//! GitHub OAuth device flow (RFC 8628) for Copilot login.
//!
//! ai-harness must register its own GitHub OAuth app and supply its client id (we do not
//! hardcode opencode's). The client id is passed in by the caller — see [`request_device_code`].

use std::time::Duration;

use serde::Deserialize;
use serde_json::Value;

const DEVICE_CODE_URL: &str = "https://github.com/login/device/code";
const ACCESS_TOKEN_URL: &str = "https://github.com/login/oauth/access_token";
const GRANT_TYPE: &str = "urn:ietf:params:oauth:grant-type:device_code";
/// Minimum scope needed to call the Copilot token-exchange endpoint.
pub const SCOPE: &str = "read:user";

#[derive(Debug, Clone, Deserialize)]
pub struct DeviceCode {
    pub device_code: String,
    pub user_code: String,
    pub verification_uri: String,
    /// Seconds between polls; GitHub requires honoring this and any `slow_down` bumps.
    #[serde(default = "default_interval")]
    pub interval: u64,
    #[serde(default)]
    pub expires_in: u64,
}

fn default_interval() -> u64 {
    5
}

/// Result of one poll of the access-token endpoint.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum PollOutcome {
    /// The user hasn't authorized yet; keep polling at the current interval.
    Pending,
    /// GitHub asked us to slow down; add 5s to the interval (RFC 8628).
    SlowDown,
    /// Authorization complete.
    Success { access_token: String },
    /// Terminal failure (expired code, denied, unknown error) with a human-readable reason.
    Failed(String),
}

#[derive(Debug, thiserror::Error)]
pub enum DeviceFlowError {
    #[error("network: {0}")]
    Network(String),
    #[error("unexpected response: {0}")]
    Unexpected(String),
}

/// Pure mapping of an access-token poll response body to a [`PollOutcome`], so the state
/// machine is testable without a live GitHub.
pub fn parse_poll_response(body: &Value) -> PollOutcome {
    if let Some(token) = body["access_token"].as_str() {
        return PollOutcome::Success {
            access_token: token.to_string(),
        };
    }
    match body["error"].as_str() {
        Some("authorization_pending") => PollOutcome::Pending,
        Some("slow_down") => PollOutcome::SlowDown,
        Some(other) => {
            let desc = body["error_description"].as_str().unwrap_or(other);
            PollOutcome::Failed(desc.to_string())
        }
        None => PollOutcome::Failed("no access_token and no error in response".to_string()),
    }
}

/// Applies a `slow_down` to a poll interval per RFC 8628 (+5s).
pub fn bump_interval(interval: u64) -> u64 {
    interval + 5
}

/// Step 1: request a device + user code for `client_id`.
pub async fn request_device_code(
    client: &reqwest::Client,
    client_id: &str,
) -> Result<DeviceCode, DeviceFlowError> {
    let resp = client
        .post(DEVICE_CODE_URL)
        .header("accept", "application/json")
        .json(&serde_json::json!({"client_id": client_id, "scope": SCOPE}))
        .send()
        .await
        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
    let value: Value = resp
        .json()
        .await
        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
    serde_json::from_value(value.clone())
        .map_err(|_| DeviceFlowError::Unexpected(value.to_string()))
}

/// Step 2 (single poll): exchange the device code for an access token, once.
pub async fn poll_once(
    client: &reqwest::Client,
    client_id: &str,
    device_code: &str,
) -> Result<PollOutcome, DeviceFlowError> {
    let resp = client
        .post(ACCESS_TOKEN_URL)
        .header("accept", "application/json")
        .json(&serde_json::json!({
            "client_id": client_id,
            "device_code": device_code,
            "grant_type": GRANT_TYPE,
        }))
        .send()
        .await
        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
    let value: Value = resp
        .json()
        .await
        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
    Ok(parse_poll_response(&value))
}

/// Step 2 (full loop): polls until success or terminal failure, honoring `interval` and
/// `slow_down`. `sleep` is injected so tests can drive it without real time.
pub async fn poll_for_token<S, Fut>(
    client: &reqwest::Client,
    client_id: &str,
    device: &DeviceCode,
    sleep: S,
) -> Result<String, DeviceFlowError>
where
    S: Fn(Duration) -> Fut,
    Fut: std::future::Future<Output = ()>,
{
    let mut interval = device.interval;
    loop {
        sleep(Duration::from_secs(interval)).await;
        match poll_once(client, client_id, &device.device_code).await? {
            PollOutcome::Pending => {}
            PollOutcome::SlowDown => interval = bump_interval(interval),
            PollOutcome::Success { access_token } => return Ok(access_token),
            PollOutcome::Failed(reason) => return Err(DeviceFlowError::Unexpected(reason)),
        }
    }
}

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

    #[test]
    fn parses_success() {
        let outcome =
            parse_poll_response(&json!({"access_token": "gho_abc", "token_type": "bearer"}));
        assert_eq!(
            outcome,
            PollOutcome::Success {
                access_token: "gho_abc".into()
            }
        );
    }

    #[test]
    fn parses_pending_and_slow_down() {
        assert_eq!(
            parse_poll_response(&json!({"error": "authorization_pending"})),
            PollOutcome::Pending
        );
        assert_eq!(
            parse_poll_response(&json!({"error": "slow_down", "interval": 10})),
            PollOutcome::SlowDown
        );
    }

    #[test]
    fn parses_terminal_errors_with_description() {
        match parse_poll_response(
            &json!({"error": "expired_token", "error_description": "code expired"}),
        ) {
            PollOutcome::Failed(msg) => assert_eq!(msg, "code expired"),
            other => panic!("expected Failed, got {other:?}"),
        }
        assert!(matches!(
            parse_poll_response(&json!({"error": "access_denied"})),
            PollOutcome::Failed(_)
        ));
        assert!(matches!(
            parse_poll_response(&json!({})),
            PollOutcome::Failed(_)
        ));
    }

    #[test]
    fn slow_down_adds_five_seconds() {
        assert_eq!(bump_interval(5), 10);
        assert_eq!(bump_interval(10), 15);
    }

    #[test]
    fn device_code_defaults_interval() {
        let dc: DeviceCode = serde_json::from_value(json!({
            "device_code": "d",
            "user_code": "WXYZ-1234",
            "verification_uri": "https://github.com/login/device"
        }))
        .unwrap();
        assert_eq!(dc.interval, 5);
    }
}