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use core::convert::TryInto;
use subtle::{Choice, CtOption};
use crate::arithmetic::util::sbb;
const LIMBS: usize = 4;
const MODULUS: [u64; LIMBS] = [
0xBFD2_5E8C_D036_4141,
0xBAAE_DCE6_AF48_A03B,
0xFFFF_FFFF_FFFF_FFFE,
0xFFFF_FFFF_FFFF_FFFF,
];
#[derive(Clone, Copy, Debug)]
pub struct Scalar(pub(crate) [u64; LIMBS]);
impl From<u64> for Scalar {
fn from(k: u64) -> Self {
Scalar([k, 0, 0, 0])
}
}
impl Scalar {
pub const fn zero() -> Scalar {
Scalar([0, 0, 0, 0])
}
pub const fn one() -> Scalar {
Scalar([1, 0, 0, 0])
}
pub fn from_bytes(bytes: [u8; 32]) -> CtOption<Self> {
let mut w = [0u64; LIMBS];
w[3] = u64::from_be_bytes(bytes[0..8].try_into().unwrap());
w[2] = u64::from_be_bytes(bytes[8..16].try_into().unwrap());
w[1] = u64::from_be_bytes(bytes[16..24].try_into().unwrap());
w[0] = u64::from_be_bytes(bytes[24..32].try_into().unwrap());
let (_, borrow) = sbb(w[0], MODULUS[0], 0);
let (_, borrow) = sbb(w[1], MODULUS[1], borrow);
let (_, borrow) = sbb(w[2], MODULUS[2], borrow);
let (_, borrow) = sbb(w[3], MODULUS[3], borrow);
let is_some = (borrow as u8) & 1;
CtOption::new(Scalar(w), Choice::from(is_some))
}
pub fn to_bytes(&self) -> [u8; 32] {
let mut ret = [0; 32];
ret[0..8].copy_from_slice(&self.0[3].to_be_bytes());
ret[8..16].copy_from_slice(&self.0[2].to_be_bytes());
ret[16..24].copy_from_slice(&self.0[1].to_be_bytes());
ret[24..32].copy_from_slice(&self.0[0].to_be_bytes());
ret
}
}