use std::collections::HashMap;
use std::io::Write;
use std::path::{Path, PathBuf};
use std::sync::RwLock;

use base64::{engine::general_purpose::STANDARD as BASE64_STANDARD, Engine as _};
use hmac::{Hmac, Mac};
use pbkdf2::pbkdf2_hmac;
use rand::{rngs::OsRng, RngCore};
use serde::{Deserialize, Serialize};
use sha2::{Digest, Sha256};
use subtle::ConstantTimeEq;

use rustdb_config::{AuthOptions, AuthUserOptions};

type HmacSha256 = Hmac<Sha256>;

const SCRAM_SHA_256: &str = "SCRAM-SHA-256";

#[derive(Debug, thiserror::Error)]
pub enum AuthError {
    #[error("authentication is disabled")]
    Disabled,
    #[error("unsupported authentication mechanism: {0}")]
    UnsupportedMechanism(String),
    #[error("invalid SCRAM payload: {0}")]
    InvalidPayload(String),
    #[error("authentication failed")]
    AuthenticationFailed,
    #[error("unknown SASL conversation")]
    UnknownConversation,
    #[error("user already exists: {0}")]
    UserAlreadyExists(String),
    #[error("user not found: {0}")]
    UserNotFound(String),
    #[error("auth metadata persistence failed: {0}")]
    Persistence(String),
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AuthAction {
    Read,
    Write,
    DbAdmin,
    UserAdmin,
    ClusterMonitor,
}

#[derive(Debug, Clone)]
pub struct AuthenticatedUser {
    pub username: String,
    pub database: String,
    pub roles: Vec<String>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
struct ScramCredential {
    salt: Vec<u8>,
    iterations: u32,
    stored_key: Vec<u8>,
    server_key: Vec<u8>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
struct AuthUser {
    username: String,
    database: String,
    roles: Vec<String>,
    scram_sha256: ScramCredential,
}

#[derive(Debug, Clone, Serialize, Deserialize, Default)]
struct PersistedAuthState {
    users: Vec<AuthUser>,
}

#[derive(Debug, Clone)]
pub struct ScramConversation {
    user: AuthenticatedUser,
    client_first_bare: String,
    server_first: String,
    nonce: String,
    stored_key: Vec<u8>,
    server_key: Vec<u8>,
}

#[derive(Debug, Clone)]
pub struct ScramStartResult {
    pub payload: Vec<u8>,
    pub conversation: ScramConversation,
}

#[derive(Debug, Clone)]
pub struct ScramContinueResult {
    pub payload: Vec<u8>,
    pub user: AuthenticatedUser,
}

#[derive(Debug)]
pub struct AuthEngine {
    enabled: bool,
    users: RwLock<HashMap<String, AuthUser>>,
    users_path: Option<PathBuf>,
    scram_iterations: u32,
}

impl AuthEngine {
    pub fn from_options(options: &AuthOptions) -> Result<Self, AuthError> {
        let users_path = options.users_path.as_ref().map(PathBuf::from);
        let mut users = if let Some(ref path) = users_path {
            load_users(path)?
        } else {
            HashMap::new()
        };

        let mut changed = false;
        for user_options in &options.users {
            let key = user_key(&user_options.database, &user_options.username);
            if !users.contains_key(&key) {
                let user = AuthUser::from_options(user_options, options.scram_iterations);
                users.insert(key, user);
                changed = true;
            }
        }

        if changed {
            if let Some(ref path) = users_path {
                persist_users(path, &users)?;
            }
        }

        Ok(Self {
            enabled: options.enabled,
            users: RwLock::new(users),
            users_path,
            scram_iterations: options.scram_iterations,
        })
    }

    pub fn disabled() -> Self {
        Self {
            enabled: false,
            users: RwLock::new(HashMap::new()),
            users_path: None,
            scram_iterations: 15000,
        }
    }

    pub fn enabled(&self) -> bool {
        self.enabled
    }

    pub fn user_count(&self) -> usize {
        self.users
            .read()
            .unwrap_or_else(|poisoned| poisoned.into_inner())
            .len()
    }

    pub fn supported_mechanisms(&self, namespace_user: &str) -> Vec<String> {
        let Some((database, username)) = namespace_user.split_once('.') else {
            return Vec::new();
        };
        let users = self.users.read().unwrap_or_else(|poisoned| poisoned.into_inner());
        if users.contains_key(&user_key(database, username)) {
            vec![SCRAM_SHA_256.to_string()]
        } else {
            Vec::new()
        }
    }

    pub fn is_authorized(
        &self,
        authenticated_users: &[AuthenticatedUser],
        target_db: &str,
        action: AuthAction,
    ) -> bool {
        authenticated_users
            .iter()
            .any(|user| user.roles.iter().any(|role| role_allows(role, user, target_db, action)))
    }

    pub fn create_user(
        &self,
        database: &str,
        username: &str,
        password: &str,
        roles: Vec<String>,
    ) -> Result<(), AuthError> {
        let key = user_key(database, username);
        let mut users = self.users.write().unwrap_or_else(|poisoned| poisoned.into_inner());
        if users.contains_key(&key) {
            return Err(AuthError::UserAlreadyExists(format!("{database}.{username}")));
        }
        let options = AuthUserOptions {
            username: username.to_string(),
            password: password.to_string(),
            database: database.to_string(),
            roles,
        };
        users.insert(key, AuthUser::from_options(&options, self.scram_iterations));
        self.persist_locked(&users)
    }

    pub fn drop_user(&self, database: &str, username: &str) -> Result<(), AuthError> {
        let key = user_key(database, username);
        let mut users = self.users.write().unwrap_or_else(|poisoned| poisoned.into_inner());
        if users.remove(&key).is_none() {
            return Err(AuthError::UserNotFound(format!("{database}.{username}")));
        }
        self.persist_locked(&users)
    }

    pub fn update_user(
        &self,
        database: &str,
        username: &str,
        password: Option<&str>,
        roles: Option<Vec<String>>,
    ) -> Result<(), AuthError> {
        let key = user_key(database, username);
        let mut users = self.users.write().unwrap_or_else(|poisoned| poisoned.into_inner());
        let user = users
            .get_mut(&key)
            .ok_or_else(|| AuthError::UserNotFound(format!("{database}.{username}")))?;
        if let Some(new_roles) = roles {
            user.roles = new_roles;
        }
        if let Some(new_password) = password {
            let options = AuthUserOptions {
                username: username.to_string(),
                password: new_password.to_string(),
                database: database.to_string(),
                roles: user.roles.clone(),
            };
            user.scram_sha256 = AuthUser::from_options(&options, self.scram_iterations).scram_sha256;
        }
        self.persist_locked(&users)
    }

    pub fn grant_roles(
        &self,
        database: &str,
        username: &str,
        roles: Vec<String>,
    ) -> Result<(), AuthError> {
        let key = user_key(database, username);
        let mut users = self.users.write().unwrap_or_else(|poisoned| poisoned.into_inner());
        let user = users
            .get_mut(&key)
            .ok_or_else(|| AuthError::UserNotFound(format!("{database}.{username}")))?;
        for role in roles {
            if !user.roles.contains(&role) {
                user.roles.push(role);
            }
        }
        self.persist_locked(&users)
    }

    pub fn revoke_roles(
        &self,
        database: &str,
        username: &str,
        roles: Vec<String>,
    ) -> Result<(), AuthError> {
        let key = user_key(database, username);
        let mut users = self.users.write().unwrap_or_else(|poisoned| poisoned.into_inner());
        let user = users
            .get_mut(&key)
            .ok_or_else(|| AuthError::UserNotFound(format!("{database}.{username}")))?;
        user.roles.retain(|role| !roles.contains(role));
        self.persist_locked(&users)
    }

    pub fn users_info(&self, database: &str, username: Option<&str>) -> Vec<AuthenticatedUser> {
        let users = self.users.read().unwrap_or_else(|poisoned| poisoned.into_inner());
        users
            .values()
            .filter(|user| user.database == database)
            .filter(|user| username.map(|name| user.username == name).unwrap_or(true))
            .map(AuthUser::to_authenticated_user)
            .collect()
    }

    pub fn list_users(&self) -> Vec<AuthenticatedUser> {
        let users = self.users.read().unwrap_or_else(|poisoned| poisoned.into_inner());
        let mut result: Vec<AuthenticatedUser> = users
            .values()
            .map(AuthUser::to_authenticated_user)
            .collect();
        result.sort_by(|a, b| a.database.cmp(&b.database).then(a.username.cmp(&b.username)));
        result
    }

    pub fn drop_users_for_database(&self, database: &str) -> Result<usize, AuthError> {
        let mut users = self.users.write().unwrap_or_else(|poisoned| poisoned.into_inner());
        let before = users.len();
        users.retain(|_, user| user.database != database);
        let dropped = before.saturating_sub(users.len());
        if dropped > 0 {
            self.persist_locked(&users)?;
        }
        Ok(dropped)
    }

    pub fn start_scram_sha256(
        &self,
        database: &str,
        payload: &[u8],
    ) -> Result<ScramStartResult, AuthError> {
        if !self.enabled {
            return Err(AuthError::Disabled);
        }

        let message = std::str::from_utf8(payload)
            .map_err(|_| AuthError::InvalidPayload("payload is not valid UTF-8".to_string()))?;
        let client_first_bare = message
            .strip_prefix("n,,")
            .ok_or_else(|| AuthError::InvalidPayload("expected SCRAM gs2 header 'n,,'".to_string()))?;
        let attrs = parse_scram_attrs(client_first_bare);
        let raw_username = attrs
            .get("n")
            .ok_or_else(|| AuthError::InvalidPayload("missing username".to_string()))?;
        let username = decode_scram_name(raw_username);
        let client_nonce = attrs
            .get("r")
            .ok_or_else(|| AuthError::InvalidPayload("missing client nonce".to_string()))?;

        let users = self.users.read().unwrap_or_else(|poisoned| poisoned.into_inner());
        let user = users
            .get(&user_key(database, &username))
            .ok_or(AuthError::AuthenticationFailed)?;

        let nonce = format!("{}{}", client_nonce, secure_base64(18));
        let server_first = format!(
            "r={},s={},i={}",
            nonce,
            BASE64_STANDARD.encode(&user.scram_sha256.salt),
            user.scram_sha256.iterations,
        );

        Ok(ScramStartResult {
            payload: server_first.as_bytes().to_vec(),
            conversation: ScramConversation {
                user: user.to_authenticated_user(),
                client_first_bare: client_first_bare.to_string(),
                server_first: server_first.clone(),
                nonce,
                stored_key: user.scram_sha256.stored_key.clone(),
                server_key: user.scram_sha256.server_key.clone(),
            },
        })
    }

    pub fn continue_scram_sha256(
        &self,
        conversation: ScramConversation,
        payload: &[u8],
    ) -> Result<ScramContinueResult, AuthError> {
        let message = std::str::from_utf8(payload)
            .map_err(|_| AuthError::InvalidPayload("payload is not valid UTF-8".to_string()))?;
        let proof_marker = ",p=";
        let proof_pos = message
            .rfind(proof_marker)
            .ok_or_else(|| AuthError::InvalidPayload("missing client proof".to_string()))?;
        let client_final_without_proof = &message[..proof_pos];
        let proof_b64 = &message[proof_pos + proof_marker.len()..];
        let attrs = parse_scram_attrs(client_final_without_proof);
        let nonce = attrs
            .get("r")
            .ok_or_else(|| AuthError::InvalidPayload("missing nonce".to_string()))?;
        if nonce != &conversation.nonce {
            return Err(AuthError::AuthenticationFailed);
        }

        let client_proof = BASE64_STANDARD
            .decode(proof_b64.as_bytes())
            .map_err(|_| AuthError::InvalidPayload("invalid client proof encoding".to_string()))?;
        if client_proof.len() != 32 || conversation.stored_key.len() != 32 {
            return Err(AuthError::AuthenticationFailed);
        }

        let auth_message = format!(
            "{},{},{}",
            conversation.client_first_bare,
            conversation.server_first,
            client_final_without_proof,
        );
        let client_signature = hmac_sha256(&conversation.stored_key, auth_message.as_bytes());
        let client_key: Vec<u8> = client_proof
            .iter()
            .zip(client_signature.iter())
            .map(|(proof_byte, signature_byte)| proof_byte ^ signature_byte)
            .collect();
        let computed_stored_key = Sha256::digest(&client_key).to_vec();

        if computed_stored_key.ct_eq(&conversation.stored_key).unwrap_u8() != 1 {
            return Err(AuthError::AuthenticationFailed);
        }

        let server_signature = hmac_sha256(&conversation.server_key, auth_message.as_bytes());
        let server_final = format!("v={}", BASE64_STANDARD.encode(server_signature));

        Ok(ScramContinueResult {
            payload: server_final.as_bytes().to_vec(),
            user: conversation.user,
        })
    }

    fn persist_locked(&self, users: &HashMap<String, AuthUser>) -> Result<(), AuthError> {
        if let Some(ref path) = self.users_path {
            persist_users(path, users)?;
        }
        Ok(())
    }
}

impl Default for AuthEngine {
    fn default() -> Self {
        Self::disabled()
    }
}

impl AuthUser {
    fn from_options(options: &AuthUserOptions, iterations: u32) -> Self {
        let salt = secure_random(24);
        let salted_password = salted_password(options.password.as_bytes(), &salt, iterations);
        let client_key = hmac_sha256(&salted_password, b"Client Key");
        let stored_key = Sha256::digest(&client_key).to_vec();
        let server_key = hmac_sha256(&salted_password, b"Server Key");

        Self {
            username: options.username.clone(),
            database: options.database.clone(),
            roles: options.roles.clone(),
            scram_sha256: ScramCredential {
                salt,
                iterations,
                stored_key,
                server_key,
            },
        }
    }

    fn to_authenticated_user(&self) -> AuthenticatedUser {
        AuthenticatedUser {
            username: self.username.clone(),
            database: self.database.clone(),
            roles: self.roles.clone(),
        }
    }
}

fn role_allows(role: &str, user: &AuthenticatedUser, target_db: &str, action: AuthAction) -> bool {
    let (role_db, role_name) = role.split_once('.').unwrap_or(("", role));
    if role_name == "root" {
        return true;
    }

    let any_database = role_name.ends_with("AnyDatabase");
    let scoped_db = if role_db.is_empty() { &user.database } else { role_db };
    if !any_database && scoped_db != target_db {
        return false;
    }

    match role_name {
        "read" | "readAnyDatabase" => action == AuthAction::Read,
        "readWrite" | "readWriteAnyDatabase" => {
            matches!(action, AuthAction::Read | AuthAction::Write)
        }
        "dbAdmin" | "dbAdminAnyDatabase" => action == AuthAction::DbAdmin,
        "userAdmin" | "userAdminAnyDatabase" => action == AuthAction::UserAdmin,
        "clusterMonitor" => action == AuthAction::ClusterMonitor,
        _ => false,
    }
}

fn load_users(path: &Path) -> Result<HashMap<String, AuthUser>, AuthError> {
    if !path.exists() {
        return Ok(HashMap::new());
    }
    let data = std::fs::read_to_string(path).map_err(|e| AuthError::Persistence(e.to_string()))?;
    let persisted: PersistedAuthState = serde_json::from_str(&data)
        .map_err(|e| AuthError::Persistence(format!("failed to parse users file: {e}")))?;
    Ok(persisted
        .users
        .into_iter()
        .map(|user| (user_key(&user.database, &user.username), user))
        .collect())
}

fn persist_users(path: &Path, users: &HashMap<String, AuthUser>) -> Result<(), AuthError> {
    if let Some(parent) = path.parent() {
        std::fs::create_dir_all(parent).map_err(|e| AuthError::Persistence(e.to_string()))?;
    }

    let mut user_list: Vec<AuthUser> = users.values().cloned().collect();
    user_list.sort_by(|a, b| a.database.cmp(&b.database).then(a.username.cmp(&b.username)));
    let payload = serde_json::to_vec_pretty(&PersistedAuthState { users: user_list })
        .map_err(|e| AuthError::Persistence(e.to_string()))?;

    let tmp_path = path.with_extension("tmp");
    {
        let mut file = std::fs::File::create(&tmp_path)
            .map_err(|e| AuthError::Persistence(e.to_string()))?;
        file.write_all(&payload)
            .map_err(|e| AuthError::Persistence(e.to_string()))?;
        file.sync_all()
            .map_err(|e| AuthError::Persistence(e.to_string()))?;
    }
    std::fs::rename(&tmp_path, path).map_err(|e| AuthError::Persistence(e.to_string()))?;
    if let Some(parent) = path.parent() {
        if let Ok(dir) = std::fs::File::open(parent) {
            let _ = dir.sync_all();
        }
    }
    Ok(())
}

fn user_key(database: &str, username: &str) -> String {
    format!("{}\0{}", database, username)
}

fn salted_password(password: &[u8], salt: &[u8], iterations: u32) -> Vec<u8> {
    let mut output = [0u8; 32];
    pbkdf2_hmac::<Sha256>(password, salt, iterations, &mut output);
    output.to_vec()
}

fn hmac_sha256(key: &[u8], message: &[u8]) -> Vec<u8> {
    let mut mac = HmacSha256::new_from_slice(key).expect("HMAC-SHA256 accepts keys of any size");
    mac.update(message);
    mac.finalize().into_bytes().to_vec()
}

fn secure_random(len: usize) -> Vec<u8> {
    let mut bytes = vec![0u8; len];
    OsRng.fill_bytes(&mut bytes);
    bytes
}

fn secure_base64(len: usize) -> String {
    BASE64_STANDARD.encode(secure_random(len))
}

fn parse_scram_attrs(input: &str) -> HashMap<String, String> {
    let mut result = HashMap::new();
    for part in input.split(',') {
        if let Some((key, value)) = part.split_once('=') {
            result.insert(key.to_string(), value.to_string());
        }
    }
    result
}

fn decode_scram_name(input: &str) -> String {
    input.replace("=2C", ",").replace("=3D", "=")
}

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

    #[test]
    fn mechanism_lookup_returns_scram_sha256() {
        let options = AuthOptions {
            enabled: true,
            users: vec![AuthUserOptions {
                username: "root".to_string(),
                password: "secret".to_string(),
                database: "admin".to_string(),
                roles: vec!["root".to_string()],
            }],
            users_path: None,
            scram_iterations: 4096,
        };
        let engine = AuthEngine::from_options(&options).unwrap();
        assert_eq!(engine.supported_mechanisms("admin.root"), vec![SCRAM_SHA_256.to_string()]);
    }

    #[test]
    fn read_write_role_allows_read_and_write_only_on_own_db() {
        let user = AuthenticatedUser {
            username: "app".to_string(),
            database: "appdb".to_string(),
            roles: vec!["readWrite".to_string()],
        };
        assert!(role_allows("readWrite", &user, "appdb", AuthAction::Read));
        assert!(role_allows("readWrite", &user, "appdb", AuthAction::Write));
        assert!(!role_allows("readWrite", &user, "other", AuthAction::Read));
        assert!(!role_allows("readWrite", &user, "appdb", AuthAction::DbAdmin));
    }
}