Refactor to use bellpepper's new conditional selects (#355)

- Function `conditionally_select_vec` is removed and replaced with `conditionally_select_slice` across multiple files for consistency.
- Duplicate functions `conditionally_select` and `conditionally_select_vec` are removed from various modules.
- Unused import, `itertools::Itertools`, removed from `src/gadgets/utils.rs`.

src/circuit.rs

@@ -7,8 +7,8 @@
 use crate::{
   constants::{NIO_NOVA_FOLD, NUM_FE_WITHOUT_IO_FOR_CRHF, NUM_HASH_BITS},
   gadgets::{
-    alloc_num_equals, alloc_scalar_as_base, alloc_zero, conditionally_select_vec, le_bits_to_num,
-    AllocatedPoint, AllocatedR1CSInstance, AllocatedRelaxedR1CSInstance,
+    alloc_num_equals, alloc_scalar_as_base, alloc_zero, le_bits_to_num, AllocatedPoint,
+    AllocatedR1CSInstance, AllocatedRelaxedR1CSInstance,
   },
   r1cs::{R1CSInstance, RelaxedR1CSInstance},
   traits::{
@@ -17,7 +17,7 @@ use crate::{
   Commitment,
 };
 use abomonation_derive::Abomonation;
-use bellpepper::gadgets::Assignment;
+use bellpepper::gadgets::{boolean_utils::conditionally_select_slice, Assignment};
 use bellpepper_core::{
   boolean::{AllocatedBit, Boolean},
   num::AllocatedNum,
@@ -318,7 +318,7 @@ impl<'a, E: Engine, SC: StepCircuit<E::Base>> NovaAugmentedCircuit<'a, E, SC> {
     );
 
     // Compute z_{i+1}
-    let z_input = conditionally_select_vec(
+    let z_input = conditionally_select_slice(
       cs.namespace(|| "select input to F"),
       &z_0,
       &z_i,

src/gadgets/ecc.rs

@@ -2,13 +2,13 @@
 #![allow(non_snake_case)]
 use crate::{
   gadgets::utils::{
-    alloc_num_equals, alloc_one, alloc_zero, conditionally_select, conditionally_select2,
-    select_num_or_one, select_num_or_zero, select_num_or_zero2, select_one_or_diff2,
-    select_one_or_num2, select_zero_or_num2,
+    alloc_num_equals, alloc_one, alloc_zero, conditionally_select2, select_num_or_one,
+    select_num_or_zero, select_num_or_zero2, select_one_or_diff2, select_one_or_num2,
+    select_zero_or_num2,
   },
   traits::Group,
 };
-use bellpepper::gadgets::Assignment;
+use bellpepper::gadgets::{boolean_utils::conditionally_select, Assignment};
 use bellpepper_core::{
   boolean::{AllocatedBit, Boolean},
   num::AllocatedNum,

src/gadgets/mod.rs

@@ -15,6 +15,5 @@ mod utils;
 #[cfg(test)]
 pub(crate) use utils::alloc_one;
 pub(crate) use utils::{
-  alloc_num_equals, alloc_scalar_as_base, alloc_zero, conditionally_select_vec, le_bits_to_num,
-  scalar_as_base,
+  alloc_num_equals, alloc_scalar_as_base, alloc_zero, le_bits_to_num, scalar_as_base,
 };

src/gadgets/r1cs.rs

@@ -8,14 +8,16 @@ use crate::{
   gadgets::{
     ecc::AllocatedPoint,
     utils::{
-      alloc_bignat_constant, alloc_one, alloc_scalar_as_base, conditionally_select,
-      conditionally_select_bignat, le_bits_to_num,
+      alloc_bignat_constant, alloc_one, alloc_scalar_as_base, conditionally_select_bignat,
+      le_bits_to_num,
     },
   },
   r1cs::{R1CSInstance, RelaxedR1CSInstance},
   traits::{commitment::CommitmentTrait, Engine, Group, ROCircuitTrait, ROConstantsCircuit},
 };
-use bellpepper::gadgets::{boolean::Boolean, num::AllocatedNum, Assignment};
+use bellpepper::gadgets::{
+  boolean::Boolean, boolean_utils::conditionally_select, num::AllocatedNum, Assignment,
+};
 use bellpepper_core::{ConstraintSystem, SynthesisError};
 use ff::Field;
 use itertools::Itertools as _;

src/gadgets/utils.rs

@@ -8,7 +8,6 @@ use bellpepper_core::{
   ConstraintSystem, LinearCombination, SynthesisError,
 };
 use ff::{Field, PrimeField, PrimeFieldBits};
-use itertools::Itertools as _;
 use num_bigint::BigInt;
 
 /// Gets as input the little indian representation of a number and spits out the number
@@ -170,49 +169,6 @@ pub fn alloc_num_equals<F: PrimeField, CS: ConstraintSystem<F>>(
   Ok(r)
 }
 
-/// If condition return a otherwise b
-pub fn conditionally_select<F: PrimeField, CS: ConstraintSystem<F>>(
-  mut cs: CS,
-  a: &AllocatedNum<F>,
-  b: &AllocatedNum<F>,
-  condition: &Boolean,
-) -> Result<AllocatedNum<F>, SynthesisError> {
-  let c = AllocatedNum::alloc(cs.namespace(|| "conditional select result"), || {
-    if *condition.get_value().get()? {
-      Ok(*a.get_value().get()?)
-    } else {
-      Ok(*b.get_value().get()?)
-    }
-  })?;
-
-  // a * condition + b*(1-condition) = c ->
-  // a * condition - b*condition = c - b
-  cs.enforce(
-    || "conditional select constraint",
-    |lc| lc + a.get_variable() - b.get_variable(),
-    |_| condition.lc(CS::one(), F::ONE),
-    |lc| lc + c.get_variable() - b.get_variable(),
-  );
-
-  Ok(c)
-}
-
-/// If condition return a otherwise b
-pub fn conditionally_select_vec<F: PrimeField, CS: ConstraintSystem<F>>(
-  mut cs: CS,
-  a: &[AllocatedNum<F>],
-  b: &[AllocatedNum<F>],
-  condition: &Boolean,
-) -> Result<Vec<AllocatedNum<F>>, SynthesisError> {
-  a.iter()
-    .zip_eq(b.iter())
-    .enumerate()
-    .map(|(i, (a, b))| {
-      conditionally_select(cs.namespace(|| format!("select_{i}")), a, b, condition)
-    })
-    .collect::<Result<Vec<AllocatedNum<F>>, SynthesisError>>()
-}
-
 /// If condition return a otherwise b where a and b are `BigNats`
 pub fn conditionally_select_bignat<F: PrimeField, CS: ConstraintSystem<F>>(
   mut cs: CS,

src/supernova/circuit.rs

@@ -15,8 +15,8 @@ use crate::{
   constants::{NIO_NOVA_FOLD, NUM_HASH_BITS},
   gadgets::{
     alloc_num_equals, alloc_scalar_as_base, alloc_zero, conditionally_select_alloc_relaxed_r1cs,
-    conditionally_select_vec, conditionally_select_vec_allocated_relaxed_r1cs_instance,
-    le_bits_to_num, AllocatedPoint, AllocatedR1CSInstance, AllocatedRelaxedR1CSInstance,
+    conditionally_select_vec_allocated_relaxed_r1cs_instance, le_bits_to_num, AllocatedPoint,
+    AllocatedR1CSInstance, AllocatedRelaxedR1CSInstance,
   },
   r1cs::{R1CSInstance, RelaxedR1CSInstance},
   traits::{commitment::CommitmentTrait, Engine, ROCircuitTrait, ROConstantsCircuit},
@@ -28,7 +28,7 @@ use bellpepper_core::{
   ConstraintSystem, SynthesisError,
 };
 
-use bellpepper::gadgets::Assignment;
+use bellpepper::gadgets::{boolean_utils::conditionally_select_slice, Assignment};
 
 use abomonation_derive::Abomonation;
 use ff::{Field, PrimeField};
@@ -619,7 +619,7 @@ impl<'a, E: Engine, SC: EnforcingStepCircuit<E::Base>> SuperNovaAugmentedCircuit
     );
 
     // Compute z_{i+1}
-    let z_input = conditionally_select_vec(
+    let z_input = conditionally_select_slice(
       cs.namespace(|| "select input to F"),
       &z_0,
       &z_i,