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use std::io;
use std::ops::{Deref, DerefMut};

use crate::{Config, Context, ContextUnit, Element, Preamble};

/// Scalar field representation with up to 256 bits.
///
/// This is agnostic to the curve choice and no canonical encoding assumption is involved.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Scalar {
    scalar: [u8; Self::LEN],
}

impl Scalar {
    /// Fixed serialized length
    pub const LEN: usize = 32;
}

impl From<[u8; Scalar::LEN]> for Scalar {
    fn from(scalar: [u8; Self::LEN]) -> Self {
        Self { scalar }
    }
}

impl AsRef<[u8]> for Scalar {
    fn as_ref(&self) -> &[u8] {
        &self.scalar
    }
}

impl Deref for Scalar {
    type Target = [u8; Self::LEN];

    fn deref(&self) -> &Self::Target {
        &self.scalar
    }
}

impl DerefMut for Scalar {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.scalar
    }
}

impl Element for Scalar {
    type Config = Config;

    fn zeroed() -> Self {
        Self::default()
    }

    fn len(config: &Self::Config) -> usize {
        if config.zeroed_scalar_values {
            0
        } else {
            Self::LEN
        }
    }

    fn to_buffer(&self, config: &Self::Config, _context: &mut ContextUnit, buf: &mut [u8]) {
        if !config.zeroed_scalar_values {
            let buf = &mut buf[..Self::LEN];

            buf.copy_from_slice(&self.scalar);
        }
    }

    fn try_from_buffer_in_place<S>(
        &mut self,
        config: &Self::Config,
        _context: &mut Context<S>,
        buf: &[u8],
    ) -> io::Result<()> {
        Self::validate_buffer_len(config, buf.len())?;

        if !config.zeroed_scalar_values {
            self.scalar.copy_from_slice(&buf[..Self::LEN]);
        }

        Ok(())
    }

    fn validate(&self, _config: &Preamble) -> io::Result<()> {
        Ok(())
    }
}

#[test]
fn bytes_encode_works() {
    let mut bytes = [0xfa; Scalar::LEN];
    let mut scalar = Scalar::from(bytes);

    assert_eq!(&bytes, scalar.as_ref());
    assert_eq!(bytes, *scalar);
    assert_eq!(&mut bytes, scalar.deref_mut());
}

#[test]
fn encode_zeroed_len_is_consitent() {
    let config = *Config::default().with_zeroed_scalar_values(false);
    let len = <Scalar as Element>::len(&config);

    assert_eq!(Scalar::LEN, len);

    let config = *Config::default().with_zeroed_scalar_values(true);
    let len = <Scalar as Element>::len(&config);

    assert_eq!(0, len);
}