//! Encode Rsa's Public Key as a Magic Public Key
//!
//! This implementation has been reverse-engineered from Mastodon's implementation, since no
//! documentation for the Magic Public Key format could be found online (Maybe I didn't look hard
//! enough).
//!
//! ### Examples
//! From private key
//! ```rust
//! # let mut rng = rand::thread_rng();
//! # let private_key = rsa::RsaPrivateKey::new(&mut rng, 2048).unwrap();
//! use rsa_magic_public_key::AsMagicPublicKey;
//! let string = private_key.as_magic_public_key();
//! ```
//! From public key
//! ```rust
//! # let mut rng = rand::thread_rng();
//! # let private_key = rsa::RsaPrivateKey::new(&mut rng, 2048).unwrap();
//! # let public_key = private_key.to_public_key();
//! use rsa_magic_public_key::AsMagicPublicKey;
//! let string = public_key.as_magic_public_key();
//! ```
//! Parsing
//! ```rust
//! # use rsa_magic_public_key::AsMagicPublicKey;
//! # let mut rng = rand::thread_rng();
//! # let private_key = rsa::RsaPrivateKey::new(&mut rng, 2048).unwrap();
//! # let magic_public_key = private_key.as_magic_public_key();
//! use rsa::RsaPublicKey;
//! use rsa_magic_public_key::FromMagicPublicKey;
//! let public_key = RsaPublicKey::from_magic_public_key(&magic_public_key).unwrap();
//! ```
use base64::{engine::general_purpose::URL_SAFE, Engine};
use num_bigint_dig::BigUint;
use rsa::{traits::PublicKeyParts, RsaPublicKey};
use thiserror::Error;

/// Helper trait to add functionality to Rsa types
pub trait AsMagicPublicKey {
    /// Produce a magic-public-key formatted string
    fn as_magic_public_key(&self) -> String;
}

/// Helper trait to add functionality to Rsa types
pub trait FromMagicPublicKey {
    /// Parse a type from a magic-public-key formatted string
    fn from_magic_public_key(magic_public_key: &str) -> Result<Self, KeyError>
    where
        Self: Sized;
}

#[derive(Debug, Error)]
/// Parsing errors
pub enum KeyError {
    /// The magic-public-key is not properly formatted
    #[error("The provided key is malformed")]
    Malformed,

    /// The magic-public-key is not Rsa
    #[error("The provided key is of the wrong kind")]
    Kind,
}

impl<T> AsMagicPublicKey for T
where
    T: PublicKeyParts,
{
    fn as_magic_public_key(&self) -> String {
        let n = URL_SAFE.encode(&self.n().to_bytes_be());
        let e = URL_SAFE.encode(&self.e().to_bytes_be());

        format!("Rsa.{}.{}", n, e)
    }
}

impl FromMagicPublicKey for RsaPublicKey {
    fn from_magic_public_key(magic_public_key: &str) -> Result<Self, KeyError> {
        let mut iter = magic_public_key.split('.');
        let kind = iter.next().ok_or(KeyError::Malformed)?;

        match kind {
            "Rsa" => (),
            _ => return Err(KeyError::Kind),
        };

        let n = iter.next().ok_or(KeyError::Malformed)?;
        let e = iter.next().ok_or(KeyError::Malformed)?;

        let n = URL_SAFE.decode(n)?;
        let e = URL_SAFE.decode(e)?;

        let n = BigUint::from_bytes_be(&n);
        let e = BigUint::from_bytes_be(&e);

        RsaPublicKey::new(n, e).map_err(|_| KeyError::Malformed)
    }
}

impl From<base64::DecodeError> for KeyError {
    fn from(_: base64::DecodeError) -> Self {
        KeyError::Malformed
    }
}

#[cfg(test)]
mod tests {
    use crate::{AsMagicPublicKey, FromMagicPublicKey};
    use rsa::{RsaPrivateKey, RsaPublicKey};

    #[test]
    fn can_complete_cycle() {
        let mut rng = rand::thread_rng();
        let rsa = RsaPrivateKey::new(&mut rng, 2048).unwrap();

        let string = rsa.as_magic_public_key();

        let res = RsaPublicKey::from_magic_public_key(&string);

        assert!(res.is_ok());
    }

    #[test]
    fn asonix_key_is_valid() {
        key_is_valid("Rsa.wEvEsHqM3twoC2F3KYMQ9YOialfVQX4StkLvhLUwFv8qpmY7ZSHHl2TWpnzlo5iWS5Pi2vC41HUGYz9XT5G74IUOyuIkjTL1FIcPJDcUFCzQjN3QZcHLPJPJVNOOOEiOk8__paOyrqJTq9ikcJDMJ8KTWQgk1leOxUVEN5uaQ-p9IBFbXC76-RqabfEoqLZagVMDSOfeC2uR9xZ1q5HkFveRTGs84QLR7FJVvx078nszx4UQGnmP0M-0sOeRJGK17IoJmhaok1XBpP6XFQ45vYeIRiaFj0Pc9GNISCW70dVXKMhv-K07orQJm6PwP8USyhq4tLkq6tcPbGRqEk3ZXw==.AQAB");
    }

    #[test]
    fn sir_boops_key_is_valid() {
        key_is_valid("Rsa.vwDujxmxoYHs64MyVB3LG5ZyBxV3ufaMRBFu42bkcTpISq1WwZ-3Zb6CI8zOO-nM-Q2llrVRYjZa4ZFnOLvMTq_Kf-Zf5wy2aCRer88gX-MsJOAtItSi412y0a_rKOuFaDYLOLeTkRvmGLgZWbsrZJOp-YWb3zQ5qsIOInkc5BwI172tMsGeFtsnbNApPV4lrmtTGaJ8RiM8MR7XANBOfOHggSt1-eAIKGIsCmINEMzs1mG9D75xKtC_sM8GfbvBclQcBstGkHAEj1VHPW0ch6Bok5_QQppicyb8UA1PAA9bznSFtKlYE4xCH8rlCDSDTBRtdnBWHKcj619Ujz4Qaw==.AQAB");
    }

    fn key_is_valid(key: &str) {
        let res = RsaPublicKey::from_magic_public_key(key);

        assert!(res.is_ok());

        assert_eq!(res.unwrap().as_magic_public_key(), key);
    }
}