// Copyright 2015-2019 Brian Smith.

//

// Permission to use, copy, modify, and/or distribute this software for any

// purpose with or without fee is hereby granted, provided that the above

// copyright notice and this permission notice appear in all copies.

//

// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES

// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF

// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY

// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES

// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION

// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN

// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

use crate::{

    arithmetic::limbs_from_hex,

    digest, error, limb,

    polyfill::slice::{self, AsChunks},

};

use core::array;

#[repr(transparent)]

pub struct Scalar([u8; SCALAR_LEN]);

pub const SCALAR_LEN: usize = 32;

impl Scalar {

    // Constructs a `Scalar` from `bytes`, failing if `bytes` encodes a scalar

    // that is not in the range [0, n).

    pub fn from_bytes_checked(bytes: [u8; SCALAR_LEN]) -> Result<Self, error::Unspecified> {

        const ORDER: [limb::Limb; SCALAR_LEN / limb::LIMB_BYTES] =

            limbs_from_hex("1000000000000000000000000000000014def9dea2f79cd65812631a5cf5d3ed");

        let order = ORDER.map(limb::Limb::from);

        let (limbs_as_bytes, _empty): (AsChunks<u8, { limb::LIMB_BYTES }>, _) =

            slice::as_chunks(&bytes);

        debug_assert!(_empty.is_empty());

        let limbs: [limb::Limb; SCALAR_LEN / limb::LIMB_BYTES] =

            array::from_fn(|i| limb::Limb::from_le_bytes(limbs_as_bytes[i]));

        limb::verify_limbs_less_than_limbs_leak_bit(&limbs, &order)?;

        Ok(Self(bytes))

    }

    // Constructs a `Scalar` from `digest` reduced modulo n.

    pub fn from_sha512_digest_reduced(digest: digest::Digest) -> Self {

        prefixed_extern! {

            fn x25519_sc_reduce(s: &mut UnreducedScalar);

        }

        let mut unreduced = [0u8; digest::SHA512_OUTPUT_LEN];

        unreduced.copy_from_slice(digest.as_ref());

        unsafe { x25519_sc_reduce(&mut unreduced) };

        Self((&unreduced[..SCALAR_LEN]).try_into().unwrap())

    }

}

#[repr(transparent)]

pub struct MaskedScalar([u8; SCALAR_LEN]);

impl MaskedScalar {

    pub fn from_bytes_masked(bytes: [u8; SCALAR_LEN]) -> Self {

        prefixed_extern! {

            fn x25519_sc_mask(a: &mut [u8; SCALAR_LEN]);

        }

        let mut r = Self(bytes);

        unsafe { x25519_sc_mask(&mut r.0) };

        r

    }

}

impl From<MaskedScalar> for Scalar {

    fn from(MaskedScalar(scalar): MaskedScalar) -> Self {

        Self(scalar)

    }

}

type UnreducedScalar = [u8; UNREDUCED_SCALAR_LEN];

const UNREDUCED_SCALAR_LEN: usize = SCALAR_LEN * 2;