// TRACES: FR-NC-12 | FR-NC-13
//! The one place the interface names a backend.
//!
//! `dr-sync` defines [`RemoteBackend`] and a capability model the engine adapts
//! to, so that a second backend can be added without touching the code that
//! uses one (ARCH §8.1). Until this module existed that boundary was
//! documentation: seven files in `dr-ui` constructed a `NextcloudBackend`
//! directly and ten functions took one by concrete type, so the abstraction
//! bought nothing it was designed for and a WebDAV or local-folder backend
//! would have had nowhere to go.
//!
//! Everything above this module works through `&dyn RemoteBackend`, and every
//! account it opens is a [`dr_sync::Account`] — configuration with no server
//! in it. Adding a backend is implementing two traits and adding a line to
//! [`registry`]; nothing else in `dr-ui` changes. See `docs/storage.md`.
//!
//! # Why the registry is built here and not in `dr-sync`
//!
//! `dr-sync` must not depend on any connector, or the engine would drag a TLS
//! stack into a build that only wanted a folder. So the crate that already
//! depends on all of them — the interface — is where the list lives. It is
//! the only file in the application that names one.

use std::sync::{Arc, OnceLock};

use dr_sync::{Account, BackendProvider, BackendRegistry, Connection, RemoteBackend, RemoteError};
use dr_sync_folder::FolderProvider;
use dr_sync_nextcloud::{NextcloudProvider, NextcloudVfs};

/// Every storage backend this build has, in the order the launch screen
/// offers them.
///
/// Built once. A provider is stateless — it holds no connection and no
/// credential — so one instance serves every thread that asks.
pub fn registry() -> &'static BackendRegistry {
    static REGISTRY: OnceLock<BackendRegistry> = OnceLock::new();
    REGISTRY.get_or_init(|| {
        let mut r = BackendRegistry::new();
        r.register(Arc::new(NextcloudProvider));
        // TRACES: FR-NC-6c
        // The folder connector does the filesystem work and knows nothing
        // about sync clients; the placeholder convention is supplied here,
        // which is the one place that may name one. Detection is per folder
        // and per connection — the same directory offers hydration while the
        // client is running and not while it is down (ARCH §9.0).
        r.register(Arc::new(FolderProvider::with_vfs_detector(|root| {
            NextcloudVfs::looks_synced(root)
                .then(|| Arc::new(NextcloudVfs::detect()) as Arc<dyn dr_sync_folder::Vfs>)
        })));
        r
    })
}

/// The connector serving an account.
///
/// Errors when this build has none — a configuration file outlives the binary
/// that wrote it, and saying *which* backend is missing beats "could not open
/// library".
pub fn provider_for(account: &Account) -> Result<&'static Arc<dyn BackendProvider>, RemoteError> {
    registry().for_account(account)
}

/// Whether an account's credential has to be fetched from secure storage.
///
/// Asked of the connector rather than inferred from the account, because an
/// empty login might be a folder library or might be a damaged record, and
/// guessing turns the second into a silent unauthenticated connection.
pub fn needs_secret(account: &Account) -> bool {
    provider_for(account).is_ok_and(|p| p.sign_in().needs_secret())
}

/// Open a connection to a configured remote.
///
/// Returns the trait object every caller should hold. The error type is
/// `dr-sync`'s rather than a connector's, so a caller handles a failure
/// without learning which backend produced it.
pub(crate) fn connect(conn: &Connection) -> Result<Box<dyn RemoteBackend>, RemoteError> {
    registry().connect(conn)
}

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

    #[test]
    fn both_backends_are_registered() {
        // The list the launch screen offers. A backend missing from here is a
        // backend the user cannot choose, however complete its connector is.
        let ids: Vec<&str> = registry().providers().iter().map(|p| p.id()).collect();
        assert!(ids.contains(&"nextcloud"), "{ids:?}");
        assert!(ids.contains(&"folder"), "{ids:?}");
    }

    #[test]
    fn only_the_backend_with_a_login_wants_a_credential() {
        assert!(needs_secret(&Account::new("nextcloud", "https://x")));
        assert!(!needs_secret(&Account::new("folder", "/mnt/photos")));
    }

    #[test]
    fn an_account_for_an_unknown_backend_names_it() {
        let e = provider_for(&Account::new("s3", "bucket"))
            .err()
            .expect("no such connector")
            .to_string();
        assert!(e.contains("s3"), "{e}");
    }
}