use axum::http::HeaderMap;
use base64::{Engine, engine::general_purpose::URL_SAFE};
use ed25519_dalek::{Signature, Verifier, VerifyingKey};
use regex::Regex;
use std::{env, sync::LazyLock};
use thiserror::Error;

use crate::util::now_millis;

pub const TOKEN_MAX_DURATION: i64 = 86_400 * 1_000;
const BEARER_PREFIX: &str = "Bearer ";

static TOKEN_RE: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"^([a-fA-F0-9]+)\|([a-fA-F0-9]+)\|([a-fA-F0-9]+)$").unwrap());

#[derive(Debug, Error)]
pub enum AuthError {
    #[error("not a valid token")]
    InvalidToken,
    #[error("invalid token format")]
    InvalidTokenFormat,
    #[error("token is expired")]
    Expired,
    #[error("signature verification failed")]
    Signature,
    #[error("this client ID is blocked")]
    Blocked,
    #[error("{0}")]
    Decode(String),
}

pub fn authenticate(token: &str) -> Result<String, AuthError> {
    let decoded = URL_SAFE
        .decode(token)
        .map_err(|err| AuthError::Decode(format!("error doing base64 decoding: {err}")))?;
    let decoded = std::str::from_utf8(&decoded)
        .map_err(|err| AuthError::Decode(format!("error decoding token as utf8: {err}")))?;
    let captures = TOKEN_RE
        .captures(decoded)
        .ok_or(AuthError::InvalidTokenFormat)?;

    let timestamp_hex = &captures[1];
    let signed_timestamp_hex = &captures[2];
    let public_key_hex = &captures[3];

    let public_key = hex::decode(public_key_hex)
        .map_err(|err| AuthError::Decode(format!("error decoding hex string: {err}")))?;
    let timestamp_bytes = hex::decode(timestamp_hex)
        .map_err(|err| AuthError::Decode(format!("error decoding hex string: {err}")))?;
    let signature_bytes = hex::decode(signed_timestamp_hex)
        .map_err(|err| AuthError::Decode(format!("error decoding hex string: {err}")))?;

    let key = VerifyingKey::from_bytes(
        public_key
            .as_slice()
            .try_into()
            .map_err(|_| AuthError::InvalidTokenFormat)?,
    )
    .map_err(|_| AuthError::InvalidTokenFormat)?;
    let signature =
        Signature::from_slice(&signature_bytes).map_err(|_| AuthError::InvalidTokenFormat)?;
    key.verify(&timestamp_bytes, &signature)
        .map_err(|_| AuthError::Signature)?;

    let timestamp = std::str::from_utf8(&timestamp_bytes)
        .map_err(|err| AuthError::Decode(format!("error parsing timestamp: {err}")))?
        .parse::<i64>()
        .map_err(|err| AuthError::Decode(format!("error parsing timestamp: {err}")))?;
    if (now_millis() - timestamp).abs() > TOKEN_MAX_DURATION {
        return Err(AuthError::Expired);
    }

    let public_key_lc = public_key_hex.to_ascii_lowercase();
    let blocked = env::var("BLOCKED_CLIENT_IDS").unwrap_or_default();
    if blocked
        .split(',')
        .any(|id| id.trim().eq_ignore_ascii_case(&public_key_lc))
    {
        return Err(AuthError::Blocked);
    }

    Ok(public_key_lc)
}

pub fn authorize(headers: &HeaderMap) -> Result<String, AuthError> {
    let token = headers
        .get("authorization")
        .and_then(|value| value.to_str().ok())
        .ok_or(AuthError::InvalidToken)?;
    let token = token
        .strip_prefix(BEARER_PREFIX)
        .ok_or(AuthError::InvalidToken)?;
    if token.is_empty() {
        return Err(AuthError::InvalidToken);
    }
    authenticate(token).map_err(|err| AuthError::Decode(format!("error authorizing: {err}")))
}

#[cfg(test)]
mod tests {
    use super::*;
    use base64::engine::general_purpose::URL_SAFE;
    use ed25519_dalek::{Signer, SigningKey};

    fn generate_token(timestamp: i64) -> (String, String) {
        let key = SigningKey::from_bytes(&[7u8; 32]);
        let timestamp_bytes = timestamp.to_string().into_bytes();
        let signature = key.sign(&timestamp_bytes);
        let public_key_hex = hex::encode(key.verifying_key().to_bytes());
        let token_hex = format!(
            "{}|{}|{}",
            hex::encode(timestamp_bytes),
            hex::encode(signature.to_bytes()),
            public_key_hex
        );
        (URL_SAFE.encode(token_hex), public_key_hex)
    }

    #[test]
    fn authenticate_accepts_valid_token() {
        let (token, public_key_hex) = generate_token(now_millis());
        let client_id = authenticate(&token).expect("token should authenticate");
        assert_eq!(client_id, public_key_hex);
    }

    #[test]
    fn authenticate_rejects_expired_token() {
        let (token, _) = generate_token(now_millis() - TOKEN_MAX_DURATION - 1);
        assert!(matches!(authenticate(&token), Err(AuthError::Expired)));
    }
}