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Implement cube root of unity for Grumpkin Fq in HonkCurve #463

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Implement cube root of unity for Grumpkin Fq in HonkCurve #463

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13 changes: 12 additions & 1 deletion co-noir/co-builder/src/honk_curve.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -200,7 +200,18 @@ impl HonkCurve<ark_bn254::Fr> for short_weierstrass::Projective<GrumpkinConfig>
val
}
fn get_cube_root_of_unity() -> Self::BaseField {
todo!("cube root of unity for grumpkin::Fq")
// Compute λ = (-1 + sqrt(-3)) / 2 in the base field
let minus_three = -ark_grumpkin::Fq::from(3u64);
let sqrt_minus_three = minus_three
.sqrt()
.expect("sqrt(-3) must exist in grumpkin::Fq");
let two_inv = ark_grumpkin::Fq::from(2u64).inverse().unwrap();
let lambda = (-ark_grumpkin::Fq::one() + sqrt_minus_three) * two_inv;

debug_assert!(lambda != ark_grumpkin::Fq::one());
debug_assert_eq!(lambda.pow([3u64]), ark_grumpkin::Fq::one());

lambda
}

fn get_bb_infinity_default() -> Self::BaseField {
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