use std::sync::LazyLock;

use bincode::{config, serde::encode_to_vec};
use ed25519::signature::{Signer, Verifier};
use ed25519_dalek::{SigningKey, VerifyingKey};
use serde::Serialize;
use tokio::sync::RwLock;
use zeroize::Zeroize;

pub type SignatureBundle =
    SignatureKeyBundle<ed25519_dalek::SigningKey, ed25519_dalek::VerifyingKey>;

static SIGNATURE_BUNDLE: LazyLock<RwLock<SignatureBundle>> =
    LazyLock::new(|| RwLock::new(SignatureKeyBundle::new()));

pub trait Verifiable {
    fn sign(&self) -> impl std::future::Future<Output = ed25519::Signature> + Send;
    fn verify<P: Serialize>(
        &self,
        signature: &ed25519::Signature,
    ) -> impl std::future::Future<Output = Result<(), ed25519::Error>> + Send;
}

#[derive(Zeroize)]
struct SignatureKeyBundle<S, V>
where
    S: Signer<ed25519::Signature>,
    V: Verifier<ed25519::Signature>,
{
    pub signing_key: S,
    pub verifying_key: V,
}

impl<S, V> SignatureKeyBundle<S, V>
where
    S: Signer<ed25519::Signature>,
    V: Verifier<ed25519::Signature>,
{
    /// Signs a serializable payload
    pub fn sign<P: Serialize>(&self, payload: &P) -> ed25519::Signature {
        self.sign_bytes(&SignatureKeyBundle::<S, V>::hash_struct(payload))
    }

    fn sign_bytes(&self, payload: &[u8]) -> ed25519::Signature {
        // NOTE: use `try_sign` if you'd like to be able to handle
        // errors from external signing services/devices (e.g. HSM/KMS)
        // <https://docs.rs/signature/latest/signature/trait.Signer.html#tymethod.try_sign>
        self.signing_key.sign(payload)
    }

    /// Verifies a serializable payload against a given signature.
    pub fn verify<P: Serialize>(
        &self,
        payload: &P,
        signature: &ed25519::Signature,
    ) -> Result<(), ed25519::Error> {
        self.verify_bytes(&SignatureKeyBundle::<S, V>::hash_struct(payload), signature)
    }

    fn verify_bytes(
        &self,
        payload: &[u8],
        signature: &ed25519::Signature,
    ) -> Result<(), ed25519::Error> {
        self.verifying_key.verify(payload, signature)
    }

    /// Serializes and hashes payload.
    /// Uses `bincode` for serialization and `blake3` for hashing.
    fn hash_struct<P: Serialize>(payload: &P) -> [u8; blake3::OUT_LEN] {
        let serialized_payload =
            encode_to_vec(payload, config::standard()).expect("Failed to serialize payload");
        let hash: blake3::Hash = blake3::hash(&serialized_payload);
        let hash_bytes = hash.as_bytes();

        *hash_bytes
    }
}

impl SignatureKeyBundle<SigningKey, VerifyingKey> {
    pub fn new() -> SignatureKeyBundle<SigningKey, VerifyingKey> {
        let mut rng = aes_gcm_siv::aead::OsRng;
        let signing_key = SigningKey::generate(&mut rng);
        let verifying_key: VerifyingKey = signing_key.verifying_key();
        Self {
            signing_key,
            verifying_key,
        }
    }
}

pub async fn sign<P: Serialize>(payload: &P) -> ed25519::Signature {
    SIGNATURE_BUNDLE.read().await.sign(payload)
}

pub async fn verify<P: Serialize>(
    payload: &P,
    signature: &ed25519::Signature,
) -> Result<(), ed25519::Error> {
    SIGNATURE_BUNDLE.read().await.verify(payload, signature)
}

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

    #[derive(Serialize)]
    struct TestPayload {
        data: String,
    }

    #[derive(Serialize)]
    struct TestPayloadEvolution {
        data: String,
        new_prop: Option<bool>,
    }

    #[test]
    fn test_sign_and_verify() {
        let key_bundle = SignatureKeyBundle::new();
        let payload = TestPayload {
            data: "test data".to_string(),
        };

        let signature = key_bundle.sign(&payload);
        assert!(key_bundle.verify(&payload, &signature).is_ok());
    }

    #[test]
    fn test_verify_with_invalid_signature() {
        let key_bundle = SignatureKeyBundle::new();
        let payload = TestPayload {
            data: "test data".to_string(),
        };

        let signature = key_bundle.sign(&payload);

        let invalid_payload = TestPayload {
            data: "tampered data".to_string(),
        };

        assert!(key_bundle.verify(&invalid_payload, &signature).is_err());
    }

    #[test]
    fn test_verify_with_different_bundles() {
        let key_bundle = SignatureKeyBundle::new();
        let payload = TestPayload {
            data: "test data".to_string(),
        };
        let signature = key_bundle.sign(&payload);

        let key_bundle = SignatureKeyBundle::new();
        assert!(key_bundle.verify(&payload, &signature).is_err());
    }

    #[test]
    fn test_sign_bytes_and_verify_bytes() {
        let key_bundle = SignatureKeyBundle::new();
        let payload = b"test bytes";

        let signature = key_bundle.sign_bytes(payload);
        assert!(key_bundle.verify_bytes(payload, &signature).is_ok());
    }

    #[test]
    fn test_verify_bytes_with_invalid_signature() {
        let key_bundle = SignatureKeyBundle::new();
        let payload = b"test bytes";

        let signature = key_bundle.sign_bytes(payload);

        let invalid_payload = b"tampered bytes";

        assert!(
            key_bundle
                .verify_bytes(invalid_payload, &signature)
                .is_err()
        );
    }
}