//! Platform secure storage for credentials (FR-NC-2, NFR-SEC-2).
//!
//! Credentials are **never** written to the catalog, to a plain file, or to
//! logs. On Linux they go to the Secret Service (GNOME Keyring, or KWallet via
//! `ksecretd`, which exposes the same D-Bus interface). On Android they belong
//! in Keystore-backed storage.
//!
//! Absence of a secrets daemon is an explicit degraded mode, not a silent
//! fallback to plaintext: a headless box or a minimal window manager may have
//! none, and quietly writing a password to disk there would be worse than
//! refusing.

use std::fmt;

/// Which secret is being stored, so one account can hold several.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SecretKind {
    /// A Nextcloud app password from Login Flow v2. Device-scoped and
    /// individually revocable — never the user's actual password.
    AppPassword,
}

impl SecretKind {
    fn as_str(self) -> &'static str {
        match self {
            SecretKind::AppPassword => "app-password",
        }
    }
}

/// Where a credential lives: one account on one server.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct SecretRef {
    pub server: String,
    pub login: String,
    pub kind: SecretKind,
}

impl SecretRef {
    pub fn app_password(server: impl Into<String>, login: impl Into<String>) -> Self {
        Self {
            server: server.into(),
            login: login.into(),
            kind: SecretKind::AppPassword,
        }
    }

    /// The key under which the platform store holds this secret.
    ///
    /// Includes the server so two accounts on different servers with the same
    /// login do not collide.
    fn entry_key(&self) -> String {
        format!("{}@{}#{}", self.login, self.server, self.kind.as_str())
    }
}

/// Deliberately opaque: the whole point is that a credential never appears in
/// a log line or an error message (NFR-SEC-2).
impl fmt::Display for SecretRef {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{} on {}", self.login, self.server)
    }
}

#[derive(Debug, thiserror::Error)]
pub enum SecretError {
    /// No secrets daemon. The app runs in a degraded mode where the user
    /// re-authenticates each session, rather than storing anything in plain.
    #[error("no platform secret store available: {0}")]
    Unavailable(String),

    #[error("secret not found")]
    NotFound,

    #[error("secret store rejected the request: {0}")]
    Denied(String),

    #[error("secret store error: {0}")]
    Other(String),
}

/// TRACES: FR-NC-2 | NFR-SEC-2 | M-2
/// Store, retrieve and delete credentials.
///
/// Implemented per platform and injected at construction, so `core/` contains
/// no `#[cfg(target_os)]` (NFR-PORT-1).
pub trait SecretStore: Send + Sync {
    fn store(&self, secret_ref: &SecretRef, secret: &str) -> Result<(), SecretError>;
    fn retrieve(&self, secret_ref: &SecretRef) -> Result<String, SecretError>;
    fn delete(&self, secret_ref: &SecretRef) -> Result<(), SecretError>;

    /// Whether the store is usable right now.
    ///
    /// Checked before offering to remember a login, so the UI can say
    /// "you will need to sign in each time" rather than failing later.
    fn is_available(&self) -> bool;
}

/// The service name entries are filed under.
const SERVICE: &str = "DarkRoom";

/// Secret Service implementation (GNOME Keyring, KWallet via ksecretd).
#[cfg(all(unix, not(target_os = "android")))]
pub struct PlatformSecretStore;

#[cfg(all(unix, not(target_os = "android")))]
impl PlatformSecretStore {
    pub fn new() -> Self {
        Self
    }

    fn entry(r: &SecretRef) -> Result<keyring::Entry, SecretError> {
        keyring::Entry::new(SERVICE, &r.entry_key()).map_err(map_err)
    }
}

#[cfg(all(unix, not(target_os = "android")))]
impl Default for PlatformSecretStore {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(all(unix, not(target_os = "android")))]
impl SecretStore for PlatformSecretStore {
    fn store(&self, secret_ref: &SecretRef, secret: &str) -> Result<(), SecretError> {
        Self::entry(secret_ref)?
            .set_password(secret)
            .map_err(map_err)
    }

    fn retrieve(&self, secret_ref: &SecretRef) -> Result<String, SecretError> {
        Self::entry(secret_ref)?.get_password().map_err(map_err)
    }

    fn delete(&self, secret_ref: &SecretRef) -> Result<(), SecretError> {
        match Self::entry(secret_ref)?.delete_credential() {
            Ok(()) => Ok(()),
            // Deleting an absent secret is the desired end state, not a
            // failure — logout must be idempotent.
            Err(keyring::Error::NoEntry) => Ok(()),
            Err(e) => Err(map_err(e)),
        }
    }

    fn is_available(&self) -> bool {
        // Probing a name that will not exist distinguishes "daemon absent"
        // from "secret absent": the former errors, the latter reports NoEntry.
        match keyring::Entry::new(SERVICE, "__availability_probe__") {
            Ok(e) => !matches!(
                e.get_password(),
                Err(keyring::Error::PlatformFailure(_)) | Err(keyring::Error::NoStorageAccess(_))
            ),
            Err(_) => false,
        }
    }
}

#[cfg(all(unix, not(target_os = "android")))]
fn map_err(e: keyring::Error) -> SecretError {
    match e {
        keyring::Error::NoEntry => SecretError::NotFound,
        keyring::Error::NoStorageAccess(e) => SecretError::Unavailable(e.to_string()),
        keyring::Error::PlatformFailure(e) => SecretError::Unavailable(e.to_string()),
        other => SecretError::Other(other.to_string()),
    }
}

/// Keystore-backed implementation (FR-PLAT-AND-1).
///
/// `android-native-keyring-store` encrypts each secret with an AES-GCM key
/// held in `AndroidKeyStore` and files the ciphertext in SharedPreferences.
/// The key never leaves the Keystore, so the preferences file is useless on
/// its own. This is the current approach rather than the deprecated
/// `EncryptedSharedPreferences` (REQ §11).
///
/// It finds the JavaVM and Context through `ndk-context`, which
/// `android-activity` initialises before `android_main` is called. Nothing
/// here is usable before that point — hence the lazy handle below.
#[cfg(target_os = "android")]
pub struct PlatformSecretStore {
    /// Built on first use, not in `new()`: construction needs the ndk-context
    /// to be live, and `new()` may run early. Cached because store names are
    /// unique — building one per call would fail on the second call.
    store: std::sync::OnceLock<Result<std::sync::Arc<android_native_keyring_store::Store>, String>>,
}

#[cfg(target_os = "android")]
impl PlatformSecretStore {
    pub fn new() -> Self {
        Self {
            store: std::sync::OnceLock::new(),
        }
    }

    fn store(&self) -> Result<&std::sync::Arc<android_native_keyring_store::Store>, SecretError> {
        self.store
            .get_or_init(|| android_native_keyring_store::Store::new().map_err(|e| e.to_string()))
            .as_ref()
            .map_err(|e| SecretError::Unavailable(e.clone()))
    }

    /// A credential specifier for one secret. Filed under the same
    /// service/key pair as the Linux path, so the two platforms agree on
    /// naming even though the backing stores differ.
    fn entry(&self, r: &SecretRef) -> Result<keyring_core::Entry, SecretError> {
        use keyring_core::api::CredentialStoreApi;

        self.store()?
            .build(SERVICE, &r.entry_key(), None)
            .map_err(map_err)
    }
}

#[cfg(target_os = "android")]
impl Default for PlatformSecretStore {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(target_os = "android")]
impl SecretStore for PlatformSecretStore {
    fn store(&self, secret_ref: &SecretRef, secret: &str) -> Result<(), SecretError> {
        self.entry(secret_ref)?
            .set_password(secret)
            .map_err(map_err)
    }

    fn retrieve(&self, secret_ref: &SecretRef) -> Result<String, SecretError> {
        self.entry(secret_ref)?.get_password().map_err(map_err)
    }

    fn delete(&self, secret_ref: &SecretRef) -> Result<(), SecretError> {
        match self.entry(secret_ref)?.delete_credential() {
            Ok(()) => Ok(()),
            // Logout must be idempotent, as on Linux.
            Err(keyring_core::Error::NoEntry) => Ok(()),
            Err(e) => Err(map_err(e)),
        }
    }

    fn is_available(&self) -> bool {
        // Unlike Linux there is no daemon to be absent: if the store builds,
        // Keystore is there. Building is the whole probe.
        self.store().is_ok()
    }
}

#[cfg(target_os = "android")]
fn map_err(e: keyring_core::Error) -> SecretError {
    match e {
        keyring_core::Error::NoEntry => SecretError::NotFound,
        keyring_core::Error::NoStorageAccess(e) => SecretError::Unavailable(e.to_string()),
        keyring_core::Error::PlatformFailure(e) => SecretError::Unavailable(e.to_string()),
        other => SecretError::Other(other.to_string()),
    }
}

/// Placeholder for platforms without an implementation yet.
///
/// Failing loudly is deliberate: a silent no-op store would look like it
/// worked and then lose the credential.
#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
pub struct PlatformSecretStore;

#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
impl PlatformSecretStore {
    pub fn new() -> Self {
        Self
    }
}

#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
impl Default for PlatformSecretStore {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(not(any(all(unix, not(target_os = "android")), target_os = "android")))]
impl SecretStore for PlatformSecretStore {
    fn store(&self, _r: &SecretRef, _s: &str) -> Result<(), SecretError> {
        Err(SecretError::Unavailable(
            "no secret store is implemented for this platform".into(),
        ))
    }
    fn retrieve(&self, _r: &SecretRef) -> Result<String, SecretError> {
        Err(SecretError::Unavailable(
            "no secret store is implemented for this platform".into(),
        ))
    }
    fn delete(&self, _r: &SecretRef) -> Result<(), SecretError> {
        Ok(())
    }
    fn is_available(&self) -> bool {
        false
    }
}

/// An in-memory store for tests and for the degraded no-daemon mode.
///
/// Credentials live only as long as the process, so a user without a secrets
/// daemon re-authenticates each session — which is the honest behaviour.
#[derive(Default)]
pub struct EphemeralSecretStore {
    entries: std::sync::Mutex<std::collections::HashMap<String, String>>,
}

impl EphemeralSecretStore {
    pub fn new() -> Self {
        Self::default()
    }
}

impl SecretStore for EphemeralSecretStore {
    fn store(&self, secret_ref: &SecretRef, secret: &str) -> Result<(), SecretError> {
        self.entries
            .lock()
            .map_err(|e| SecretError::Other(e.to_string()))?
            .insert(secret_ref.entry_key(), secret.to_string());
        Ok(())
    }

    fn retrieve(&self, secret_ref: &SecretRef) -> Result<String, SecretError> {
        self.entries
            .lock()
            .map_err(|e| SecretError::Other(e.to_string()))?
            .get(&secret_ref.entry_key())
            .cloned()
            .ok_or(SecretError::NotFound)
    }

    fn delete(&self, secret_ref: &SecretRef) -> Result<(), SecretError> {
        self.entries
            .lock()
            .map_err(|e| SecretError::Other(e.to_string()))?
            .remove(&secret_ref.entry_key());
        Ok(())
    }

    fn is_available(&self) -> bool {
        true
    }
}

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

    #[test]
    fn keys_separate_accounts_across_servers() {
        // Same login on two servers must not collide, or signing into the
        // second would overwrite the first.
        let a = SecretRef::app_password("https://a.example", "duncan");
        let b = SecretRef::app_password("https://b.example", "duncan");
        assert_ne!(a.entry_key(), b.entry_key());
    }

    #[test]
    fn keys_separate_logins_on_one_server() {
        let a = SecretRef::app_password("https://a.example", "duncan");
        let b = SecretRef::app_password("https://a.example", "someone");
        assert_ne!(a.entry_key(), b.entry_key());
    }

    #[test]
    fn display_never_reveals_the_secret_or_the_key() {
        let r = SecretRef::app_password("https://cloud.example", "duncan");
        let shown = r.to_string();
        assert!(shown.contains("duncan"));
        assert!(
            !shown.contains("app-password"),
            "internal key must not leak"
        );
    }

    #[test]
    fn ephemeral_round_trips() {
        let s = EphemeralSecretStore::new();
        let r = SecretRef::app_password("https://cloud.example", "duncan");

        assert!(matches!(s.retrieve(&r), Err(SecretError::NotFound)));
        s.store(&r, "token-value").unwrap();
        assert_eq!(s.retrieve(&r).unwrap(), "token-value");
    }

    #[test]
    fn deleting_is_idempotent() {
        // Logout must succeed whether or not a credential is present.
        let s = EphemeralSecretStore::new();
        let r = SecretRef::app_password("https://cloud.example", "duncan");
        assert!(s.delete(&r).is_ok());
        s.store(&r, "x").unwrap();
        assert!(s.delete(&r).is_ok());
        assert!(matches!(s.retrieve(&r), Err(SecretError::NotFound)));
    }

    #[test]
    fn storing_twice_overwrites() {
        let s = EphemeralSecretStore::new();
        let r = SecretRef::app_password("https://cloud.example", "duncan");
        s.store(&r, "first").unwrap();
        s.store(&r, "second").unwrap();
        assert_eq!(s.retrieve(&r).unwrap(), "second");
    }
}