//! Login Flow v2 (FR-NC-1).
//!
//! The app never sees the user's password. It asks the server for a login URL,
//! opens that in the **system browser**, and polls until the server hands back
//! an app password scoped to this device.

use std::time::Duration;

use dr_sync::RemoteError;
use serde::{Deserialize, Serialize};

/// The flow's poll token is valid for 20 minutes.
const FLOW_TIMEOUT: Duration = Duration::from_secs(20 * 60);
const POLL_INTERVAL: Duration = Duration::from_secs(2);

/// What the server returns when a flow is started.
#[derive(Debug, Clone, Deserialize)]
pub struct LoginFlow {
    /// Open this in the system browser — never an embedded webview, which
    /// would defeat the point of not handling the password.
    #[serde(rename = "login")]
    pub login_url: String,
    #[serde(rename = "poll")]
    pub poll: PollInfo,
    /// The server the flow was started against — the address the user typed,
    /// already normalised. Not part of the response: [`begin`] fills it in so
    /// [`poll`] can put it in the credentials instead of the server's answer.
    #[serde(skip)]
    pub server: String,
}

#[derive(Debug, Clone, Deserialize)]
pub struct PollInfo {
    pub token: String,
    pub endpoint: String,
}

/// Credentials issued at the end of the flow.
#[derive(Debug, Clone, Deserialize, Serialize)]
pub struct AppCredentials {
    pub server: String,
    #[serde(rename = "loginName")]
    pub login_name: String,
    /// Device-scoped and individually revocable — not the user's password.
    #[serde(rename = "appPassword")]
    pub app_password: String,
}

/// TRACES: FR-NC-1 | M-1
/// Begin a login flow.
///
/// The `User-Agent` names the resulting app password in the user's security
/// settings, so it should identify the device for per-device revocation.
pub async fn begin(
    client: &reqwest::Client,
    server: &str,
    user_agent: &str,
) -> Result<LoginFlow, RemoteError> {
    let url = format!("{}/index.php/login/v2", server.trim_end_matches('/'));
    let resp = client
        .post(&url)
        .header(reqwest::header::USER_AGENT, user_agent)
        .send()
        .await
        .map_err(super::map_send_error)?;

    if !resp.status().is_success() {
        return Err(RemoteError::Server {
            status: resp.status().as_u16(),
            detail: "login flow could not be started".into(),
        });
    }

    let mut flow = resp
        .json::<LoginFlow>()
        .await
        .map_err(|e| RemoteError::Protocol(e.to_string()))?;

    // Both URLs are the server's to choose, and neither may be trusted as
    // sent. The login URL is handed to the operating system to open, where a
    // `file:` or UNC path is a program launch rather than a web page; the poll
    // endpoint is where the app password comes back from.
    flow.login_url = upgraded("login URL", &flow.login_url)?;
    flow.poll.endpoint = upgraded("poll endpoint", &flow.poll.endpoint)?;
    flow.server = server.trim_end_matches('/').to_string();
    Ok(flow)
}

/// TRACES: NFR-SEC-3
/// A URL the server sent, upgraded to HTTPS, or refused.
///
/// `http` is upgraded rather than refused: a Nextcloud behind a TLS-terminating
/// proxy without `overwriteprotocol` builds every absolute URL it returns with
/// `http`, and the same path over `https` is the one that works. Any other
/// scheme is refused, because the only thing it could be for is reaching
/// something that is not this server.
///
/// The host is not checked. A server reached by its LAN address may answer with
/// its public name, and nothing here is safer for refusing that: the account
/// is stored under the address the user typed (see [`poll`]), not under
/// anything the server said.
pub(crate) fn upgraded(what: &str, url: &str) -> Result<String, RemoteError> {
    let mut parsed = url::Url::parse(url).map_err(|e| {
        RemoteError::Protocol(format!(
            "the server sent a {what} that is not a URL ({e}): {url}"
        ))
    })?;
    match parsed.scheme() {
        "https" => {}
        "http" => parsed.set_scheme("https").map_err(|()| {
            RemoteError::Protocol(format!("{what} cannot be upgraded to https: {url}"))
        })?,
        other => {
            return Err(RemoteError::Protocol(format!(
                "the server sent a {what} using {other}:, and only https is accepted: {url}"
            )))
        }
    }
    if parsed.host_str().is_none_or(str::is_empty) {
        return Err(RemoteError::Protocol(format!(
            "the server sent a {what} with no host: {url}"
        )));
    }
    Ok(parsed.into())
}

/// Poll until the user finishes authenticating in the browser.
///
/// The server returns 404 while pending and 200 exactly once — so a dropped
/// success response means restarting the flow, and the result must be
/// persisted immediately.
pub async fn poll(
    client: &reqwest::Client,
    flow: &LoginFlow,
) -> Result<AppCredentials, RemoteError> {
    let deadline = std::time::Instant::now() + FLOW_TIMEOUT;

    while std::time::Instant::now() < deadline {
        let resp = client
            .post(&flow.poll.endpoint)
            // One field; encoding it by hand avoids pulling in reqwest's
            // form feature for a single call.
            .header(
                reqwest::header::CONTENT_TYPE,
                "application/x-www-form-urlencoded",
            )
            .body(format!("token={}", urlencode(&flow.poll.token)))
            .send()
            .await
            .map_err(super::map_send_error)?;

        match resp.status().as_u16() {
            200 => {
                let creds = resp
                    .json::<AppCredentials>()
                    .await
                    .map_err(|e| RemoteError::Protocol(e.to_string()))?;
                return Ok(under_typed_server(creds, &flow.server));
            }
            // Still waiting for the user.
            404 => tokio::time::sleep(POLL_INTERVAL).await,
            s => {
                return Err(RemoteError::Server {
                    status: s,
                    detail: "unexpected status while polling".into(),
                })
            }
        }
    }

    Err(RemoteError::AuthFailed)
}

/// TRACES: NFR-SEC-3
/// Credentials filed under the address the user typed, not the one the server
/// reports.
///
/// That report is the server's idea of its own URL, and behind a proxy
/// without `overwriteprotocol` it says `http://` — which, stored, would send
/// the app password in the clear on every request from then on. The typed
/// address has just carried the whole flow, so it is known to reach the
/// server.
fn under_typed_server(mut creds: AppCredentials, server: &str) -> AppCredentials {
    if creds.server.trim_end_matches('/') != server {
        log::info!(
            "server reports itself as {}; keeping {server}",
            creds.server
        );
    }
    creds.server = server.to_string();
    creds
}

/// Percent-encode a form value.
fn urlencode(s: &str) -> String {
    s.bytes()
        .map(|b| match b {
            b'A'..=b'Z' | b'a'..=b'z' | b'0'..=b'9' | b'-' | b'_' | b'.' | b'~' => {
                (b as char).to_string()
            }
            _ => format!("%{b:02X}"),
        })
        .collect()
}

/// TRACES: FR-NC-1 | M-4
/// Revoke the app password on logout (FR-NC-1).
pub async fn revoke(
    client: &reqwest::Client,
    server: &str,
    login: &str,
    password: &str,
) -> Result<(), RemoteError> {
    let url = format!(
        "{}/ocs/v2.php/core/apppassword",
        server.trim_end_matches('/')
    );
    client
        .delete(&url)
        .basic_auth(login, Some(password))
        .header("OCS-APIRequest", "true")
        .send()
        .await
        .map_err(super::map_send_error)?;
    Ok(())
}

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

    #[test]
    fn login_flow_response_deserialises() {
        // The shape Nextcloud actually returns.
        let json = r#"{
            "poll": {"token": "abc", "endpoint": "https://cloud.example/login/v2/poll"},
            "login": "https://cloud.example/login/v2/flow/xyz"
        }"#;
        let flow: LoginFlow = serde_json::from_str(json).unwrap();
        assert_eq!(flow.poll.token, "abc");
        assert!(flow.login_url.contains("/login/v2/flow/"));
    }

    /// TRACES: NFR-SEC-3
    #[test]
    fn server_urls_are_upgraded_to_https_or_refused() {
        assert_eq!(
            upgraded("login URL", "http://cloud.example/login/v2/flow/xyz").unwrap(),
            "https://cloud.example/login/v2/flow/xyz"
        );
        // A non-default port stays, and only the scheme changes.
        assert_eq!(
            upgraded("poll endpoint", "http://cloud.example:8443/login/v2/poll").unwrap(),
            "https://cloud.example:8443/login/v2/poll"
        );
        assert_eq!(
            upgraded("login URL", "https://cloud.example/x").unwrap(),
            "https://cloud.example/x"
        );
        // What `rundll32 url.dll,FileProtocolHandler` would run, and what
        // `xdg-open` would hand to whatever claims it.
        for hostile in [
            "file:///C:/Windows/System32/calc.exe",
            "\\\\evil\\share\\x.exe",
            "C:\\x.exe",
            "javascript:alert(1)",
            "-v",
            "",
        ] {
            assert!(upgraded("login URL", hostile).is_err(), "{hostile:?}");
        }
    }

    /// TRACES: NFR-SEC-3
    #[test]
    fn credentials_keep_the_typed_server_not_the_reported_one() {
        let reported = AppCredentials {
            server: "http://cloud.example".into(),
            login_name: "duncan".into(),
            app_password: "secret-token".into(),
        };
        let c = under_typed_server(reported, "https://cloud.example");
        assert_eq!(c.server, "https://cloud.example");
        assert_eq!(c.app_password, "secret-token");
    }

    #[test]
    fn form_values_are_encoded() {
        assert_eq!(urlencode("abc123"), "abc123");
        assert_eq!(urlencode("a b+c"), "a%20b%2Bc");
    }

    #[test]
    fn credentials_deserialise_with_camel_case_keys() {
        let json = r#"{
            "server": "https://cloud.example",
            "loginName": "duncan",
            "appPassword": "secret-token"
        }"#;
        let c: AppCredentials = serde_json::from_str(json).unwrap();
        assert_eq!(c.login_name, "duncan");
        assert_eq!(c.app_password, "secret-token");
    }
}