//! Login tokens in the OS keychain (cinny #105, step 1).
//!
//! Step 1 only MIRRORS the session tokens into the keychain (Windows
//! Credential Manager): the web client keeps reading its session from
//! localStorage exactly as before, so nothing about login changes and a
//! keychain problem can't log anyone out. Once the mirror has proven itself on
//! real installs, step 2 switches reads to the keychain and drops the tokens
//! from localStorage.
//!
//! Only the secrets are stored (user id + device id to match them to the
//! session, the access token and the refresh token); the rest of the session
//! stays in localStorage. Windows caps a credential at 2560 bytes, so the
//! serialized value is limited well below that.
//!
//! Other platforms: not supported yet (Linux Secret Service can prompt to
//! unlock a wallet at startup; that needs its own testing), and the commands
//! say so instead of failing.

use serde::{Deserialize, Serialize};

#[cfg_attr(not(any(target_os = "windows", test)), allow(dead_code))]
pub(crate) const SERVICE: &str = "Lotus Chat";
#[cfg_attr(not(any(target_os = "windows", test)), allow(dead_code))]
pub(crate) const ACCOUNT: &str = "session";
/// Serialized-length cap (chars). Windows' limit is 2560 bytes; staying under
/// 1200 chars keeps us safe whether the value is stored as UTF-8 or UTF-16.
pub(crate) const MAX_LEN: usize = 1200;

#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq)]
#[serde(rename_all = "camelCase")]
pub struct SessionTokens {
    pub user_id: String,
    pub device_id: String,
    pub access_token: String,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub refresh_token: Option<String>,
}

pub(crate) fn store(entry: &keyring::Entry, tokens: &SessionTokens) -> Result<(), String> {
    if tokens.user_id.is_empty() || tokens.device_id.is_empty() || tokens.access_token.is_empty() {
        return Err("incomplete session".into());
    }
    let value = serde_json::to_string(tokens).map_err(|e| e.to_string())?;
    if value.chars().count() > MAX_LEN {
        return Err(format!(
            "session too large for the keychain ({} chars)",
            value.chars().count()
        ));
    }
    entry.set_password(&value).map_err(|e| e.to_string())
}

pub(crate) fn load(entry: &keyring::Entry) -> Result<Option<SessionTokens>, String> {
    match entry.get_password() {
        Ok(value) => serde_json::from_str(&value)
            .map(Some)
            .map_err(|e| format!("unreadable keychain entry: {e}")),
        Err(keyring::Error::NoEntry) => Ok(None),
        Err(e) => Err(e.to_string()),
    }
}

pub(crate) fn clear(entry: &keyring::Entry) -> Result<(), String> {
    match entry.delete_credential() {
        Ok(()) | Err(keyring::Error::NoEntry) => Ok(()),
        Err(e) => Err(e.to_string()),
    }
}

#[cfg(target_os = "windows")]
fn entry() -> Result<keyring::Entry, String> {
    keyring::Entry::new(SERVICE, ACCOUNT).map_err(|e| e.to_string())
}

#[cfg(not(target_os = "windows"))]
fn entry() -> Result<keyring::Entry, String> {
    Err("not supported on this platform".into())
}

/// Keychain calls can block (credential store, AV scanners): keep them off
/// the main thread.
async fn blocking<T: Send + 'static>(
    f: impl FnOnce() -> Result<T, String> + Send + 'static,
) -> Result<T, String> {
    tauri::async_runtime::spawn_blocking(f)
        .await
        .map_err(|e| e.to_string())?
}

/// Whether this platform stores the session in the keychain.
#[tauri::command]
pub fn secure_session_supported() -> bool {
    cfg!(target_os = "windows")
}

#[tauri::command]
pub async fn secure_session_set(tokens: SessionTokens) -> Result<(), String> {
    blocking(move || store(&entry()?, &tokens)).await
}

#[tauri::command]
pub async fn secure_session_get() -> Result<Option<SessionTokens>, String> {
    blocking(|| load(&entry()?)).await
}

#[tauri::command]
pub async fn secure_session_clear() -> Result<(), String> {
    blocking(|| clear(&entry()?)).await
}

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

    fn mock_entry() -> keyring::Entry {
        keyring::set_default_credential_builder(keyring::mock::default_credential_builder());
        keyring::Entry::new(SERVICE, ACCOUNT).unwrap()
    }

    fn tokens() -> SessionTokens {
        SessionTokens {
            user_id: "@alice:lotusguild.org".into(),
            device_id: "ABCDEFGHIJ".into(),
            access_token: "syt_YWxpY2U_abcdefghijklmnopqrst_0AbCdE".into(),
            refresh_token: None,
        }
    }

    #[test]
    fn round_trip_and_clear() {
        let e = mock_entry();
        assert_eq!(load(&e).unwrap(), None);
        store(&e, &tokens()).unwrap();
        assert_eq!(load(&e).unwrap(), Some(tokens()));
        let mut oidc = tokens();
        oidc.refresh_token = Some("mar_refresh_token_value_0123456789".into());
        store(&e, &oidc).unwrap();
        assert_eq!(load(&e).unwrap(), Some(oidc));
        clear(&e).unwrap();
        assert_eq!(load(&e).unwrap(), None);
        // Clearing an empty keychain is fine (logout twice, or never mirrored).
        clear(&e).unwrap();
    }

    #[test]
    fn rejects_incomplete_or_oversized_sessions() {
        let e = mock_entry();
        let mut t = tokens();
        t.access_token = String::new();
        assert!(store(&e, &t).is_err());
        let mut big = tokens();
        big.access_token = "x".repeat(MAX_LEN);
        assert!(store(&e, &big).unwrap_err().contains("too large"));
        assert_eq!(load(&e).unwrap(), None, "nothing written on error");
    }

    #[test]
    fn serialized_shape_matches_the_web_client() {
        let json = serde_json::to_value(tokens()).unwrap();
        assert_eq!(json["userId"], "@alice:lotusguild.org");
        assert_eq!(json["deviceId"], "ABCDEFGHIJ");
        assert!(json.get("refreshToken").is_none());
        let back: SessionTokens = serde_json::from_str(
            r#"{"userId":"@a:b","deviceId":"D","accessToken":"t","refreshToken":"r"}"#,
        )
        .unwrap();
        assert_eq!(back.refresh_token.as_deref(), Some("r"));
    }

    #[test]
    fn a_realistic_oidc_session_fits() {
        let e = mock_entry();
        let t = SessionTokens {
            user_id: format!("@{}:matrix.lotusguild.org", "a".repeat(60)),
            device_id: "X".repeat(40),
            access_token: "mat_".to_string() + &"A".repeat(200),
            refresh_token: Some("mar_".to_string() + &"B".repeat(200)),
        };
        store(&e, &t).unwrap();
    }
}