,7 +155,7 @@ pub struct EccConfig {
     /// Selector used to enforce switching logic on LSB in variable-base scalar mul
     pub q_mul_lsb: Selector,
     /// Variable-base scalar multiplication (overflow check)
-    pub q_mul_overflow: Selector,
+    pub mul_overflow: mul::overflow::Config,
 
     /// Fixed-base full-width scalar multiplication
     pub q_mul_fixed_full: Selector,
@@ -221,9 +223,6 @@ impl EccChip {
         // mul_fixed::base_field_element::Config:
         // - [advices[6], advices[7], advices[8]]: canon_advices
         //
-        // mul::overflow::Config:
-        // - [advices[0], advices[1], advices[2]]: advices
-        //
         // mul::incomplete::Config
         // - advices[4]: lambda1
         // - advices[9]: z
@@ -259,6 +258,8 @@ impl EccChip {
             meta, advices[6], advices[7], advices[0], advices[1], advices[8], advices[2],
         );
         let mul_complete = mul::complete::Config::configure(meta, advices[9], add);
+        let mul_overflow =
+            mul::overflow::Config::configure(meta, range_check, advices[6..9].try_into().unwrap());
 
         let config = EccConfig {
             advices,
@@ -269,7 +270,7 @@ impl EccChip {
             mul_hi,
             mul_lo,
             mul_complete,
-            q_mul_overflow: meta.selector(),
+            mul_overflow,
             q_mul_lsb: meta.selector(),
             q_mul_fixed_full: meta.selector(),
             q_mul_fixed_short: meta.selector(),

src/circuit/gadget/ecc/chip/mul.rs

@@ -20,7 +20,8 @@ use pasta_curves::pallas;
 pub(crate) mod complete;
 // TODO: Undo this pub(crate).
 pub(crate) mod incomplete;
-mod overflow;
+// TODO: Undo this pub(crate).
+pub(crate) mod overflow;
 
 /// Number of bits for which complete addition needs to be used in variable-base
 /// scalar multiplication
@@ -69,7 +70,7 @@ impl From<&EccConfig> for Config {
             hi_config: ecc_config.mul_hi,
             lo_config: ecc_config.mul_lo,
             complete_config: ecc_config.mul_complete,
-            overflow_config: ecc_config.into(),
+            overflow_config: ecc_config.mul_overflow,
         };
 
         assert_eq!(
@@ -112,8 +113,6 @@ impl From<&EccConfig> for Config {
 
 impl Config {
     pub(super) fn create_gate(&self, meta: &mut ConstraintSystem<pallas::Base>) {
-        self.overflow_config.create_gate(meta);
-
         // If `lsb` is 0, (x, y) = (x_p, -y_p). If `lsb` is 1, (x, y) = (0,0).
         meta.create_gate("LSB check", |meta| {
             let q_mul_lsb = meta.query_selector(self.q_mul_lsb);

src/circuit/gadget/ecc/chip/mul/overflow.rs

@@ -1,4 +1,4 @@
-use super::super::{copy, CellValue, EccConfig, Var};
+use super::super::{copy, CellValue, Var};
 use super::Z;
 use crate::{
     circuit::gadget::utilities::lookup_range_check::LookupRangeCheckConfig, constants::T_Q,
@@ -15,6 +15,7 @@ use pasta_curves::{arithmetic::FieldExt, pallas};
 
 use std::iter;
 
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub struct Config {
     // Selector to check z_0 = alpha + t_q (mod p)
     q_mul_overflow: Selector,
@@ -24,24 +25,29 @@ pub struct Config {
     advices: [Column<Advice>; 3],
 }
 
-impl From<&EccConfig> for Config {
-    fn from(ecc_config: &EccConfig) -> Self {
-        Self {
-            q_mul_overflow: ecc_config.q_mul_overflow,
-            lookup_config: ecc_config.lookup_config,
-            // Use advice columns that don't conflict with the either the incomplete
-            // additions in fixed-base scalar mul, or the lookup range checks.
-            advices: [
-                ecc_config.advices[6],
-                ecc_config.advices[7],
-                ecc_config.advices[8],
-            ],
-        }
-    }
+impl Config {
+    // TODO: Make this pub(super).
+    pub(crate) fn configure(
+        meta: &mut ConstraintSystem<pallas::Base>,
+        lookup_config: LookupRangeCheckConfig<pallas::Base, { sinsemilla::K }>,
+        advices: [Column<Advice>; 3],
+    ) -> Self {
+        for advice in advices.iter() {
+            meta.enable_equality((*advice).into());
         }
 
-impl Config {
-    pub(super) fn create_gate(&self, meta: &mut ConstraintSystem<pallas::Base>) {
+        let config = Self {
+            q_mul_overflow: meta.selector(),
+            lookup_config,
+            advices,
+        };
+
+        config.create_gate(meta);
+
+        config
+    }
+
+    fn create_gate(&self, meta: &mut ConstraintSystem<pallas::Base>) {
         meta.create_gate("overflow checks", |meta| {
             let q_mul_overflow = meta.query_selector(self.q_mul_overflow);