//! Retrying a request whose server went away, with a bounded, jittered backoff.
//!
//! Retrying is safe because nothing is recorded until a completion is final: a retry sends the same
//! bytes again. Only errors that mean "the server went away" are retried, and only within the first
//! window and attempt budget, so a dead server is not chased forever.

use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};

use super::{ChatEvent, ChatRequest, Client, Completion, InferError};

pub fn is_retryable(e: &InferError) -> bool {
    match e {
        InferError::Connect(_) => true, // the server went away and may be back
        InferError::StreamClosedEarly => true,
        InferError::Stalled => true,
        InferError::LoadTimeout => true,
        InferError::Http { status: 503, .. } => true,
        InferError::Busy => false, // the slot is busy; the gate already said so
        InferError::WaitTimeout => false, // it has already waited
        InferError::ThinkingOverrun => false,
        InferError::Protocol(_) => false,
        InferError::Http { .. } => false, // any other status would fail again
    }
}

pub fn backoff_ms(schedule: &[u64], retry: u32, jitter: i32) -> u64 {
    // Retry numbers start at 1; 0 is treated as 1.
    let retry = retry.max(1);
    // The base is schedule[retry - 1], or the last entry past the end, or 0 for an empty schedule.
    let index = usize::try_from(retry - 1).unwrap_or(usize::MAX);
    let base = schedule
        .get(index)
        .copied()
        .or_else(|| schedule.last().copied())
        .unwrap_or(0);
    // Jitter is clamped to -1000..=1000 and moves the wait by up to a quarter of the base.
    let jitter = jitter.clamp(-1000, 1000);
    let value = i128::from(base) + i128::from(base / 4) * i128::from(jitter) / 1000;
    u64::try_from(value.clamp(0, i128::from(u64::MAX))).unwrap_or(u64::MAX)
}

impl Client {
    pub fn chat_with_retry(
        &self,
        req: &ChatRequest,
        on_event: &mut dyn FnMut(&ChatEvent),
    ) -> Result<Completion, InferError> {
        let start = Instant::now();
        let mut attempt: u32 = 1;
        loop {
            match self.chat(req, on_event) {
                Ok(completion) => return Ok(completion),
                Err(e) => {
                    if !is_retryable(&e) || attempt >= self.cfg.limits.retry_attempts {
                        return Err(e);
                    }
                    let wait = backoff_ms(&self.cfg.limits.retry_backoff_ms, attempt, now_jitter());
                    if elapsed_ms(start) + wait > self.cfg.limits.retry_window_ms {
                        return Err(e);
                    }
                    on_event(&ChatEvent::Retrying {
                        attempt: attempt + 1,
                        after_ms: wait,
                        error: e.to_string(),
                    });
                    std::thread::sleep(Duration::from_millis(wait));
                    attempt += 1;
                }
            }
        }
    }
}

fn now_jitter() -> i32 {
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|since| since.subsec_nanos())
        .unwrap_or(0);
    // A nanosecond count reduced to -1000..=1000 is random enough for jitter.
    i32::try_from(nanos % 2000).unwrap_or(999) - 1000
}

fn elapsed_ms(start: Instant) -> u64 {
    start.elapsed().as_millis().try_into().unwrap_or(u64::MAX)
}