use rand::Rng;

pub fn compute_next_attempt_at(now: i64, attempt_count: i64) -> i64 {
    let capped_attempts = attempt_count.min(30) as u32;
    let base_delay_ms = 1000_i64.saturating_mul(1 << capped_attempts);
    let capped_delay_ms = base_delay_ms.min(3_600_000);

    let jitter_factor = rand::thread_rng().gen_range(0.9..=1.1);
    let delay_with_jitter = (capped_delay_ms as f64 * jitter_factor) as i64;

    now.saturating_add(delay_with_jitter)
}

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

    const MAX_DELAY_MS: i64 = 3_600_000;

    #[test]
    fn test_exponential_curve() {
        let now = 1_000_000_000_i64;
        for attempt in 1..=10 {
            let result = compute_next_attempt_at(now, attempt);
            let delay = result - now;
            let expected_base = 1000_i64 * (1 << attempt);
            let min_expected = (expected_base as f64 * 0.89) as i64;
            let max_expected = (expected_base as f64 * 1.11) as i64;
            assert!(
                delay >= min_expected && delay <= max_expected,
                "attempt {attempt}: delay {delay} not in [{min_expected}, {max_expected}]"
            );
        }
    }

    #[test]
    fn test_cap_at_one_hour() {
        let now = 1_000_000_000_i64;
        for attempt in [20, 25, 30, 50, 100] {
            let result = compute_next_attempt_at(now, attempt);
            let delay = result - now;
            let max_with_jitter = (MAX_DELAY_MS as f64 * 1.11) as i64;
            assert!(
                delay <= max_with_jitter,
                "attempt {attempt}: delay {delay} exceeds cap {max_with_jitter}"
            );
        }
    }

    #[test]
    fn test_jitter_range() {
        let now = 1_000_000_000_i64;
        let attempt = 5;
        let base = 1000_i64 * (1 << attempt);
        let min_delay = (base as f64 * 0.89) as i64;
        let max_delay = (base as f64 * 1.11) as i64;

        for _ in 0..100 {
            let result = compute_next_attempt_at(now, attempt);
            let delay = result - now;
            assert!(
                delay >= min_delay && delay <= max_delay,
                "delay {delay} not in jitter range [{min_delay}, {max_delay}]"
            );
        }
    }

    #[test]
    fn test_first_retry_is_about_two_seconds() {
        let now = 1_000_000_000_i64;
        let result = compute_next_attempt_at(now, 1);
        let delay = result - now;
        assert!(
            (1800..=2200).contains(&delay),
            "first retry delay: {delay}ms"
        );
    }

    #[test]
    fn test_overflow_safety() {
        let now = i64::MAX / 2;
        let result = compute_next_attempt_at(now, i64::MAX);
        assert!(result > now);
    }

    #[test]
    fn test_saturating_add_prevents_overflow() {
        let now = i64::MAX - 10;
        let result = compute_next_attempt_at(now, 30);
        assert_eq!(result, i64::MAX);
    }
}