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remove: multiexp_serial for halo2curves version #282

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Feb 22, 2024
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remove: multiexp_serial for halo2curves version #282

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91 changes: 1 addition & 90 deletions halo2_backend/src/arithmetic.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@ use group::{
use halo2_common::multicore;
pub use halo2_middleware::ff::Field;

use halo2curves::msm::multiexp_serial;
pub use halo2curves::{CurveAffine, CurveExt};

/// This represents an element of a group with basic operations that can be
Expand All @@ -25,96 +26,6 @@ where
{
}

fn multiexp_serial<C: CurveAffine>(coeffs: &[C::Scalar], bases: &[C], acc: &mut C::Curve) {
let coeffs: Vec<_> = coeffs.iter().map(|a| a.to_repr()).collect();

let c = if bases.len() < 4 {
1
} else if bases.len() < 32 {
3
} else {
(f64::from(bases.len() as u32)).ln().ceil() as usize
};

fn get_at<F: PrimeField>(segment: usize, c: usize, bytes: &F::Repr) -> usize {
let skip_bits = segment * c;
let skip_bytes = skip_bits / 8;

if skip_bytes >= (F::NUM_BITS as usize + 7) / 8 {
return 0;
}

let mut v = [0; 8];
for (v, o) in v.iter_mut().zip(bytes.as_ref()[skip_bytes..].iter()) {
*v = *o;
}

let mut tmp = u64::from_le_bytes(v);
tmp >>= skip_bits - (skip_bytes * 8);
tmp %= 1 << c;

tmp as usize
}

let segments = (C::Scalar::NUM_BITS as usize / c) + 1;

for current_segment in (0..segments).rev() {
for _ in 0..c {
*acc = acc.double();
}

#[derive(Clone, Copy)]
enum Bucket<C: CurveAffine> {
None,
Affine(C),
Projective(C::Curve),
}

impl<C: CurveAffine> Bucket<C> {
fn add_assign(&mut self, other: &C) {
*self = match *self {
Bucket::None => Bucket::Affine(*other),
Bucket::Affine(a) => Bucket::Projective(a + *other),
Bucket::Projective(mut a) => {
a += *other;
Bucket::Projective(a)
}
}
}

fn add(self, mut other: C::Curve) -> C::Curve {
match self {
Bucket::None => other,
Bucket::Affine(a) => {
other += a;
other
}
Bucket::Projective(a) => other + a,
}
}
}

let mut buckets: Vec<Bucket<C>> = vec![Bucket::None; (1 << c) - 1];

for (coeff, base) in coeffs.iter().zip(bases.iter()) {
let coeff = get_at::<C::Scalar>(current_segment, c, coeff);
if coeff != 0 {
buckets[coeff - 1].add_assign(base);
}
}

// Summation by parts
// e.g. 3a + 2b + 1c = a +
// (a) + b +
// ((a) + b) + c
let mut running_sum = C::Curve::identity();
for exp in buckets.into_iter().rev() {
running_sum = exp.add(running_sum);
*acc += &running_sum;
}
}
}

/// Performs a small multi-exponentiation operation.
/// Uses the double-and-add algorithm with doublings shared across points.
pub fn small_multiexp<C: CurveAffine>(coeffs: &[C::Scalar], bases: &[C]) -> C::Curve {
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