use hmac::{Hmac,NewMac,Mac};
use sha2::Sha256;

const SIGNED_KEY_LEN: usize = 32;
const BASE64_DIGEST_LEN: usize = 44;

/// A convenience wrapper around HMAC.
///
/// This code was adapted from the [`cookie`] crate which does not make the sign and verify functions public
/// forcing the use of [`CookieJar`](cookie::CookieJar)s, which are akward to work with without a high-level framework.
// Thanks to Sergio Benitez for writing the original code and releasing it under MIT!
pub struct Key (pub [u8; SIGNED_KEY_LEN]);

impl Key {
    const fn zero() -> Self {
        Key ( [0; SIGNED_KEY_LEN])
    }

    /// Attempts to generate signing/encryption keys from a secure, random
    /// source. Keys are generated nondeterministically. If randomness cannot be
    /// retrieved from the underlying operating system, returns `None`.
    pub fn try_generate() -> Option<Key> {
        use rand::RngCore;

        let mut rng = rand::thread_rng();
        let mut both_keys = [0; SIGNED_KEY_LEN];
        rng.try_fill_bytes(&mut both_keys).ok()?;
        Some(Key::from(&both_keys))
    }

    /// Creates a new Key from a 32 byte cryptographically random string.
    pub fn from(key: &[u8]) -> Key {
        if key.len() < SIGNED_KEY_LEN {
            panic!("bad key length: expected >= 32 bytes, found {}", key.len());
        }

        let mut output = Key::zero();
        output.0.copy_from_slice(&key[..SIGNED_KEY_LEN]);
        output
    }

    /// Signs the value providing integrity and authenticity.
    pub fn sign(&self, value: &str) -> String {
        // Compute HMAC-SHA256 of the cookie's value.
        let mut mac = Hmac::<Sha256>::new_varkey(&self.0).expect("good key");
        mac.update(value.as_bytes());

        // Cookie's new value is [MAC | original-value].
        let mut new_value = base64::encode(&mac.finalize().into_bytes());
        new_value.push_str(value);
        new_value
    }

    /// Extracts the value from a string signed with [`Key::sign`].
    /// Fails if the string is malformed or the signature doesn't match.
    pub fn verify(&self, value: &str) -> Result<String, String> {
        if value.len() < BASE64_DIGEST_LEN {
            return Err("length of value is <= BASE64_DIGEST_LEN".to_string());
        }

        // Split [MAC | original-value] into its two parts.
        let (digest_str, value) = value.split_at(BASE64_DIGEST_LEN);
        let digest = base64::decode(digest_str).map_err(|_| "bad base64 digest")?;

        // Perform the verification.
        let mut mac = Hmac::<Sha256>::new_varkey(&self.0).expect("good key");
        mac.update(value.as_bytes());
        mac.verify(&digest)
            .map(|_| value.to_string())
            .map_err(|_| "value did not verify".to_string())
    }
}