//! Claude subscription usage watch: polls `GET /api/oauth/usage` with the
//! agent's own OAuth access token and records each window's utilization
//! (`five_hour`, `seven_day`, …) as an OTEL gauge.
//!
//! Endpoint, headers and response shape are the ones the bundled `claude`
//! CLI uses for its own `/usage` view: `Authorization: Bearer <token>` plus
//! `anthropic-beta: oauth-2025-04-20`; the body is an object whose window
//! entries are `{ utilization: 0-100 | null, resets_at: ISO 8601 | null }`.
//!
//! ⚠️ Read-only use of the current access token. This task never refreshes
//! it: `claude` owns refresh-token rotation, and a second refresher racing it
//! can invalidate the session and log the agent out. An expired token is
//! skipped until `claude`'s next turn refreshes it.
//!
//! Agents without an OAuth session (API-key backends, not yet logged in)
//! have nothing to poll with and skip quietly.

use std::path::Path;
use std::time::Duration;

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

const POLL_INTERVAL: Duration = Duration::from_mins(5);
const REQUEST_TIMEOUT: Duration = Duration::from_secs(10);
const USAGE_URL: &str = "https://api.anthropic.com/api/oauth/usage";
const OAUTH_BETA: &str = "oauth-2025-04-20";
/// The credentials file `claude` itself reads its OAuth session from.
const CREDENTIALS_FILE: &str = ".credentials.json";

/// No `Debug`/`Display`, so the token cannot end up in a log line by
/// accident.
struct AccessToken(String);

enum Token {
    /// No usable OAuth session in the credentials file.
    Absent,
    /// Past `expiresAt`; a request would only 401.
    Expired,
    Usable(AccessToken),
}

#[derive(Deserialize)]
struct CredentialsFile {
    #[serde(rename = "claudeAiOauth")]
    oauth: Option<OauthEntry>,
}

#[derive(Deserialize)]
struct OauthEntry {
    #[serde(rename = "accessToken")]
    access_token: Option<String>,
    /// Unix milliseconds.
    #[serde(rename = "expiresAt")]
    expires_at: Option<i64>,
}

/// One usage window as reported by the endpoint.
#[derive(Debug, PartialEq)]
struct Window {
    name: String,
    percent: f64,
    /// Unix seconds; `None` when the server sent no (or an unparsable) reset.
    resets_at: Option<u64>,
}

/// Background loop. Detached task — runs for the harness's lifetime; every
/// failure is logged and retried on the next tick, never fatal.
pub async fn run() {
    if crate::login::using_api_key() {
        tracing::debug!("claude usage watch: api-key agent, no OAuth token to poll with");
        return;
    }
    if !crate::otel_turn_metrics::enabled() {
        return;
    }
    let client = match reqwest::Client::builder().timeout(REQUEST_TIMEOUT).build() {
        Ok(c) => c,
        Err(e) => {
            tracing::warn!(error = %e, "claude usage watch: failed to build http client");
            return;
        }
    };
    let creds = crate::paths::claude_dir().join(CREDENTIALS_FILE);
    let mut skip_logged = false;
    // Separate streak from `skip_logged`: a token can be usable while the
    // response still carries no recognisable window (endpoint change,
    // unexpected account state), and that deserves its own once-per-streak
    // warning rather than silence.
    let mut empty_logged = false;
    let mut tick = tokio::time::interval(POLL_INTERVAL);
    tick.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
    loop {
        tick.tick().await;
        let token = match read_token(&creds).await {
            Token::Usable(t) => t,
            Token::Absent | Token::Expired if skip_logged => continue,
            Token::Absent => {
                tracing::debug!("claude usage watch: no OAuth access token, skipping");
                skip_logged = true;
                continue;
            }
            Token::Expired => {
                tracing::debug!("claude usage watch: OAuth access token expired, skipping");
                skip_logged = true;
                continue;
            }
        };
        skip_logged = false;
        match fetch(&client, &token).await {
            Ok(body) => {
                let windows = windows(&body);
                if should_warn_empty(&mut empty_logged, windows.len()) {
                    tracing::warn!(
                        "claude usage watch: usage response had no recognisable windows"
                    );
                }
                for w in windows {
                    crate::otel_turn_metrics::record_claude_usage(&w.name, w.percent, w.resets_at);
                }
            }
            Err(e) => tracing::warn!("claude usage watch: {e}"),
        }
    }
}

async fn read_token(path: &Path) -> Token {
    let Ok(bytes) = tokio::fs::read(path).await else {
        return Token::Absent;
    };
    let now_ms = chrono::Utc::now().timestamp_millis();
    token_from(&bytes, now_ms)
}

/// Parse errors are deliberately dropped: a serde message can quote the
/// offending value, and this file holds secrets.
fn token_from(bytes: &[u8], now_ms: i64) -> Token {
    let Ok(file) = serde_json::from_slice::<CredentialsFile>(bytes) else {
        return Token::Absent;
    };
    let Some(entry) = file.oauth else {
        return Token::Absent;
    };
    let Some(token) = entry.access_token.filter(|t| !t.is_empty()) else {
        return Token::Absent;
    };
    if entry.expires_at.is_some_and(|exp| exp <= now_ms) {
        return Token::Expired;
    }
    Token::Usable(AccessToken(token))
}

/// Errors carry the status or the transport error only — never the response
/// body, never the token.
async fn fetch(client: &reqwest::Client, token: &AccessToken) -> Result<Value, String> {
    let resp = client
        .get(USAGE_URL)
        .bearer_auth(&token.0)
        .header("anthropic-beta", OAUTH_BETA)
        .header(reqwest::header::CONTENT_TYPE, "application/json")
        .send()
        .await
        .map_err(|e| format!("request failed: {e}"))?;
    let status = resp.status();
    if !status.is_success() {
        return Err(format!("request rejected: {status}"));
    }
    resp.json::<Value>()
        .await
        .map_err(|e| format!("unreadable response: {e}"))
}

/// Streak-deduped decision for the empty-windows warning: updates
/// `empty_logged` and returns whether this tick should actually log. Split
/// from [`run`] so the once-per-streak logic is testable without a timer.
fn should_warn_empty(empty_logged: &mut bool, window_count: usize) -> bool {
    if window_count > 0 {
        *empty_logged = false;
        return false;
    }
    if *empty_logged {
        return false;
    }
    *empty_logged = true;
    true
}

/// Window entries are the top-level objects carrying both `utilization` and
/// `resets_at`; other top-level keys (`extra_usage`, `limits`, …) are not
/// windows. A window whose `utilization` is `null` is not reported.
fn windows(body: &Value) -> Vec<Window> {
    let Some(obj) = body.as_object() else {
        return Vec::new();
    };
    obj.iter()
        .filter_map(|(name, entry)| {
            let entry = entry.as_object()?;
            let resets_at = entry.get("resets_at")?;
            let percent = entry.get("utilization")?.as_f64()?;
            let resets_at = resets_at
                .as_str()
                .and_then(|s| chrono::DateTime::parse_from_rfc3339(s).ok())
                .and_then(|t| u64::try_from(t.timestamp()).ok());
            Some(Window {
                name: name.clone(),
                percent,
                resets_at,
            })
        })
        .collect()
}

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

    /// Shape of a real response, field names from the `claude` CLI bundle.
    const FIXTURE: &str = r#"{
        "five_hour": { "utilization": 42.0, "resets_at": "2026-09-30T18:00:00.123456+00:00" },
        "seven_day": { "utilization": 7, "resets_at": "2026-10-04T09:00:00Z" },
        "seven_day_oauth_apps": null,
        "seven_day_opus": { "utilization": null, "resets_at": null },
        "seven_day_sonnet": { "utilization": 3.5, "resets_at": null },
        "extra_usage": { "is_enabled": false, "utilization": null },
        "limits": [ { "kind": "session", "percent": 42.0, "resets_at": null } ]
    }"#;

    #[test]
    fn parses_windows_and_skips_non_windows() {
        let body: Value = serde_json::from_str(FIXTURE).expect("fixture");
        assert_eq!(
            windows(&body),
            vec![
                Window {
                    name: "five_hour".into(),
                    percent: 42.0,
                    resets_at: Some(1_790_791_200),
                },
                Window {
                    name: "seven_day".into(),
                    percent: 7.0,
                    resets_at: Some(1_791_104_400),
                },
                Window {
                    name: "seven_day_sonnet".into(),
                    percent: 3.5,
                    resets_at: None,
                },
            ]
        );
    }

    #[test]
    fn non_object_body_yields_nothing() {
        assert!(windows(&Value::Null).is_empty());
        assert!(windows(&serde_json::json!([1, 2])).is_empty());
    }

    #[test]
    fn token_usable_before_expiry() {
        let f = br#"{"claudeAiOauth":{"accessToken":"t","expiresAt":2000,"refreshToken":"r"}}"#;
        assert!(matches!(token_from(f, 1000), Token::Usable(t) if t.0 == "t"));
    }

    #[test]
    fn token_expired_is_not_used() {
        let f = br#"{"claudeAiOauth":{"accessToken":"t","expiresAt":1000}}"#;
        assert!(matches!(token_from(f, 1000), Token::Expired));
    }

    /// Empty responses warn once per streak; a non-empty response in
    /// between resets it, so a second empty streak warns again.
    #[test]
    fn empty_windows_warn_once_per_streak() {
        let mut empty_logged = false;
        assert!(should_warn_empty(&mut empty_logged, 0));
        assert!(!should_warn_empty(&mut empty_logged, 0));
        assert!(!should_warn_empty(&mut empty_logged, 2));
        assert!(should_warn_empty(&mut empty_logged, 0));
    }

    #[test]
    fn missing_or_malformed_credentials_are_absent() {
        assert!(matches!(token_from(b"{}", 0), Token::Absent));
        assert!(matches!(
            token_from(br#"{"claudeAiOauth":{"accessToken":""}}"#, 0),
            Token::Absent
        ));
        assert!(matches!(token_from(b"not json", 0), Token::Absent));
    }
}