//! Bounded, sliding-TTL protocol detection cache with periodic re-probing and failure suppression.
//!
//! Caches the detected protocol (H1, H2, or H3) per backend endpoint and requested
//! domain (host:port + requested_host). This prevents cache oscillation when multiple
//! frontend domains share the same backend but differ in protocol support.
//!
//! ## Sliding TTL
//!
//! Each cache hit refreshes the entry's expiry timer (`last_accessed_at`). Entries
//! remain valid for up to 1 day of continuous use. Every 5 minutes, the next request
//! triggers an inline ALPN re-probe to verify the cached protocol is still correct.
//!
//! ## Upgrade signals
//!
//! - ALPN (TLS handshake) → detects H2 vs H1
//! - Alt-Svc (response header) → advertises H3
//!
//! ## Protocol transitions
//!
//! All protocol changes are logged at `info!()` level with the reason:
//! "Protocol transition: H1 → H2 because periodic ALPN re-probe"
//!
//! ## Failure suppression
//!
//! When a protocol fails, `record_failure()` prevents upgrade signals from
//! re-introducing it until an escalating cooldown expires (5s → 10s → ... → 300s).
//! Within-request escalation is allowed via `can_retry()` after a 5s minimum gap.
//!
//! ## Total failure eviction
//!
//! When all protocols (H3, H2, H1) fail for a backend, the cache entry is evicted
//! entirely via `evict()`, forcing a fresh probe on the next request.
//!
//! Cascading: when a lower protocol also fails, higher protocol cooldowns are
//! reduced to 5s remaining (not instant clear), preventing tight retry loops.

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

use dashmap::DashMap;
use tracing::{debug, info};

/// Sliding TTL for cached protocol detection results.
/// Entries that haven't been accessed for this duration are evicted.
/// Each `get()` call refreshes the timer (sliding window).
const PROTOCOL_CACHE_TTL: Duration = Duration::from_secs(86400); // 1 day

/// Interval between inline ALPN re-probes for H1/H2 entries.
/// When a cached entry's `last_probed_at` exceeds this, the next request
/// triggers an ALPN re-probe to verify the backend still speaks the same protocol.
const PROTOCOL_REPROBE_INTERVAL: Duration = Duration::from_secs(300); // 5 minutes

/// Maximum number of entries in the protocol cache.
const PROTOCOL_CACHE_MAX_ENTRIES: usize = 4096;

/// Background cleanup interval.
const PROTOCOL_CACHE_CLEANUP_INTERVAL: Duration = Duration::from_secs(60);

/// Minimum cooldown between retry attempts of a failed protocol.
const PROTOCOL_FAILURE_COOLDOWN: Duration = Duration::from_secs(5);

/// Maximum cooldown (escalation ceiling).
const PROTOCOL_FAILURE_MAX_COOLDOWN: Duration = Duration::from_secs(300);

/// Consecutive failure count at which cooldown reaches maximum.
/// 5s × 2^5 = 160s, 5s × 2^6 = 320s → capped at 300s.
const PROTOCOL_FAILURE_ESCALATION_CAP: u32 = 6;

/// Detected backend protocol.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum DetectedProtocol {
    H1,
    H2,
    H3,
}

impl std::fmt::Display for DetectedProtocol {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            DetectedProtocol::H1 => write!(f, "H1"),
            DetectedProtocol::H2 => write!(f, "H2"),
            DetectedProtocol::H3 => write!(f, "H3"),
        }
    }
}

/// Result of a protocol cache lookup.
#[derive(Debug, Clone, Copy)]
pub struct CachedProtocol {
    pub protocol: DetectedProtocol,
    /// For H3: the port advertised by Alt-Svc (may differ from TCP port).
    pub h3_port: Option<u16>,
    /// True if the entry's `last_probed_at` exceeds `PROTOCOL_REPROBE_INTERVAL`.
    /// Caller should perform an inline ALPN re-probe and call `update_probe_result()`.
    /// Always `false` for H3 entries (H3 is discovered via Alt-Svc, not ALPN).
    pub needs_reprobe: bool,
}

/// Key for the protocol cache: (host, port, requested_host).
#[derive(Clone, Debug, Hash, Eq, PartialEq)]
pub struct ProtocolCacheKey {
    pub host: String,
    pub port: u16,
    /// The incoming request's domain (Host header / :authority).
    /// Distinguishes protocol detection when multiple domains share the same backend.
    pub requested_host: Option<String>,
}

/// A cached protocol detection result with timestamps.
struct CachedEntry {
    protocol: DetectedProtocol,
    /// When this protocol was first detected (or last changed).
    detected_at: Instant,
    /// Last time any request used this entry (sliding-window TTL).
    last_accessed_at: Instant,
    /// Last time an ALPN re-probe was performed for this entry.
    last_probed_at: Instant,
    /// For H3: the port advertised by Alt-Svc (may differ from TCP port).
    h3_port: Option<u16>,
}

/// Failure record for a single protocol level.
#[derive(Debug, Clone)]
struct FailureRecord {
    /// When the failure was last recorded.
    failed_at: Instant,
    /// Current cooldown duration. Escalates on consecutive failures.
    cooldown: Duration,
    /// Number of consecutive failures (for escalation).
    consecutive_failures: u32,
}

/// Per-key failure state. Tracks failures at each upgradeable protocol level.
/// H1 is never tracked (it's the protocol floor — nothing to fall back to).
#[derive(Debug, Clone, Default)]
struct FailureState {
    h2: Option<FailureRecord>,
    h3: Option<FailureRecord>,
}

impl FailureState {
    fn is_empty(&self) -> bool {
        self.h2.is_none() && self.h3.is_none()
    }

    fn all_expired(&self) -> bool {
        let h2_expired = self
            .h2
            .as_ref()
            .map(|r| r.failed_at.elapsed() >= r.cooldown)
            .unwrap_or(true);
        let h3_expired = self
            .h3
            .as_ref()
            .map(|r| r.failed_at.elapsed() >= r.cooldown)
            .unwrap_or(true);
        h2_expired && h3_expired
    }

    fn get(&self, protocol: DetectedProtocol) -> Option<&FailureRecord> {
        match protocol {
            DetectedProtocol::H2 => self.h2.as_ref(),
            DetectedProtocol::H3 => self.h3.as_ref(),
            DetectedProtocol::H1 => None,
        }
    }

    fn get_mut(&mut self, protocol: DetectedProtocol) -> &mut Option<FailureRecord> {
        match protocol {
            DetectedProtocol::H2 => &mut self.h2,
            DetectedProtocol::H3 => &mut self.h3,
            DetectedProtocol::H1 => unreachable!("H1 failures are never recorded"),
        }
    }
}

/// Snapshot of a single protocol cache entry, suitable for metrics/UI display.
#[derive(Debug, Clone)]
pub struct ProtocolCacheEntry {
    pub host: String,
    pub port: u16,
    pub domain: Option<String>,
    pub protocol: String,
    pub h3_port: Option<u16>,
    pub age_secs: u64,
    pub last_accessed_secs: u64,
    pub last_probed_secs: u64,
    pub h2_suppressed: bool,
    pub h3_suppressed: bool,
    pub h2_cooldown_remaining_secs: Option<u64>,
    pub h3_cooldown_remaining_secs: Option<u64>,
    pub h2_consecutive_failures: Option<u32>,
    pub h3_consecutive_failures: Option<u32>,
}

/// Exponential backoff: PROTOCOL_FAILURE_COOLDOWN × 2^(n-1), capped at MAX.
fn escalate_cooldown(consecutive: u32) -> Duration {
    let base = PROTOCOL_FAILURE_COOLDOWN.as_secs();
    let exp = consecutive.saturating_sub(1).min(63) as u64;
    let secs = base.saturating_mul(1u64.checked_shl(exp as u32).unwrap_or(u64::MAX));
    Duration::from_secs(secs.min(PROTOCOL_FAILURE_MAX_COOLDOWN.as_secs()))
}

/// Bounded, sliding-TTL protocol detection cache with failure suppression.
///
/// Memory safety guarantees:
/// - Hard cap at `PROTOCOL_CACHE_MAX_ENTRIES` — cannot grow unboundedly.
/// - Sliding TTL expiry — entries age out after 1 day without access.
/// - Background cleanup task — proactively removes expired entries every 60s.
/// - `clear()` — called on route updates to discard stale detections.
/// - `Drop` — aborts the background task to prevent dangling tokio tasks.
pub struct ProtocolCache {
    cache: Arc<DashMap<ProtocolCacheKey, CachedEntry>>,
    /// Generic protocol failure suppression map. Tracks per-protocol failure
    /// records (H2, H3) for each cache key. Used to prevent upgrade signals
    /// (ALPN, Alt-Svc) from re-introducing failed protocols.
    failures: Arc<DashMap<ProtocolCacheKey, FailureState>>,
    cleanup_handle: Option<tokio::task::JoinHandle<()>>,
}

impl ProtocolCache {
    /// Create a new protocol cache and start the background cleanup task.
    pub fn new() -> Self {
        let cache: Arc<DashMap<ProtocolCacheKey, CachedEntry>> = Arc::new(DashMap::new());
        let failures: Arc<DashMap<ProtocolCacheKey, FailureState>> = Arc::new(DashMap::new());
        let cache_clone = Arc::clone(&cache);
        let failures_clone = Arc::clone(&failures);
        let cleanup_handle = tokio::spawn(async move {
            Self::cleanup_loop(cache_clone, failures_clone).await;
        });

        Self {
            cache,
            failures,
            cleanup_handle: Some(cleanup_handle),
        }
    }

    /// Look up the cached protocol for a backend endpoint.
    ///
    /// Returns `None` if not cached or expired (caller should probe via ALPN).
    /// On hit, refreshes `last_accessed_at` (sliding TTL) and sets `needs_reprobe`
    /// if the entry hasn't been probed in over 5 minutes (H1/H2 only).
    pub fn get(&self, key: &ProtocolCacheKey) -> Option<CachedProtocol> {
        let mut entry = self.cache.get_mut(key)?;
        if entry.last_accessed_at.elapsed() < PROTOCOL_CACHE_TTL {
            // Refresh sliding TTL
            entry.last_accessed_at = Instant::now();

            // H3 is the ceiling — can't ALPN-probe for H3 (discovered via Alt-Svc).
            // Only H1/H2 entries trigger periodic re-probing.
            let needs_reprobe = entry.protocol != DetectedProtocol::H3
                && entry.last_probed_at.elapsed() >= PROTOCOL_REPROBE_INTERVAL;

            Some(CachedProtocol {
                protocol: entry.protocol,
                h3_port: entry.h3_port,
                needs_reprobe,
            })
        } else {
            // Expired — remove and return None to trigger re-probe
            drop(entry); // release DashMap ref before remove
            self.cache.remove(key);
            None
        }
    }

    /// Insert a detected protocol into the cache.
    /// Returns `false` if suppressed due to active failure suppression.
    ///
    /// **Key semantic**: only suppresses if the protocol being inserted matches
    /// a suppressed protocol. H1 inserts are NEVER suppressed — downgrades
    /// always succeed.
    pub fn insert(&self, key: ProtocolCacheKey, protocol: DetectedProtocol, reason: &str) -> bool {
        if self.is_suppressed(&key, protocol) {
            debug!(
                host = %key.host, port = %key.port, domain = ?key.requested_host,
                protocol = ?protocol,
                "Protocol cache insert suppressed — recent failure"
            );
            return false;
        }
        self.insert_internal(key, protocol, None, reason);
        true
    }

    /// Insert an H3 detection result with the Alt-Svc advertised port.
    /// Returns `false` if H3 is suppressed.
    pub fn insert_h3(&self, key: ProtocolCacheKey, h3_port: u16, reason: &str) -> bool {
        if self.is_suppressed(&key, DetectedProtocol::H3) {
            debug!(
                host = %key.host, port = %key.port, domain = ?key.requested_host,
                "H3 upgrade suppressed — recent failure"
            );
            return false;
        }
        self.insert_internal(key, DetectedProtocol::H3, Some(h3_port), reason);
        true
    }

    /// Update the cache after an inline ALPN re-probe completes.
    ///
    /// Always updates `last_probed_at`. If the protocol changed, logs the transition
    /// and updates the entry. Returns `Some(new_protocol)` if changed, `None` if unchanged.
    pub fn update_probe_result(
        &self,
        key: &ProtocolCacheKey,
        probed_protocol: DetectedProtocol,
        reason: &str,
    ) -> Option<DetectedProtocol> {
        if let Some(mut entry) = self.cache.get_mut(key) {
            let old_protocol = entry.protocol;
            entry.last_probed_at = Instant::now();
            entry.last_accessed_at = Instant::now();

            if old_protocol != probed_protocol {
                info!(
                    host = %key.host, port = %key.port, domain = ?key.requested_host,
                    old = %old_protocol, new = %probed_protocol, reason = %reason,
                    "Protocol transition"
                );
                entry.protocol = probed_protocol;
                entry.detected_at = Instant::now();
                // Clear h3_port if downgrading from H3
                if old_protocol == DetectedProtocol::H3 && probed_protocol != DetectedProtocol::H3 {
                    entry.h3_port = None;
                }
                return Some(probed_protocol);
            }

            debug!(
                host = %key.host, port = %key.port, domain = ?key.requested_host,
                protocol = %old_protocol, reason = %reason,
                "Re-probe confirmed — no protocol change"
            );
            None
        } else {
            // Entry was evicted between the get() and the probe completing.
            // Insert as a fresh entry.
            self.insert_internal(key.clone(), probed_protocol, None, reason);
            Some(probed_protocol)
        }
    }

    /// Record a protocol failure. Future `insert()` calls for this protocol
    /// will be suppressed until the escalating cooldown expires.
    ///
    /// Cooldown escalation: 5s → 10s → 20s → 40s → 80s → 160s → 300s.
    /// Consecutive counter resets if the previous failure is older than 2× its cooldown.
    ///
    /// Cascading: when H2 fails, H3 cooldown is reduced to 5s remaining.
    /// H1 failures are ignored (H1 is the protocol floor).
    pub fn record_failure(&self, key: ProtocolCacheKey, protocol: DetectedProtocol) {
        if protocol == DetectedProtocol::H1 {
            return; // H1 is the floor — nothing to suppress
        }

        let mut entry = self.failures.entry(key.clone()).or_default();

        let record = entry.get_mut(protocol);
        let (consecutive, new_cooldown) = match record {
            Some(existing)
                if existing.failed_at.elapsed() < existing.cooldown.saturating_mul(2) =>
            {
                // Still within the "recent" window — escalate
                let c = existing
                    .consecutive_failures
                    .saturating_add(1)
                    .min(PROTOCOL_FAILURE_ESCALATION_CAP);
                (c, escalate_cooldown(c))
            }
            _ => {
                // First failure or old failure that expired long ago — reset
                (1, PROTOCOL_FAILURE_COOLDOWN)
            }
        };

        *record = Some(FailureRecord {
            failed_at: Instant::now(),
            cooldown: new_cooldown,
            consecutive_failures: consecutive,
        });

        // Cascading: when H2 fails, reduce H3 cooldown to 5s remaining
        if protocol == DetectedProtocol::H2 {
            Self::reduce_cooldown_to(entry.h3.as_mut(), PROTOCOL_FAILURE_COOLDOWN);
        }

        info!(
            host = %key.host, port = %key.port, domain = ?key.requested_host,
            protocol = ?protocol,
            consecutive = consecutive,
            cooldown_secs = new_cooldown.as_secs(),
            "Protocol failure recorded — suppressing for {:?}", new_cooldown
        );
    }

    /// Check whether a protocol is currently suppressed for the given key.
    /// Returns `true` if the protocol failed within its cooldown period.
    /// H1 is never suppressed.
    pub fn is_suppressed(&self, key: &ProtocolCacheKey, protocol: DetectedProtocol) -> bool {
        if protocol == DetectedProtocol::H1 {
            return false;
        }
        self.failures
            .get(key)
            .and_then(|entry| {
                entry
                    .get(protocol)
                    .map(|r| r.failed_at.elapsed() < r.cooldown)
            })
            .unwrap_or(false)
    }

    /// Check whether a protocol can be retried (for within-request escalation).
    /// Returns `true` if there's no failure record OR if ≥5s have passed since
    /// the last attempt. More permissive than `is_suppressed`.
    pub fn can_retry(&self, key: &ProtocolCacheKey, protocol: DetectedProtocol) -> bool {
        if protocol == DetectedProtocol::H1 {
            return true;
        }
        match self.failures.get(key) {
            Some(entry) => match entry.get(protocol) {
                Some(r) => r.failed_at.elapsed() >= PROTOCOL_FAILURE_COOLDOWN,
                None => true, // no failure record
            },
            None => true,
        }
    }

    /// Record a retry attempt WITHOUT escalating the cooldown.
    /// Resets the `failed_at` timestamp to prevent rapid retries (5s gate).
    /// Called before an escalation attempt. If the attempt fails,
    /// `record_failure` should be called afterward with proper escalation.
    pub fn record_retry_attempt(&self, key: &ProtocolCacheKey, protocol: DetectedProtocol) {
        if protocol == DetectedProtocol::H1 {
            return;
        }
        if let Some(mut entry) = self.failures.get_mut(key) {
            if let Some(ref mut r) = entry.get_mut(protocol) {
                r.failed_at = Instant::now();
            }
        }
    }

    /// Clear the failure record for a protocol (it recovered).
    /// Called when an escalation retry succeeds.
    pub fn clear_failure(&self, key: &ProtocolCacheKey, protocol: DetectedProtocol) {
        if protocol == DetectedProtocol::H1 {
            return;
        }
        if let Some(mut entry) = self.failures.get_mut(key) {
            *entry.get_mut(protocol) = None;
            if entry.is_empty() {
                drop(entry);
                self.failures.remove(key);
            }
        }
    }

    /// Evict a cache entry entirely. Called when all protocol probes (H3, H2, H1)
    /// have failed for a backend.
    pub fn evict(&self, key: &ProtocolCacheKey) {
        self.cache.remove(key);
        self.failures.remove(key);
        info!(
            host = %key.host, port = %key.port, domain = ?key.requested_host,
            "Cache entry evicted — all protocols failed"
        );
    }

    /// Clear all entries. Called on route updates to discard stale detections.
    pub fn clear(&self) {
        self.cache.clear();
        self.failures.clear();
    }

    /// Snapshot all non-expired cache entries for metrics/UI display.
    pub fn snapshot(&self) -> Vec<ProtocolCacheEntry> {
        self.cache
            .iter()
            .filter(|entry| entry.value().last_accessed_at.elapsed() < PROTOCOL_CACHE_TTL)
            .map(|entry| {
                let key = entry.key();
                let val = entry.value();
                let failure_info = self.failures.get(key);

                let (h2_sup, h2_cd, h2_cons) =
                    Self::suppression_info(failure_info.as_deref().and_then(|f| f.h2.as_ref()));
                let (h3_sup, h3_cd, h3_cons) =
                    Self::suppression_info(failure_info.as_deref().and_then(|f| f.h3.as_ref()));

                ProtocolCacheEntry {
                    host: key.host.clone(),
                    port: key.port,
                    domain: key.requested_host.clone(),
                    protocol: match val.protocol {
                        DetectedProtocol::H1 => "h1".to_string(),
                        DetectedProtocol::H2 => "h2".to_string(),
                        DetectedProtocol::H3 => "h3".to_string(),
                    },
                    h3_port: val.h3_port,
                    age_secs: val.detected_at.elapsed().as_secs(),
                    last_accessed_secs: val.last_accessed_at.elapsed().as_secs(),
                    last_probed_secs: val.last_probed_at.elapsed().as_secs(),
                    h2_suppressed: h2_sup,
                    h3_suppressed: h3_sup,
                    h2_cooldown_remaining_secs: h2_cd,
                    h3_cooldown_remaining_secs: h3_cd,
                    h2_consecutive_failures: h2_cons,
                    h3_consecutive_failures: h3_cons,
                }
            })
            .collect()
    }

    // --- Internal helpers ---

    /// Insert a protocol detection result with an optional H3 port.
    /// Logs protocol transitions when overwriting an existing entry.
    /// No suppression check — callers must check before calling.
    fn insert_internal(
        &self,
        key: ProtocolCacheKey,
        protocol: DetectedProtocol,
        h3_port: Option<u16>,
        reason: &str,
    ) {
        // Check for existing entry to log protocol transitions
        if let Some(existing) = self.cache.get(&key) {
            if existing.protocol != protocol {
                info!(
                    host = %key.host, port = %key.port, domain = ?key.requested_host,
                    old = %existing.protocol, new = %protocol, reason = %reason,
                    "Protocol transition"
                );
            }
            drop(existing);
        }

        // Evict oldest entry if at capacity
        if self.cache.len() >= PROTOCOL_CACHE_MAX_ENTRIES && !self.cache.contains_key(&key) {
            let oldest = self
                .cache
                .iter()
                .min_by_key(|entry| entry.value().last_accessed_at)
                .map(|entry| entry.key().clone());
            if let Some(oldest_key) = oldest {
                self.cache.remove(&oldest_key);
            }
        }

        let now = Instant::now();
        self.cache.insert(
            key,
            CachedEntry {
                protocol,
                detected_at: now,
                last_accessed_at: now,
                last_probed_at: now,
                h3_port,
            },
        );
    }

    /// Reduce a failure record's remaining cooldown to `target`, if it currently
    /// has MORE than `target` remaining. Never increases cooldown.
    fn reduce_cooldown_to(record: Option<&mut FailureRecord>, target: Duration) {
        if let Some(r) = record {
            let elapsed = r.failed_at.elapsed();
            if elapsed < r.cooldown {
                let remaining = r.cooldown - elapsed;
                if remaining > target {
                    // Shrink cooldown so it expires in `target` from now
                    r.cooldown = elapsed + target;
                }
            }
        }
    }

    /// Extract suppression info from a failure record for metrics.
    fn suppression_info(record: Option<&FailureRecord>) -> (bool, Option<u64>, Option<u32>) {
        match record {
            Some(r) => {
                let elapsed = r.failed_at.elapsed();
                let suppressed = elapsed < r.cooldown;
                let remaining = if suppressed {
                    Some((r.cooldown - elapsed).as_secs())
                } else {
                    None
                };
                (suppressed, remaining, Some(r.consecutive_failures))
            }
            None => (false, None, None),
        }
    }

    /// Background cleanup loop.
    async fn cleanup_loop(
        cache: Arc<DashMap<ProtocolCacheKey, CachedEntry>>,
        failures: Arc<DashMap<ProtocolCacheKey, FailureState>>,
    ) {
        let mut interval = tokio::time::interval(PROTOCOL_CACHE_CLEANUP_INTERVAL);
        loop {
            interval.tick().await;

            // Clean expired cache entries (sliding TTL based on last_accessed_at)
            let expired: Vec<ProtocolCacheKey> = cache
                .iter()
                .filter(|entry| entry.value().last_accessed_at.elapsed() >= PROTOCOL_CACHE_TTL)
                .map(|entry| entry.key().clone())
                .collect();

            if !expired.is_empty() {
                debug!(
                    "Protocol cache cleanup: removing {} expired entries",
                    expired.len()
                );
                for key in expired {
                    cache.remove(&key);
                }
            }

            // Clean fully-expired failure entries
            let expired_failures: Vec<ProtocolCacheKey> = failures
                .iter()
                .filter(|entry| entry.value().all_expired())
                .map(|entry| entry.key().clone())
                .collect();

            if !expired_failures.is_empty() {
                debug!(
                    "Protocol cache cleanup: removing {} expired failure entries",
                    expired_failures.len()
                );
                for key in expired_failures {
                    failures.remove(&key);
                }
            }

            // Safety net: cap failures map at 2× max entries
            if failures.len() > PROTOCOL_CACHE_MAX_ENTRIES * 2 {
                let oldest: Vec<ProtocolCacheKey> = failures
                    .iter()
                    .filter(|e| e.value().all_expired())
                    .map(|e| e.key().clone())
                    .take(failures.len() - PROTOCOL_CACHE_MAX_ENTRIES)
                    .collect();
                for key in oldest {
                    failures.remove(&key);
                }
            }
        }
    }
}

impl Drop for ProtocolCache {
    fn drop(&mut self) {
        if let Some(handle) = self.cleanup_handle.take() {
            handle.abort();
        }
    }
}