use std::{array::TryFromSliceError, sync::LazyLock};

use axum::{
    extract::FromRequestParts,
    http::{request::Parts, StatusCode},
    response::IntoResponse,
    RequestPartsExt,
};
use axum_extra::{
    headers::{authorization::Bearer, Authorization},
    TypedHeader,
};
use jsonwebtoken::{decode, encode, DecodingKey, EncodingKey, Header, Validation};
use rand::{rngs::OsRng, RngCore};
use serde::{Deserialize, Serialize};
use subtle::ConstantTimeEq;

use crate::{db, Pool};

pub const HASH_LENGTH: usize = 64;
pub const SALT_LENGTH: usize = 64;

static PARAMS: LazyLock<scrypt::Params> =
    LazyLock::new(|| scrypt::Params::new(14, 8, 1, HASH_LENGTH).unwrap());
static KEYS: LazyLock<Keys> = LazyLock::new(|| {
    let secret = std::env::var("JWT_SECRET").expect("JWT_SECRET must be set");
    Keys::from_secret(secret.as_bytes())
});

struct Keys {
    encoding_key: EncodingKey,
    decoding_key: DecodingKey,
}

impl Keys {
    fn from_secret(secret: &[u8]) -> Self {
        Self {
            encoding_key: EncodingKey::from_secret(secret),
            decoding_key: DecodingKey::from_secret(secret),
        }
    }
}

/// Forces the evaluation of the keys. They will be created upon first use otherwise
pub fn force_init_keys() {
    LazyLock::force(&KEYS);
}

/// Hashes the bytes with Scrypt with the given salt
#[must_use]
fn hash_scrypt(bytes: &[u8], salt: &[u8]) -> [u8; HASH_LENGTH] {
    let mut hash = [0; HASH_LENGTH];
    scrypt::scrypt(bytes, salt, &PARAMS, &mut hash).unwrap();
    hash
}

/// Verifieble scrypt hashed bytes
pub struct HashedBytes {
    pub hash: [u8; HASH_LENGTH],
    pub salt: [u8; SALT_LENGTH],
}

impl HashedBytes {
    /// Hashes the bytes
    #[must_use]
    pub fn hash_bytes(bytes: &[u8]) -> Self {
        let mut salt = [0; 64];
        OsRng.fill_bytes(&mut salt);
        Self {
            hash: hash_scrypt(bytes, &salt),
            salt,
        }
    }

    /// Parses the bytes where the first `HASH_LENGTH` bytes are the hash and the latter `SALT_LENGTH` bytes are the salt
    pub fn from_bytes(bytes: &[u8]) -> Result<Self, TryFromSliceError> {
        let (hash, salt) = bytes.split_at(HASH_LENGTH);
        let result = Self {
            hash: hash.try_into()?,
            salt: salt.try_into()?,
        };
        Ok(result)
    }

    #[must_use]
    pub fn verify(&self, bytes: &[u8]) -> bool {
        let hash = hash_scrypt(bytes, self.salt.as_ref());
        hash.ct_eq(self.hash.as_ref()).into()
    }

    pub fn as_bytes(&self) -> Vec<u8> {
        let mut result = Vec::with_capacity(self.hash.len() + self.salt.len());
        result.extend_from_slice(&self.hash);
        result.extend_from_slice(&self.salt);
        result
    }
}

pub async fn authenticate_user(
    username: &str,
    password: &str,
    pool: &Pool,
) -> anyhow::Result<Option<i32>> {
    let Some((user_id, hash)) = db::users::get_hash(username, pool).await? else {
        return Ok(None);
    };
    let hash = HashedBytes::from_bytes(&hash)?;
    Ok(hash.verify(password.as_bytes()).then_some(user_id))
}

#[derive(Debug, Serialize)]
pub struct Token {
    access_token: String,
    token_type: &'static str,
}

#[derive(Serialize, Deserialize, Debug)]
pub struct Claims {
    pub user_id: i32,
    pub exp: i64,
}

impl Claims {
    pub fn encode(self) -> Result<Token, Error> {
        let access_token = encode(
            &Header::new(jsonwebtoken::Algorithm::HS256),
            &self,
            &KEYS.encoding_key,
        )
        .map_err(|_| Error::TokenCreation)?;
        let token = Token {
            access_token,
            token_type: "Bearer",
        };
        Ok(token)
    }
}

#[derive(Debug)]
pub enum Error {
    WrongCredentials,
    TokenCreation,
    InvalidToken,
}

impl IntoResponse for Error {
    fn into_response(self) -> axum::response::Response {
        let (status, error_message) = match self {
            Error::WrongCredentials => (StatusCode::UNAUTHORIZED, "Wrong credentials"),
            Error::TokenCreation => (StatusCode::INTERNAL_SERVER_ERROR, "Token creation error"),
            Error::InvalidToken => (StatusCode::BAD_REQUEST, "Invalid token"),
        };
        (status, error_message).into_response()
    }
}

#[axum::async_trait]
impl<T> FromRequestParts<T> for Claims {
    type Rejection = Error;

    async fn from_request_parts(parts: &mut Parts, _state: &T) -> Result<Self, Self::Rejection> {
        let TypedHeader(Authorization(bearer)) = parts
            .extract::<TypedHeader<Authorization<Bearer>>>()
            .await
            .map_err(|_| Error::InvalidToken)?;
        // Decode the user data
        let token_data =
            decode::<Claims>(bearer.token(), &KEYS.decoding_key, &Validation::default())
                .map_err(|_| Error::InvalidToken)?;

        Ok(token_data.claims)
    }
}