Fix ifft_g1

Added fft/ifft g1 test

nomos-da/kzgrs/src/fft.rs

@@ -1,7 +1,11 @@
-use ark_bls12_381::{Fr, G1Affine};
-use ark_ec::CurveGroup;
+use ark_bls12_381::{Bls12_381, Fr, G1Affine};
+use ark_ec::pairing::Pairing;
+use ark_ec::{AffineRepr, CurveGroup};
+use ark_ff::{BigInt, BigInteger, FftField, Field, PrimeField};
+use blst::BLS12_381_G1;
 
 pub fn fft_g1(vals: &[G1Affine], roots_of_unity: &[Fr]) -> Vec<G1Affine> {
+    debug_assert_eq!(vals.len(), roots_of_unity.len());
     if vals.len() == 1 {
         return vals.to_vec();
     }
@@ -47,3 +51,42 @@ pub fn fft_g1(vals: &[G1Affine], roots_of_unity: &[Fr]) -> Vec<G1Affine> {
         })
         .collect()
 }
+
+pub fn ifft_g1(vals: &[G1Affine], roots_of_unity: &[Fr]) -> Vec<G1Affine> {
+    debug_assert_eq!(vals.len(), roots_of_unity.len());
+    let mut mod_min_2 = BigInt::new(<Fr as PrimeField>::MODULUS.0);
+    mod_min_2.sub_with_borrow(&BigInt::<4>::from(2u64));
+    let invlen = Fr::from(vals.len() as u64).pow(mod_min_2).into_bigint();
+    fft_g1(vals, roots_of_unity)
+        .into_iter()
+        .map(|g| g.mul_bigint(invlen).into_affine())
+        .collect()
+}
+
+#[cfg(test)]
+mod test {
+    use crate::fft::{fft_g1, ifft_g1};
+    use ark_bls12_381::{Fr, G1Affine};
+    use ark_ec::{AffineRepr, CurveGroup};
+    use ark_ff::{BigInt, FftField, Field};
+
+    #[test]
+    fn test_fft_ifft_g1() {
+        for size in [16usize, 32, 64, 128, 256, 512, 1024, 2048, 4096] {
+            let primitive_root = <Fr as FftField>::get_root_of_unity(size as u64).unwrap();
+            let roots_of_unity: Vec<_> = (1..=size)
+                .map(|i| primitive_root.pow::<ark_ff::BigInt<4>>(BigInt::from(i as u64)))
+                .collect();
+            let buff: Vec<G1Affine> = (0..size)
+                .map(|i| {
+                    G1Affine::identity()
+                        .mul_bigint(BigInt::<4>::from(i as u64))
+                        .into_affine()
+                })
+                .collect();
+            let fft = fft_g1(&buff, &roots_of_unity);
+            let ifft = ifft_g1(&fft, &roots_of_unity);
+            assert_eq!(buff, ifft);
+        }
+    }
+}