// -*- mode: rust; -*-
//
// This file is part of curve25519-dalek.
// Copyright (c) 2016-2021 isis lovecruft
// Copyright (c) 2016-2019 Henry de Valence
// See LICENSE for licensing information.
//
// Authors:
// - isis agora lovecruft <isis@patternsinthevoid.net>
// - Henry de Valence <hdevalence@hdevalence.ca>

#![allow(non_snake_case)]

use core::cmp::Ordering;

use crate::backend::vector::BASEPOINT_ODD_LOOKUP_TABLE;
use crate::backend::vector::{CachedPoint, ExtendedPoint};
use crate::edwards::EdwardsPoint;
use crate::scalar::Scalar;
use crate::traits::Identity;
use crate::window::NafLookupTable5;

/// Compute \\(aA + bB\\) in variable time, where \\(B\\) is the Ed25519 basepoint.
pub fn mul(a: &Scalar, A: &EdwardsPoint, b: &Scalar) -> EdwardsPoint {
    let a_naf = a.non_adjacent_form(5);
    let b_naf = b.non_adjacent_form(8);

    // Find starting index
    let mut i: usize = 255;
    for j in (0..256).rev() {
        i = j;
        if a_naf[i] != 0 || b_naf[i] != 0 {
            break;
        }
    }

    let table_A = NafLookupTable5::<CachedPoint>::from(A);
    let table_B = &BASEPOINT_ODD_LOOKUP_TABLE;

    let mut Q = ExtendedPoint::identity();

    loop {
        Q = Q.double();

        match a_naf[i].cmp(&0) {
            Ordering::Greater => {
                Q = &Q + &table_A.select(a_naf[i] as usize);
            }
            Ordering::Less => {
                Q = &Q - &table_A.select(-a_naf[i] as usize);
            }
            Ordering::Equal => {}
        }

        match b_naf[i].cmp(&0) {
            Ordering::Greater => {
                Q = &Q + &table_B.select(b_naf[i] as usize);
            }
            Ordering::Less => {
                Q = &Q - &table_B.select(-b_naf[i] as usize);
            }
            Ordering::Equal => {}
        }

        if i == 0 {
            break;
        }
        i -= 1;
    }

    Q.into()
}