//! Local spanning tree state for a node.

use std::collections::HashMap;
use std::fmt;
use std::time::Duration;

use super::{CoordEntry, ParentDeclaration, TreeCoordinate, TreeError};
use crate::time::{Instant, instant_now};
use crate::{Identity, NodeAddr};

/// Local spanning tree state for a node.
///
/// Contains this node's declaration, coordinates, and view of peers'
/// tree positions. State is bounded by O(P × D) where P is peer count
/// and D is tree depth.
pub struct TreeState {
    /// This node's NodeAddr.
    my_node_addr: NodeAddr,
    /// This node's current parent declaration.
    my_declaration: ParentDeclaration,
    /// This node's current coordinates (computed from declaration chain).
    pub(super) my_coords: TreeCoordinate,
    /// The current elected root (smallest reachable node_addr).
    pub(super) root: NodeAddr,
    /// Each peer's most recent parent declaration.
    peer_declarations: HashMap<NodeAddr, ParentDeclaration>,
    /// Each peer's full ancestry to root.
    peer_ancestry: HashMap<NodeAddr, TreeCoordinate>,
    /// Hysteresis factor for cost-based parent re-selection (0.0-1.0).
    parent_hysteresis: f64,
    /// Hold-down period after parent switch (0 = disabled).
    hold_down: Duration,
    /// Timestamp of last parent switch (for hold-down enforcement).
    last_parent_switch: Option<Instant>,
    /// Number of parent switches in current flap window.
    flap_count: u32,
    /// Start of the current flap counting window.
    flap_window_start: Option<Instant>,
    /// If dampened, suppressed until this instant.
    flap_dampening_until: Option<Instant>,
    /// Flap threshold: max switches before dampening engages.
    flap_threshold: u32,
    /// Flap window duration.
    flap_window: Duration,
    /// Dampening duration when threshold exceeded.
    flap_dampening_duration: Duration,
}

impl TreeState {
    /// Create initial tree state for a node (as root candidate).
    ///
    /// The node starts as its own root until it learns of a smaller node_addr.
    /// Initial sequence is 1 per protocol spec; timestamp is current Unix time.
    pub fn new(my_node_addr: NodeAddr) -> Self {
        let timestamp = crate::time::now_secs();
        let my_declaration = ParentDeclaration::self_root(my_node_addr, 1, timestamp);
        let my_coords = TreeCoordinate::root_with_meta(my_node_addr, 1, timestamp);

        Self {
            my_node_addr,
            my_declaration,
            my_coords,
            root: my_node_addr,
            peer_declarations: HashMap::new(),
            peer_ancestry: HashMap::new(),
            parent_hysteresis: 0.0,
            hold_down: Duration::ZERO,
            last_parent_switch: None,
            flap_count: 0,
            flap_window_start: None,
            flap_dampening_until: None,
            flap_threshold: 4,
            flap_window: Duration::from_secs(60),
            flap_dampening_duration: Duration::from_secs(120),
        }
    }

    /// Get this node's NodeAddr.
    pub fn my_node_addr(&self) -> &NodeAddr {
        &self.my_node_addr
    }

    /// Get this node's current declaration.
    pub fn my_declaration(&self) -> &ParentDeclaration {
        &self.my_declaration
    }

    /// Get this node's current coordinates.
    pub fn my_coords(&self) -> &TreeCoordinate {
        &self.my_coords
    }

    /// Get the current root.
    pub fn root(&self) -> &NodeAddr {
        &self.root
    }

    /// Check if this node is currently the root.
    pub fn is_root(&self) -> bool {
        self.root == self.my_node_addr
    }

    /// Get coordinates for a peer, if known.
    pub fn peer_coords(&self, peer_id: &NodeAddr) -> Option<&TreeCoordinate> {
        self.peer_ancestry.get(peer_id)
    }

    /// Get declaration for a peer, if known.
    pub fn peer_declaration(&self, peer_id: &NodeAddr) -> Option<&ParentDeclaration> {
        self.peer_declarations.get(peer_id)
    }

    /// Number of known peers.
    pub fn peer_count(&self) -> usize {
        self.peer_declarations.len()
    }

    /// Iterate over all peer node IDs.
    pub fn peer_ids(&self) -> impl Iterator<Item = &NodeAddr> {
        self.peer_declarations.keys()
    }

    /// Add or update a peer's tree state.
    ///
    /// Returns true if the state was updated (new or fresher declaration).
    pub fn update_peer(
        &mut self,
        declaration: ParentDeclaration,
        ancestry: TreeCoordinate,
    ) -> bool {
        let peer_id = *declaration.node_addr();

        // Check if this is a fresh update
        if let Some(existing) = self.peer_declarations.get(&peer_id)
            && !declaration.is_fresher_than(existing)
        {
            return false;
        }

        self.peer_declarations.insert(peer_id, declaration);
        self.peer_ancestry.insert(peer_id, ancestry);
        true
    }

    /// Remove a peer from the tree state.
    pub fn remove_peer(&mut self, peer_id: &NodeAddr) {
        self.peer_declarations.remove(peer_id);
        self.peer_ancestry.remove(peer_id);
    }

    /// Update this node's parent selection.
    ///
    /// Call this when switching parents. Updates the declaration and coordinates.
    /// Returns true if flap dampening was just engaged due to this switch.
    /// Only records a flap when the parent actually changes.
    pub fn set_parent(&mut self, parent_id: NodeAddr, sequence: u64, timestamp: u64) -> bool {
        let parent_changed = self.is_root() || *self.my_declaration.parent_id() != parent_id;
        self.my_declaration =
            ParentDeclaration::new(self.my_node_addr, parent_id, sequence, timestamp);
        self.last_parent_switch = Some(instant_now());
        // Record switch for flap detection only when parent actually changes;
        // coordinates will be recomputed when ancestry is available
        if parent_changed {
            self.record_parent_switch()
        } else {
            false
        }
    }

    /// Update this node's coordinates based on current parent's ancestry.
    ///
    /// Defensive: if extending the parent's ancestry would put `self` at the
    /// minimum (because `self` is smaller than the parent's root), the
    /// declaration is demoted to self-root in place. The caller is responsible
    /// for re-signing the declaration after this call (do `set_parent → recompute_coords → sign_declaration`,
    /// not `set_parent → sign_declaration → recompute_coords`).
    pub fn recompute_coords(&mut self) {
        if self.my_declaration.is_root() {
            self.my_coords = TreeCoordinate::root_with_meta(
                self.my_node_addr,
                self.my_declaration.sequence(),
                self.my_declaration.timestamp(),
            );
            self.root = self.my_node_addr;
            return;
        }

        let parent_id = self.my_declaration.parent_id();
        if let Some(parent_coords) = self.peer_ancestry.get(parent_id) {
            let parent_root = *parent_coords.root_id();
            if self.my_node_addr <= parent_root {
                // Prepending self would put a smaller-or-equal node at depth 0,
                // breaking the "advertised root = min path entry" invariant.
                // Demote to self-root rather than emit a path peers will reject.
                let seq = self.my_declaration.sequence();
                let ts = self.my_declaration.timestamp();
                self.my_declaration = ParentDeclaration::self_root(self.my_node_addr, seq, ts);
                self.my_coords = TreeCoordinate::root_with_meta(self.my_node_addr, seq, ts);
                self.root = self.my_node_addr;
                return;
            }
            // Our coords = [self_entry] ++ parent_coords entries
            let self_entry = CoordEntry::new(
                self.my_node_addr,
                self.my_declaration.sequence(),
                self.my_declaration.timestamp(),
            );
            let mut entries = vec![self_entry];
            entries.extend_from_slice(parent_coords.entries());
            self.my_coords = TreeCoordinate::new(entries).expect("non-empty path");
            self.root = *self.my_coords.root_id();
        }
    }

    /// Smallest root_id visible across known peers.
    pub fn smallest_visible_root(&self) -> Option<NodeAddr> {
        self.peer_ancestry.values().map(|c| *c.root_id()).min()
    }

    /// Whether this node should be the tree root: either there are no peers,
    /// or our NodeAddr is `<=` every visible root.
    pub fn should_be_root(&self) -> bool {
        match self.smallest_visible_root() {
            Some(sr) => self.my_node_addr <= sr,
            None => true,
        }
    }

    /// Promote self to root with an incremented sequence number.
    ///
    /// Caller must `sign_declaration` afterwards before sending the result.
    pub fn become_root(&mut self) {
        let new_seq = self.my_declaration.sequence() + 1;
        let timestamp = crate::time::now_secs();
        self.my_declaration = ParentDeclaration::self_root(self.my_node_addr, new_seq, timestamp);
        self.recompute_coords();
    }

    /// Calculate tree distance to a peer.
    pub fn distance_to_peer(&self, peer_id: &NodeAddr) -> Option<usize> {
        self.peer_ancestry
            .get(peer_id)
            .map(|coords| self.my_coords.distance_to(coords))
    }

    /// Find the best next hop toward a destination using greedy tree routing.
    ///
    /// Returns the peer that minimizes tree distance to the destination,
    /// but only if that peer is strictly closer than we are (prevents
    /// routing loops at local minima). Tie-breaks equal distance by
    /// smallest node_addr.
    ///
    /// Returns `None` if:
    /// - No peers have coordinates
    /// - Destination is in a different tree (different root)
    /// - No peer is closer to the destination than we are
    pub fn find_next_hop(&self, dest_coords: &TreeCoordinate) -> Option<NodeAddr> {
        if self.my_coords.root_id() != dest_coords.root_id() {
            return None;
        }

        let my_distance = self.my_coords.distance_to(dest_coords);

        let mut best: Option<(NodeAddr, usize)> = None;

        for (peer_id, peer_coords) in &self.peer_ancestry {
            let distance = peer_coords.distance_to(dest_coords);

            let dominated = match &best {
                None => true,
                Some((best_id, best_dist)) => {
                    distance < *best_dist || (distance == *best_dist && peer_id < best_id)
                }
            };

            if dominated {
                best = Some((*peer_id, distance));
            }
        }

        match best {
            Some((peer_id, distance)) if distance < my_distance => Some(peer_id),
            _ => None,
        }
    }

    /// Set the parent hysteresis factor (0.0-1.0).
    pub fn set_parent_hysteresis(&mut self, hysteresis: f64) {
        self.parent_hysteresis = hysteresis.clamp(0.0, 1.0);
    }

    /// Set the hold-down duration after parent switches.
    pub fn set_hold_down(&mut self, secs: u64) {
        self.hold_down = Duration::from_secs(secs);
    }

    /// Configure flap dampening parameters.
    pub fn set_flap_dampening(&mut self, threshold: u32, window_secs: u64, dampening_secs: u64) {
        self.flap_threshold = threshold;
        self.flap_window = Duration::from_secs(window_secs);
        self.flap_dampening_duration = Duration::from_secs(dampening_secs);
    }

    /// Record a parent switch for flap detection.
    /// Returns true if dampening was just engaged.
    pub fn record_parent_switch(&mut self) -> bool {
        let now = instant_now();

        // Reset window if expired or not started
        match self.flap_window_start {
            Some(start) if now.duration_since(start) < self.flap_window => {
                self.flap_count += 1;
            }
            _ => {
                self.flap_window_start = Some(now);
                self.flap_count = 1;
            }
        }

        // Check threshold
        if self.flap_count >= self.flap_threshold && self.flap_dampening_until.is_none() {
            self.flap_dampening_until = Some(now + self.flap_dampening_duration);
            return true;
        }
        false
    }

    /// Check if flap dampening is currently active.
    pub fn is_flap_dampened(&self) -> bool {
        match self.flap_dampening_until {
            Some(until) => instant_now() < until,
            None => false,
        }
    }

    /// Evaluate whether to switch parents based on current peer tree state.
    ///
    /// Uses effective_depth (depth + link_cost) for parent comparison.
    /// `peer_costs` maps each peer's NodeAddr to its link cost (from local
    /// MMP measurements). Missing entries default to 1.0 (optimistic).
    ///
    /// Returns `Some(peer_node_addr)` if a parent switch is recommended,
    /// or `None` if the current parent is adequate.
    pub fn evaluate_parent(&self, peer_costs: &HashMap<NodeAddr, f64>) -> Option<NodeAddr> {
        if self.peer_ancestry.is_empty() {
            return None;
        }

        // Find the smallest root visible across all peers
        let mut smallest_root: Option<NodeAddr> = None;
        for coords in self.peer_ancestry.values() {
            let peer_root = coords.root_id();
            smallest_root = Some(match smallest_root {
                None => *peer_root,
                Some(current) => {
                    if *peer_root < current {
                        *peer_root
                    } else {
                        current
                    }
                }
            });
        }

        let smallest_root = smallest_root?;

        // If our own NodeAddr is smaller than (or equal to) the smallest visible
        // root, we are the network's smallest node and must be root. Returning
        // `None` lets the caller promote us via `become_root` / `should_be_root`.
        // Picking any peer here would produce an invalid path, since prepending
        // `self` to that peer's ancestry would put `self` at depth 0 and the
        // peer's larger root at the tail — violating "advertised root = min path
        // entry" and getting rejected by recipients' `validate_semantics`.
        if self.my_node_addr <= smallest_root {
            return None;
        }

        // Among peers that reach the smallest root, find the lowest effective_depth.
        // effective_depth(peer) = peer.depth + link_cost_to_peer
        let mut best_peer: Option<(NodeAddr, f64)> = None; // (peer_addr, effective_depth)
        for (peer_id, coords) in &self.peer_ancestry {
            if *coords.root_id() != smallest_root {
                continue;
            }
            // Reject candidates whose ancestry contains us (would create a loop)
            if coords.contains(&self.my_node_addr) {
                continue;
            }
            // If any peer has MMP cost data, only consider measured peers.
            // This prevents freshly connected peers (no SRTT, default cost 1.0)
            // from appearing artificially cheap. During cold start (no peer has
            // MMP data, peer_costs is empty), fall back to default cost 1.0.
            let cost = match peer_costs.get(peer_id) {
                Some(&c) => c,
                None if peer_costs.is_empty() => 1.0,
                None => continue,
            };
            let eff_depth = coords.depth() as f64 + cost;
            match &best_peer {
                None => best_peer = Some((*peer_id, eff_depth)),
                Some((best_id, best_eff)) => {
                    if eff_depth < *best_eff || (eff_depth == *best_eff && peer_id < best_id) {
                        best_peer = Some((*peer_id, eff_depth));
                    }
                }
            }
        }

        let (best_peer_id, best_eff_depth) = best_peer?;

        // If already using this peer as parent, no switch needed
        if *self.my_declaration.parent_id() == best_peer_id && !self.is_root() {
            return None;
        }

        // --- Mandatory switches (bypass hold-down and hysteresis) ---

        // If our current parent is gone from peer_ancestry, our path is broken — always switch
        if !self.is_root()
            && !self
                .peer_ancestry
                .contains_key(self.my_declaration.parent_id())
        {
            return Some(best_peer_id);
        }

        // Switching roots (smaller root found) → always switch
        if smallest_root < self.root || (self.is_root() && smallest_root < self.my_node_addr) {
            return Some(best_peer_id);
        }

        // We're root but shouldn't be (peers have a smaller root) — always switch
        if self.is_root() {
            return Some(best_peer_id);
        }

        // --- Hold-down: suppress non-mandatory re-evaluation after recent switch ---

        if !self.hold_down.is_zero()
            && self
                .last_parent_switch
                .is_some_and(|last| last.elapsed() < self.hold_down)
        {
            return None;
        }

        // --- Flap dampening: suppress after excessive parent switches ---

        if self.is_flap_dampened() {
            return None;
        }

        // --- Same root, cost-aware comparison with hysteresis ---

        // Current parent's effective_depth.
        // If peer_costs is non-empty but current parent has no entry,
        // treat as maximally expensive so any measured candidate can win.
        // If peer_costs is empty (cold start), use default cost 1.0.
        let current_parent_cost = peer_costs
            .get(self.my_declaration.parent_id())
            .copied()
            .unwrap_or(if peer_costs.is_empty() {
                1.0
            } else {
                f64::INFINITY
            });
        let current_parent_coords = self.peer_ancestry.get(self.my_declaration.parent_id());
        let current_parent_eff = match current_parent_coords {
            Some(coords) => coords.depth() as f64 + current_parent_cost,
            None => return Some(best_peer_id), // Parent has no coords — treat as lost
        };

        // Apply hysteresis: only switch if candidate is significantly better
        if best_eff_depth < current_parent_eff * (1.0 - self.parent_hysteresis) {
            return Some(best_peer_id);
        }

        None
    }

    /// Handle loss of current parent.
    ///
    /// Tries to find an alternative parent among remaining peers.
    /// If none available, becomes its own root (increments sequence).
    ///
    /// Returns `true` if the tree state changed (caller should re-announce).
    pub fn handle_parent_lost(&mut self, peer_costs: &HashMap<NodeAddr, f64>) -> bool {
        // Try to find an alternative parent
        if let Some(new_parent) = self.evaluate_parent(peer_costs) {
            let timestamp = crate::time::now_secs();
            let new_seq = self.my_declaration.sequence() + 1;
            self.set_parent(new_parent, new_seq, timestamp);
            self.recompute_coords();
            return true;
        }

        // No alternative: become own root
        let timestamp = crate::time::now_secs();
        let new_seq = self.my_declaration.sequence() + 1;
        self.my_declaration = ParentDeclaration::self_root(self.my_node_addr, new_seq, timestamp);
        self.recompute_coords();
        true
    }

    /// Sign this node's declaration with the given identity.
    ///
    /// The identity's node_addr must match this TreeState's node_addr.
    pub fn sign_declaration(&mut self, identity: &Identity) -> Result<(), TreeError> {
        self.my_declaration.sign(identity)
    }

    /// Check if this node's declaration is signed.
    pub fn is_declaration_signed(&self) -> bool {
        self.my_declaration.is_signed()
    }
}

impl fmt::Debug for TreeState {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TreeState")
            .field("my_node_addr", &self.my_node_addr)
            .field("root", &self.root)
            .field("is_root", &self.is_root())
            .field("depth", &self.my_coords.depth())
            .field("peers", &self.peer_count())
            .finish()
    }
}
