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use curve25519_entropic::{
    constants::ED25519_BASEPOINT_TABLE, edwards::CompressedEdwardsY, scalar::Scalar,
};

use sha2::{Digest, Sha512};

use crate::{errors::VRFError, utils::WEAK_KEYS};

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct SecretKey {
    pub(crate) bytes: [u8; 32],
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct PublicKey {
    pub(crate) point: CompressedEdwardsY,
}

impl SecretKey {
    #[must_use]
    pub fn new(bytes: &[u8; 32]) -> Self {
        SecretKey { bytes: *bytes }
    }

    #[must_use]
    pub fn from_slice(bytes: &[u8]) -> Self {
        let mut b = [0u8; 32];
        b.copy_from_slice(bytes);
        SecretKey { bytes: b }
    }

    #[must_use]
    pub fn as_bytes(&self) -> &[u8] {
        &self.bytes
    }

    #[must_use]
    pub fn to_bytes(&self) -> [u8; 32] {
        self.bytes
    }

    /// Extracts the public key and scalar of this [`SecretKey`].
    ///
    /// # Errors
    ///
    /// This function will return an error if the secret key is invalid.
    pub fn extract_public_key_and_scalar(&self) -> Result<(PublicKey, Scalar), VRFError> {
        let mut hasher = Sha512::new();
        hasher.update(&self.bytes);
        let hash: [u8; 64] = hasher.finalize().into();
        let mut digest: [u8; 32] = [0u8; 32];
        digest.copy_from_slice(&hash[..32]);
        digest[0] &= 0xF8;
        digest[31] &= 0x7F;
        digest[31] |= 0x40;

        let scalar = Scalar::from_bits(digest);

        let point = &scalar * &ED25519_BASEPOINT_TABLE;
        if point.mul_by_cofactor().compress() == CompressedEdwardsY::default() {
            return Err(VRFError::InvalidSecretKey {});
        }
        let pk = PublicKey {
            point: point.compress(),
        };
        Ok((pk, scalar))
    }
}

impl PublicKey {
    #[must_use]
    pub fn new(point: CompressedEdwardsY) -> Self {
        PublicKey { point }
    }

    #[must_use]
    pub fn from_bytes(bytes: &[u8]) -> Self {
        let mut b = [0u8; 32];
        b.copy_from_slice(bytes);
        PublicKey {
            point: CompressedEdwardsY::from_slice(&b),
        }
    }

    #[must_use]
    pub fn as_bytes(&self) -> &[u8] {
        self.point.as_bytes()
    }

    #[must_use]
    pub fn to_bytes(&self) -> [u8; 32] {
        self.point.to_bytes()
    }

    #[must_use]
    pub fn as_point(&self) -> &CompressedEdwardsY {
        &self.point
    }

    /// Validates this [`PublicKey`].
    ///
    /// # Errors
    ///
    /// This function will return an error if the public key multiplied by the cofactor is the identity of the curve.
    pub fn validate(&self) -> Result<(), VRFError> {
        if self.point.decompress().is_some() {
            if WEAK_KEYS.contains(self.point.as_bytes()) {
                return Err(VRFError::InvalidPublicKey {});
            }
        } else {
            return Err(VRFError::InvalidPublicKey {});
        }
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_extract_pk_scalar() {
        let secret_key =
            hex::decode("9d61b19deffd5a60ba844af492ec2cc44449c5697b326919703bac031cae7f60")
                .unwrap();
        let public_key =
            hex::decode("d75a980182b10ab7d54bfed3c964073a0ee172f3daa62325af021a68f707511a")
                .unwrap();
        let secret_scalar =
            hex::decode("307c83864f2833cb427a2ef1c00a013cfdff2768d980c0a3a520f006904de94f")
                .unwrap();
        let secret_key = SecretKey::from_slice(&secret_key);
        let (pk, scalar) = secret_key.extract_public_key_and_scalar().unwrap();
        assert_eq!(pk.as_bytes(), public_key.as_slice());
        assert_eq!(scalar.as_bytes(), secret_scalar.as_slice());
    }
}