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use super::Params;
use crate::arithmetic::{best_multiexp, CurveAffine};
use ff::Field;
use group::Group;
use std::collections::BTreeMap;
#[derive(Debug, Clone)]
pub struct MSM<'a, C: CurveAffine> {
pub(crate) params: &'a Params<C>,
g_scalars: Option<Vec<C::Scalar>>,
w_scalar: Option<C::Scalar>,
u_scalar: Option<C::Scalar>,
other: BTreeMap<C::Base, (C::Scalar, C::Base)>,
}
impl<'a, C: CurveAffine> MSM<'a, C> {
pub fn new(params: &'a Params<C>) -> Self {
let g_scalars = None;
let w_scalar = None;
let u_scalar = None;
let other = BTreeMap::new();
MSM {
params,
g_scalars,
w_scalar,
u_scalar,
other,
}
}
pub fn add_msm(&mut self, other: &Self) {
for (x, (scalar, y)) in other.other.iter() {
self.other
.entry(*x)
.and_modify(|(our_scalar, our_y)| {
if our_y == y {
*our_scalar += *scalar;
} else {
assert!(*our_y == -*y);
*our_scalar -= *scalar;
}
})
.or_insert((*scalar, *y));
}
if let Some(g_scalars) = &other.g_scalars {
self.add_to_g_scalars(g_scalars);
}
if let Some(w_scalar) = &other.w_scalar {
self.add_to_w_scalar(*w_scalar);
}
if let Some(u_scalar) = &other.u_scalar {
self.add_to_u_scalar(*u_scalar);
}
}
pub fn append_term(&mut self, scalar: C::Scalar, point: C) {
if !bool::from(point.is_identity()) {
let xy = point.coordinates().unwrap();
let x = *xy.x();
let y = *xy.y();
self.other
.entry(x)
.and_modify(|(our_scalar, our_y)| {
if *our_y == y {
*our_scalar += scalar;
} else {
assert!(*our_y == -y);
*our_scalar -= scalar;
}
})
.or_insert((scalar, y));
}
}
pub fn add_constant_term(&mut self, constant: C::Scalar) {
if let Some(g_scalars) = self.g_scalars.as_mut() {
g_scalars[0] += &constant;
} else {
let mut g_scalars = vec![C::Scalar::zero(); self.params.n as usize];
g_scalars[0] += &constant;
self.g_scalars = Some(g_scalars);
}
}
pub fn add_to_g_scalars(&mut self, scalars: &[C::Scalar]) {
assert_eq!(scalars.len(), self.params.n as usize);
if let Some(g_scalars) = &mut self.g_scalars {
for (g_scalar, scalar) in g_scalars.iter_mut().zip(scalars.iter()) {
*g_scalar += scalar;
}
} else {
self.g_scalars = Some(scalars.to_vec());
}
}
pub fn add_to_w_scalar(&mut self, scalar: C::Scalar) {
self.w_scalar = self.w_scalar.map_or(Some(scalar), |a| Some(a + &scalar));
}
pub fn add_to_u_scalar(&mut self, scalar: C::Scalar) {
self.u_scalar = self.u_scalar.map_or(Some(scalar), |a| Some(a + &scalar));
}
pub fn scale(&mut self, factor: C::Scalar) {
if let Some(g_scalars) = &mut self.g_scalars {
for g_scalar in g_scalars {
*g_scalar *= &factor;
}
}
for other in self.other.values_mut() {
other.0 *= factor;
}
self.w_scalar = self.w_scalar.map(|a| a * &factor);
self.u_scalar = self.u_scalar.map(|a| a * &factor);
}
pub fn eval(self) -> bool {
let len = self.g_scalars.as_ref().map(|v| v.len()).unwrap_or(0)
+ self.w_scalar.map(|_| 1).unwrap_or(0)
+ self.u_scalar.map(|_| 1).unwrap_or(0)
+ self.other.len();
let mut scalars: Vec<C::Scalar> = Vec::with_capacity(len);
let mut bases: Vec<C> = Vec::with_capacity(len);
scalars.extend(self.other.values().map(|(scalar, _)| scalar));
bases.extend(
self.other
.iter()
.map(|(x, (_, y))| C::from_xy(*x, *y).unwrap()),
);
if let Some(w_scalar) = self.w_scalar {
scalars.push(w_scalar);
bases.push(self.params.w);
}
if let Some(u_scalar) = self.u_scalar {
scalars.push(u_scalar);
bases.push(self.params.u);
}
if let Some(g_scalars) = &self.g_scalars {
scalars.extend(g_scalars);
bases.extend(self.params.g.iter());
}
assert_eq!(scalars.len(), len);
bool::from(best_multiexp(&scalars, &bases).is_identity())
}
}
#[cfg(test)]
mod tests {
use crate::poly::commitment::{Params, MSM};
use group::Curve;
use pasta_curves::{arithmetic::CurveAffine, EpAffine, Fp, Fq};
#[test]
fn msm_arithmetic() {
let base = EpAffine::from_xy(-Fp::one(), Fp::from(2)).unwrap();
let base_viol = (base + base).to_affine();
let params = Params::new(4);
let mut a: MSM<EpAffine> = MSM::new(¶ms);
a.append_term(Fq::one(), base);
assert!(!a.clone().eval());
a.append_term(Fq::one(), base);
assert!(!a.clone().eval());
a.append_term(-Fq::one(), base_viol);
assert!(a.clone().eval());
let b = a.clone();
a.append_term(Fq::from(4), -base);
assert!(!a.clone().eval());
a.append_term(Fq::from(2), base_viol);
assert!(a.clone().eval());
a.scale(Fq::from(3));
a.add_msm(&b);
assert!(a.clone().eval());
let mut c: MSM<EpAffine> = MSM::new(¶ms);
c.append_term(Fq::from(2), base);
c.append_term(Fq::one(), -base_viol);
assert!(c.clone().eval());
a.add_msm(&c);
assert!(a.eval());
}
}