Dori/bitreverse permutation public (#960)

* refactor: share bitreverse permutation code

Signed-off-by: Dori Medini <dori@starkware.co>

* feat: export bitreverse_permutation_in_place for public use

Signed-off-by: Dori Medini <dori@starkware.co>

* chore: added entry to changelog

Signed-off-by: Dori Medini <dori@starkware.co>

---------

Signed-off-by: Dori Medini <dori@starkware.co>

Author: dorimedini-starkware

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Pending
 
+- Consolidated logic into `bitreverse_permutation_in_place` and made it public.
+
 ### Breaking changes
 
 ### Features

poly/src/domain/mixed_radix.rs

@@ -12,7 +12,7 @@
 
 pub use crate::domain::utils::Elements;
 use crate::domain::{
-    utils::{best_fft, bitreverse},
+    utils::{best_fft, bitreverse_permutation_in_place},
     DomainCoeff, EvaluationDomain,
 };
 use ark_ff::{fields::utils::k_adicity, FftField};
@@ -354,13 +354,7 @@ pub(crate) fn serial_mixed_radix_fft<T: DomainCoeff<F>, F: FftField>(
             m *= q;
         }
     } else {
-        // swapping in place (from Storer's book)
-        for k in 0..n {
-            let rk = bitreverse(k as u32, two_adicity) as usize;
-            if k < rk {
-                a.swap(k, rk);
-            }
-        }
+        bitreverse_permutation_in_place(a, two_adicity);
     }
 
     for _ in 0..two_adicity {

poly/src/domain/radix2/mod.rs

@@ -4,7 +4,7 @@
 //!
 //! `Radix2EvaluationDomain` supports FFTs of size at most `2^F::TWO_ADICITY`.
 
-pub use crate::domain::utils::Elements;
+pub use crate::domain::utils::{bitreverse_permutation_in_place, Elements};
 use crate::domain::{DomainCoeff, EvaluationDomain};
 use ark_ff::FftField;
 use ark_serialize::{CanonicalDeserialize, CanonicalSerialize};
@@ -442,12 +442,7 @@ mod tests {
             assert_eq!(n, 1 << log_n);
 
             // swap coefficients in place
-            for k in 0..n {
-                let rk = crate::domain::utils::bitreverse(k, log_n);
-                if k < rk {
-                    a.swap(rk as usize, k as usize);
-                }
-            }
+            crate::domain::utils::bitreverse_permutation_in_place(a, log_n);
 
             let mut m = 1;
             for _i in 1..=log_n {

poly/src/domain/utils.rs

@@ -19,6 +19,18 @@ pub(crate) fn bitreverse(mut n: u32, l: u32) -> u32 {
     r
 }
 
+#[inline]
+pub fn bitreverse_permutation_in_place<T>(a: &mut [T], width: u32) {
+    // swapping in place (from Storer's book)
+    let n = a.len();
+    for k in 0..n {
+        let rk = bitreverse(k as u32, width) as usize;
+        if k < rk {
+            a.swap(k, rk);
+        }
+    }
+}
+
 pub(crate) fn compute_powers_serial<F: Field>(size: usize, root: F) -> Vec<F> {
     compute_powers_and_mul_by_const_serial(size, root, F::one())
 }