//! Relay Health Tracking for GRASP-02 Proactive Sync
//!
//! This module implements health tracking for relay connections, including:
//! - Health state machine (Healthy -> Degraded -> Dead)
//! - Exponential backoff with configurable max delay
//! - Dead relay detection after 24h of continuous failures
//!
//! ## Health States
//!
//! - **Healthy**: Working connection, no recent failures
//! - **Degraded**: Connection failed, retrying with backoff
//! - **Dead**: 24h+ of continuous failures, minimal retry (once per day)

use std::sync::Arc;
use std::time::{Duration, Instant};

use dashmap::DashMap;

use crate::config::Config;

/// Duration threshold before a relay is considered dead (24 hours)
const DEAD_THRESHOLD_HOURS: u64 = 24;

/// How often dead relays are retried (once per 24 hours)
const DEAD_RETRY_INTERVAL_HOURS: u64 = 24;

/// Default maximum backoff duration in seconds (1 hour)
const DEFAULT_MAX_BACKOFF_SECS: u64 = 3600;

/// Base backoff duration in seconds
const BASE_BACKOFF_SECS: u64 = 5;

/// Health state of a relay connection
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum HealthState {
    /// Working connection, no recent failures
    Healthy,
    /// Connection failed, retrying with exponential backoff
    Degraded,
    /// 24h+ of continuous failures, minimal retry
    Dead,
}

impl std::fmt::Display for HealthState {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            HealthState::Healthy => write!(f, "healthy"),
            HealthState::Degraded => write!(f, "degraded"),
            HealthState::Dead => write!(f, "dead"),
        }
    }
}

/// Health information for a single relay
#[derive(Debug, Clone)]
pub struct RelayHealth {
    /// Current health state
    pub state: HealthState,
    /// Number of consecutive connection failures
    pub consecutive_failures: u32,
    /// Time of the first failure in the current failure streak
    pub first_failure_time: Option<Instant>,
    /// Time of the last failure
    pub last_failure_time: Option<Instant>,
    /// Time of the last successful connection
    pub last_success_time: Option<Instant>,
    /// Next time a connection attempt should be made
    pub next_retry_at: Option<Instant>,
}

impl Default for RelayHealth {
    fn default() -> Self {
        Self {
            state: HealthState::Healthy,
            consecutive_failures: 0,
            first_failure_time: None,
            last_failure_time: None,
            last_success_time: None,
            next_retry_at: None,
        }
    }
}

impl RelayHealth {
    /// Create a new RelayHealth with healthy state
    pub fn new() -> Self {
        Self::default()
    }
}

/// Thread-safe relay health tracker using DashMap
#[derive(Debug)]
pub struct RelayHealthTracker {
    health: DashMap<String, RelayHealth>,
    max_backoff_secs: u64,
}

impl RelayHealthTracker {
    /// Create a new RelayHealthTracker
    pub fn new(config: &Config) -> Self {
        Self {
            health: DashMap::new(),
            max_backoff_secs: config.sync_max_backoff_secs,
        }
    }

    /// Create a new RelayHealthTracker with default settings
    pub fn with_defaults() -> Self {
        Self {
            health: DashMap::new(),
            max_backoff_secs: DEFAULT_MAX_BACKOFF_SECS,
        }
    }

    /// Create a new RelayHealthTracker with custom max backoff
    pub fn with_max_backoff(max_backoff_secs: u64) -> Self {
        Self {
            health: DashMap::new(),
            max_backoff_secs,
        }
    }

    /// Record a successful connection to a relay
    ///
    /// Resets the relay to Healthy state and clears failure counters.
    pub fn record_success(&self, relay_url: &str) {
        let now = Instant::now();
        let mut entry = self.health.entry(relay_url.to_string()).or_default();
        let health = entry.value_mut();

        let old_state = health.state;

        // Reset to healthy state
        health.state = HealthState::Healthy;
        health.consecutive_failures = 0;
        health.first_failure_time = None;
        health.last_failure_time = None;
        health.last_success_time = Some(now);
        health.next_retry_at = None;

        if old_state != HealthState::Healthy {
            tracing::info!(
                "Relay {} recovered to healthy (was {:?})",
                relay_url,
                old_state
            );
        }
    }

    /// Record a connection failure for a relay
    ///
    /// Increments failure counter, updates state, and calculates next retry time.
    pub fn record_failure(&self, relay_url: &str) {
        let now = Instant::now();
        let mut entry = self.health.entry(relay_url.to_string()).or_default();
        let health = entry.value_mut();

        let old_state = health.state;

        // Set first_failure_time if this is a new failure streak
        if health.first_failure_time.is_none() {
            health.first_failure_time = Some(now);
        }

        health.consecutive_failures = health.consecutive_failures.saturating_add(1);
        health.last_failure_time = Some(now);

        // Check if we should transition to Dead state
        if let Some(first_failure) = health.first_failure_time {
            let failure_duration = now.duration_since(first_failure);
            let dead_threshold = Duration::from_secs(DEAD_THRESHOLD_HOURS * 3600);

            if failure_duration >= dead_threshold {
                health.state = HealthState::Dead;
                // Dead relays retry once per day
                health.next_retry_at =
                    Some(now + Duration::from_secs(DEAD_RETRY_INTERVAL_HOURS * 3600));

                if old_state != HealthState::Dead {
                    tracing::warn!(
                        "Relay {} marked dead after 24h failures ({} consecutive failures)",
                        relay_url,
                        health.consecutive_failures
                    );
                }
            } else {
                // Degraded state with exponential backoff
                health.state = HealthState::Degraded;
                let backoff = Self::get_backoff_duration(
                    health.consecutive_failures,
                    self.max_backoff_secs,
                );
                health.next_retry_at = Some(now + backoff);

                if old_state != HealthState::Degraded {
                    tracing::warn!(
                        "Relay {} degraded, backoff {:?}",
                        relay_url,
                        backoff
                    );
                } else {
                    tracing::debug!(
                        "Relay {} failure #{}, backoff {:?}",
                        relay_url,
                        health.consecutive_failures,
                        backoff
                    );
                }
            }
        }
    }

    /// Check if a connection attempt should be made to a relay
    ///
    /// Returns true if:
    /// - The relay has no health record (first attempt)
    /// - The relay is healthy
    /// - The backoff period has elapsed
    pub fn should_attempt_connection(&self, relay_url: &str) -> bool {
        let entry = self.health.get(relay_url);

        match entry {
            None => true, // No record, allow first attempt
            Some(entry) => {
                let health = entry.value();

                match health.state {
                    HealthState::Healthy => true,
                    HealthState::Degraded | HealthState::Dead => {
                        // Check if backoff period has elapsed
                        match health.next_retry_at {
                            None => true,
                            Some(next_retry) => Instant::now() >= next_retry,
                        }
                    }
                }
            }
        }
    }

    /// Get the current health state of a relay
    pub fn get_state(&self, relay_url: &str) -> HealthState {
        self.health
            .get(relay_url)
            .map(|entry| entry.value().state)
            .unwrap_or(HealthState::Healthy)
    }

    /// Check if a relay is marked as dead
    pub fn is_dead(&self, relay_url: &str) -> bool {
        self.get_state(relay_url) == HealthState::Dead
    }

    /// Get the remaining backoff duration for a relay
    ///
    /// Returns None if no backoff is active.
    pub fn get_remaining_backoff(&self, relay_url: &str) -> Option<Duration> {
        let entry = self.health.get(relay_url)?;
        let health = entry.value();
        let next_retry = health.next_retry_at?;
        let now = Instant::now();

        if now >= next_retry {
            None
        } else {
            Some(next_retry - now)
        }
    }

    /// Get the consecutive failure count for a relay
    pub fn get_failure_count(&self, relay_url: &str) -> u32 {
        self.health
            .get(relay_url)
            .map(|entry| entry.value().consecutive_failures)
            .unwrap_or(0)
    }

    /// Calculate the backoff duration based on failure count
    ///
    /// Uses exponential backoff: base * 2^failures, capped at max_backoff
    pub fn get_backoff_duration(consecutive_failures: u32, max_backoff_secs: u64) -> Duration {
        let backoff_secs = BASE_BACKOFF_SECS
            .saturating_mul(2u64.saturating_pow(consecutive_failures.saturating_sub(1)));
        Duration::from_secs(backoff_secs.min(max_backoff_secs))
    }

    /// Get all tracked relay URLs
    pub fn get_tracked_relays(&self) -> Vec<String> {
        self.health.iter().map(|entry| entry.key().clone()).collect()
    }

    /// Get a clone of the health info for a relay
    pub fn get_health(&self, relay_url: &str) -> Option<RelayHealth> {
        self.health.get(relay_url).map(|entry| entry.value().clone())
    }
}

/// Create a shared RelayHealthTracker wrapped in Arc
pub fn create_health_tracker(config: &Config) -> Arc<RelayHealthTracker> {
    Arc::new(RelayHealthTracker::new(config))
}

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

    #[test]
    fn test_health_state_display() {
        assert_eq!(HealthState::Healthy.to_string(), "healthy");
        assert_eq!(HealthState::Degraded.to_string(), "degraded");
        assert_eq!(HealthState::Dead.to_string(), "dead");
    }

    #[test]
    fn test_default_health_is_healthy() {
        let health = RelayHealth::default();
        assert_eq!(health.state, HealthState::Healthy);
        assert_eq!(health.consecutive_failures, 0);
        assert!(health.first_failure_time.is_none());
    }

    #[test]
    fn test_should_attempt_connection_new_relay() {
        let tracker = RelayHealthTracker::with_defaults();
        assert!(tracker.should_attempt_connection("wss://new-relay.example.com"));
    }

    #[test]
    fn test_record_success_resets_to_healthy() {
        let tracker = RelayHealthTracker::with_defaults();
        let url = "wss://test-relay.example.com";

        // Simulate a few failures
        tracker.record_failure(url);
        tracker.record_failure(url);
        assert_eq!(tracker.get_state(url), HealthState::Degraded);
        assert_eq!(tracker.get_failure_count(url), 2);

        // Record success
        tracker.record_success(url);
        assert_eq!(tracker.get_state(url), HealthState::Healthy);
        assert_eq!(tracker.get_failure_count(url), 0);
        assert!(tracker.should_attempt_connection(url));
    }

    #[test]
    fn test_backoff_increases_exponentially() {
        // failure 1: 5s
        assert_eq!(
            RelayHealthTracker::get_backoff_duration(1, 3600),
            Duration::from_secs(5)
        );
        // failure 2: 10s
        assert_eq!(
            RelayHealthTracker::get_backoff_duration(2, 3600),
            Duration::from_secs(10)
        );
        // failure 3: 20s
        assert_eq!(
            RelayHealthTracker::get_backoff_duration(3, 3600),
            Duration::from_secs(20)
        );
        // failure 4: 40s
        assert_eq!(
            RelayHealthTracker::get_backoff_duration(4, 3600),
            Duration::from_secs(40)
        );
        // failure 5: 80s
        assert_eq!(
            RelayHealthTracker::get_backoff_duration(5, 3600),
            Duration::from_secs(80)
        );
    }

    #[test]
    fn test_backoff_capped_at_max() {
        let max_backoff = 3600u64;
        // After many failures, should cap at max_backoff (1 hour)
        assert_eq!(
            RelayHealthTracker::get_backoff_duration(20, max_backoff),
            Duration::from_secs(max_backoff)
        );
    }

    #[test]
    fn test_degraded_state_after_failure() {
        let tracker = RelayHealthTracker::with_defaults();
        let url = "wss://test-relay.example.com";

        tracker.record_failure(url);
        assert_eq!(tracker.get_state(url), HealthState::Degraded);
        assert_eq!(tracker.get_failure_count(url), 1);
    }

    #[test]
    fn test_backoff_blocks_immediate_reconnection() {
        let tracker = RelayHealthTracker::with_defaults();
        let url = "wss://test-relay.example.com";

        tracker.record_failure(url);

        // Immediately after failure, should not attempt (backoff active)
        assert!(!tracker.should_attempt_connection(url));

        // Remaining backoff should be some positive duration
        let remaining = tracker.get_remaining_backoff(url);
        assert!(remaining.is_some());
        assert!(remaining.unwrap() > Duration::ZERO);
    }

    #[test]
    fn test_is_dead() {
        let tracker = RelayHealthTracker::with_defaults();
        let url = "wss://test-relay.example.com";

        // Initially not dead
        assert!(!tracker.is_dead(url));

        // After a failure, still not dead (just degraded)
        tracker.record_failure(url);
        assert!(!tracker.is_dead(url));
        assert_eq!(tracker.get_state(url), HealthState::Degraded);
    }

    #[test]
    fn test_get_tracked_relays() {
        let tracker = RelayHealthTracker::with_defaults();

        tracker.record_success("wss://relay1.example.com");
        tracker.record_failure("wss://relay2.example.com");

        let tracked = tracker.get_tracked_relays();
        assert_eq!(tracked.len(), 2);
        assert!(tracked.contains(&"wss://relay1.example.com".to_string()));
        assert!(tracked.contains(&"wss://relay2.example.com".to_string()));
    }

    #[test]
    fn test_custom_max_backoff() {
        let custom_max = 60u64; // 1 minute max
        let tracker = RelayHealthTracker::with_max_backoff(custom_max);
        let url = "wss://test-relay.example.com";

        // Simulate many failures
        for _ in 0..20 {
            tracker.record_failure(url);
        }

        // The remaining backoff should respect the custom max
        // Note: We can't easily test the internal backoff calculation here,
        // but we can verify the tracker was created with the custom setting
        assert_eq!(tracker.max_backoff_secs, custom_max);
    }

    #[test]
    fn test_get_health_returns_clone() {
        let tracker = RelayHealthTracker::with_defaults();
        let url = "wss://test-relay.example.com";

        tracker.record_success(url);
        let health = tracker.get_health(url);

        assert!(health.is_some());
        let health = health.unwrap();
        assert_eq!(health.state, HealthState::Healthy);
        assert!(health.last_success_time.is_some());
    }

    #[test]
    fn test_get_health_nonexistent() {
        let tracker = RelayHealthTracker::with_defaults();
        let health = tracker.get_health("wss://nonexistent.example.com");
        assert!(health.is_none());
    }
}