//! Transaction routing to shards.
//!
//! Routes transactions to appropriate shards based on account addresses.

use std::collections::HashMap;

use serde::{Deserialize, Serialize};
use synor_types::Hash256;

use crate::{ShardConfig, ShardId};

/// Routing table entry.
#[derive(Clone, Debug, Serialize, Deserialize)]
pub struct RoutingEntry {
    /// Address prefix.
    pub prefix: Vec<u8>,
    /// Assigned shard.
    pub shard_id: ShardId,
    /// Entry weight (for load balancing).
    pub weight: u32,
}

/// Routing table for shard assignment.
#[derive(Clone, Debug)]
pub struct RoutingTable {
    /// Entries sorted by prefix.
    entries: Vec<RoutingEntry>,
    /// Cache for recent lookups.
    cache: HashMap<Hash256, ShardId>,
    /// Max cache size.
    max_cache_size: usize,
}

impl RoutingTable {
    /// Creates a new routing table.
    pub fn new() -> Self {
        Self {
            entries: Vec::new(),
            cache: HashMap::new(),
            max_cache_size: 10000,
        }
    }

    /// Adds a routing entry.
    pub fn add_entry(&mut self, entry: RoutingEntry) {
        self.entries.push(entry);
        self.entries.sort_by(|a, b| a.prefix.cmp(&b.prefix));
    }

    /// Looks up the shard for an address.
    pub fn lookup(&mut self, address: &Hash256) -> Option<ShardId> {
        // Check cache first
        if let Some(&shard) = self.cache.get(address) {
            return Some(shard);
        }

        // Search entries
        let addr_bytes = address.as_bytes();
        for entry in &self.entries {
            if addr_bytes.starts_with(&entry.prefix) {
                // Update cache
                if self.cache.len() < self.max_cache_size {
                    self.cache.insert(*address, entry.shard_id);
                }
                return Some(entry.shard_id);
            }
        }

        None
    }

    /// Clears the cache.
    pub fn clear_cache(&mut self) {
        self.cache.clear();
    }
}

impl Default for RoutingTable {
    fn default() -> Self {
        Self::new()
    }
}

/// Routes transactions to shards.
pub struct ShardRouter {
    /// Configuration.
    config: ShardConfig,
    /// Custom routing table (optional).
    routing_table: Option<RoutingTable>,
    /// Shard load (for load balancing).
    shard_load: HashMap<ShardId, u64>,
}

impl ShardRouter {
    /// Creates a new router with the given configuration.
    pub fn new(config: ShardConfig) -> Self {
        let mut shard_load = HashMap::new();
        for i in 0..config.num_shards {
            shard_load.insert(i, 0);
        }

        Self {
            config,
            routing_table: None,
            shard_load,
        }
    }

    /// Sets a custom routing table.
    pub fn with_routing_table(mut self, table: RoutingTable) -> Self {
        self.routing_table = Some(table);
        self
    }

    /// Routes an address to a shard.
    pub fn route(&self, address: &Hash256) -> ShardId {
        // Check custom routing table first
        if let Some(ref table) = self.routing_table {
            let mut table = table.clone();
            if let Some(shard) = table.lookup(address) {
                return shard;
            }
        }

        // Default: hash-based routing
        self.config.shard_for_address(address)
    }

    /// Routes a transaction with sender and receiver.
    /// Returns (sender_shard, receiver_shard, is_cross_shard).
    pub fn route_transaction(
        &self,
        sender: &Hash256,
        receiver: &Hash256,
    ) -> (ShardId, ShardId, bool) {
        let sender_shard = self.route(sender);
        let receiver_shard = self.route(receiver);
        let is_cross_shard = sender_shard != receiver_shard;

        (sender_shard, receiver_shard, is_cross_shard)
    }

    /// Records transaction load on a shard.
    pub fn record_transaction(&mut self, shard_id: ShardId) {
        if let Some(load) = self.shard_load.get_mut(&shard_id) {
            *load += 1;
        }
    }

    /// Resets load counters.
    pub fn reset_load(&mut self) {
        for load in self.shard_load.values_mut() {
            *load = 0;
        }
    }

    /// Gets the current load for a shard.
    pub fn get_load(&self, shard_id: ShardId) -> u64 {
        self.shard_load.get(&shard_id).copied().unwrap_or(0)
    }

    /// Gets the least loaded shard.
    pub fn least_loaded_shard(&self) -> ShardId {
        self.shard_load
            .iter()
            .min_by_key(|(_, &load)| load)
            .map(|(&id, _)| id)
            .unwrap_or(0)
    }

    /// Gets the most loaded shard.
    pub fn most_loaded_shard(&self) -> ShardId {
        self.shard_load
            .iter()
            .max_by_key(|(_, &load)| load)
            .map(|(&id, _)| id)
            .unwrap_or(0)
    }

    /// Checks if load is imbalanced (for resharding trigger).
    pub fn is_load_imbalanced(&self, threshold: f64) -> bool {
        if self.shard_load.is_empty() {
            return false;
        }

        let loads: Vec<u64> = self.shard_load.values().copied().collect();
        let avg = loads.iter().sum::<u64>() as f64 / loads.len() as f64;

        if avg == 0.0 {
            return false;
        }

        // Check if any shard is significantly above average
        loads.iter().any(|&load| (load as f64 / avg) > threshold)
    }

    /// Returns load distribution stats.
    pub fn load_stats(&self) -> LoadStats {
        let loads: Vec<u64> = self.shard_load.values().copied().collect();
        let total: u64 = loads.iter().sum();
        let avg = if loads.is_empty() {
            0.0
        } else {
            total as f64 / loads.len() as f64
        };
        let min = loads.iter().min().copied().unwrap_or(0);
        let max = loads.iter().max().copied().unwrap_or(0);

        LoadStats {
            total_transactions: total,
            average_per_shard: avg,
            min_load: min,
            max_load: max,
            num_shards: loads.len(),
        }
    }
}

/// Load distribution statistics.
#[derive(Clone, Debug)]
pub struct LoadStats {
    /// Total transactions routed.
    pub total_transactions: u64,
    /// Average per shard.
    pub average_per_shard: f64,
    /// Minimum shard load.
    pub min_load: u64,
    /// Maximum shard load.
    pub max_load: u64,
    /// Number of shards.
    pub num_shards: usize,
}

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

    #[test]
    fn test_routing_deterministic() {
        let config = ShardConfig::with_shards(32);
        let router = ShardRouter::new(config);

        let addr = Hash256::from_bytes([1u8; 32]);
        let shard1 = router.route(&addr);
        let shard2 = router.route(&addr);

        assert_eq!(shard1, shard2);
    }

    #[test]
    fn test_routing_distribution() {
        let config = ShardConfig::with_shards(8);
        let router = ShardRouter::new(config);

        let mut counts = [0u32; 8];
        for i in 0..1000u32 {
            let mut bytes = [0u8; 32];
            bytes[0..4].copy_from_slice(&i.to_le_bytes());
            let addr = Hash256::from_bytes(bytes);
            let shard = router.route(&addr) as usize;
            counts[shard] += 1;
        }

        // Check all shards have some load
        for (i, count) in counts.iter().enumerate() {
            assert!(*count > 0, "Shard {} has no transactions", i);
        }
    }

    #[test]
    fn test_cross_shard_detection() {
        let config = ShardConfig::with_shards(32);
        let router = ShardRouter::new(config);

        // Same shard
        let sender = Hash256::from_bytes([1u8; 32]);
        let receiver = Hash256::from_bytes([1u8; 32]); // Same address
        let (_, _, is_cross) = router.route_transaction(&sender, &receiver);
        assert!(!is_cross);

        // Different shards
        let mut other_bytes = [0u8; 32];
        other_bytes[0] = 255; // Different first bytes
        let other = Hash256::from_bytes(other_bytes);
        let sender_shard = router.route(&sender);
        let other_shard = router.route(&other);
        if sender_shard != other_shard {
            let (_, _, is_cross) = router.route_transaction(&sender, &other);
            assert!(is_cross);
        }
    }

    #[test]
    fn test_load_tracking() {
        let config = ShardConfig::with_shards(4);
        let mut router = ShardRouter::new(config);

        router.record_transaction(0);
        router.record_transaction(0);
        router.record_transaction(1);

        assert_eq!(router.get_load(0), 2);
        assert_eq!(router.get_load(1), 1);
        assert_eq!(router.least_loaded_shard(), 2); // or 3, both have 0
        assert_eq!(router.most_loaded_shard(), 0);
    }

    #[test]
    fn test_load_imbalance() {
        let config = ShardConfig::with_shards(4);
        let mut router = ShardRouter::new(config);

        // Even load
        for _ in 0..10 {
            router.record_transaction(0);
            router.record_transaction(1);
            router.record_transaction(2);
            router.record_transaction(3);
        }
        assert!(!router.is_load_imbalanced(2.0));

        // Reset and create imbalance
        router.reset_load();
        for _ in 0..100 {
            router.record_transaction(0);
        }
        router.record_transaction(1);
        assert!(router.is_load_imbalanced(2.0));
    }
}