/**
 * Shard Manager for Horizontal Scaling
 * Implements consistent hashing for data distribution across shards
 */

import { createHash } from 'crypto'
import { EventEmitter } from 'events'

export interface ShardConfig {
  shardCount?: number
  replicationFactor?: number
  virtualNodes?: number
  autoRebalance?: boolean
}

export interface Shard {
  id: string
  nodeId: string
  virtualNodes: string[]
  itemCount: number
  sizeBytes: number
  status: 'active' | 'rebalancing' | 'offline'
}

export interface ShardAssignment {
  shardId: string
  nodeId: string
  replicas: string[]
}

/**
 * Consistent Hash Ring for shard distribution
 */
class ConsistentHashRing {
  private ring: Map<number, string> = new Map()
  private virtualNodes: number
  private sortedKeys: number[] = []

  constructor(virtualNodes: number = 150) {
    this.virtualNodes = virtualNodes
  }

  /**
   * Add a node to the hash ring
   */
  addNode(nodeId: string): void {
    for (let i = 0; i < this.virtualNodes; i++) {
      const virtualNodeId = `${nodeId}:${i}`
      const hash = this.hash(virtualNodeId)
      this.ring.set(hash, nodeId)
    }
    this.updateSortedKeys()
  }

  /**
   * Remove a node from the hash ring
   */
  removeNode(nodeId: string): void {
    const keysToRemove: number[] = []
    
    for (const [hash, node] of this.ring) {
      if (node === nodeId) {
        keysToRemove.push(hash)
      }
    }
    
    for (const key of keysToRemove) {
      this.ring.delete(key)
    }
    
    this.updateSortedKeys()
  }

  /**
   * Get the node responsible for a given key
   */
  getNode(key: string): string | null {
    if (this.ring.size === 0) return null
    
    const hash = this.hash(key)
    
    // Find the first node with hash >= key hash
    for (const nodeHash of this.sortedKeys) {
      if (nodeHash >= hash) {
        return this.ring.get(nodeHash) || null
      }
    }
    
    // Wrap around to the first node
    return this.ring.get(this.sortedKeys[0]) || null
  }

  /**
   * Get N nodes for replication
   */
  getNodes(key: string, count: number): string[] {
    if (this.ring.size === 0) return []
    
    const nodes = new Set<string>()
    const hash = this.hash(key)
    
    // Start from the primary node position
    let startIdx = 0
    for (let i = 0; i < this.sortedKeys.length; i++) {
      if (this.sortedKeys[i] >= hash) {
        startIdx = i
        break
      }
    }
    
    // Collect unique nodes
    let idx = startIdx
    while (nodes.size < count && nodes.size < this.getUniqueNodeCount()) {
      const nodeHash = this.sortedKeys[idx % this.sortedKeys.length]
      const node = this.ring.get(nodeHash)
      if (node) {
        nodes.add(node)
      }
      idx++
    }
    
    return Array.from(nodes)
  }

  /**
   * Get unique node count
   */
  private getUniqueNodeCount(): number {
    return new Set(this.ring.values()).size
  }

  /**
   * Update sorted keys for efficient lookup
   */
  private updateSortedKeys(): void {
    this.sortedKeys = Array.from(this.ring.keys()).sort((a, b) => a - b)
  }

  /**
   * Hash function for consistent hashing
   */
  private hash(key: string): number {
    const hash = createHash('md5').update(key).digest()
    return hash.readUInt32BE(0)
  }

  /**
   * Get all nodes in the ring
   */
  getAllNodes(): string[] {
    return Array.from(new Set(this.ring.values()))
  }
}

/**
 * Shard Manager for distributing data across multiple nodes
 */
export class ShardManager extends EventEmitter {
  private hashRing: ConsistentHashRing
  private shards: Map<string, Shard> = new Map()
  private nodeToShards: Map<string, Set<string>> = new Map()
  private shardCount: number
  private replicationFactor: number
  private autoRebalance: boolean

  constructor(config: ShardConfig = {}) {
    super()
    
    this.shardCount = config.shardCount || 64
    this.replicationFactor = config.replicationFactor || 3
    this.autoRebalance = config.autoRebalance ?? true
    
    this.hashRing = new ConsistentHashRing(config.virtualNodes || 150)
    
    // Initialize shards
    this.initializeShards()
  }

  /**
   * Initialize shard configuration
   */
  private initializeShards(): void {
    for (let i = 0; i < this.shardCount; i++) {
      const shardId = `shard-${i.toString().padStart(3, '0')}`
      this.shards.set(shardId, {
        id: shardId,
        nodeId: '',
        virtualNodes: [],
        itemCount: 0,
        sizeBytes: 0,
        status: 'offline'
      })
    }
  }

  /**
   * Add a node to the cluster
   */
  addNode(nodeId: string): void {
    this.hashRing.addNode(nodeId)
    this.nodeToShards.set(nodeId, new Set())
    
    // Assign shards to the new node
    this.rebalanceShards()
    
    this.emit('nodeAdded', { nodeId })
  }

  /**
   * Remove a node from the cluster
   */
  removeNode(nodeId: string): void {
    const affectedShards = this.nodeToShards.get(nodeId) || new Set()
    
    this.hashRing.removeNode(nodeId)
    this.nodeToShards.delete(nodeId)
    
    // Reassign affected shards
    for (const shardId of affectedShards) {
      const shard = this.shards.get(shardId)
      if (shard) {
        shard.status = 'rebalancing'
      }
    }
    
    this.rebalanceShards()
    
    this.emit('nodeRemoved', { nodeId, affectedShards: Array.from(affectedShards) })
  }

  /**
   * Get shard assignment for a key
   */
  getShardForKey(key: string): ShardAssignment | null {
    const shardId = this.getShardId(key)
    const nodes = this.hashRing.getNodes(shardId, this.replicationFactor)
    
    if (nodes.length === 0) return null
    
    return {
      shardId,
      nodeId: nodes[0],
      replicas: nodes.slice(1)
    }
  }

  /**
   * Get nodes responsible for a shard
   */
  getNodesForShard(shardId: string): string[] {
    const shard = this.shards.get(shardId)
    if (!shard) return []
    
    // Return primary node and replicas
    const nodes = this.hashRing.getNodes(shardId, this.replicationFactor)
    return nodes
  }

  /**
   * Get total number of shards
   */
  getTotalShards(): number {
    return this.shardCount
  }

  /**
   * Update shard assignment to a new node
   */
  updateShardAssignment(shardId: string, newNodeId: string): void {
    const shard = this.shards.get(shardId)
    if (!shard) {
      throw new Error(`Shard ${shardId} not found`)
    }
    
    // Remove from old node
    if (shard.nodeId) {
      const oldNodeShards = this.nodeToShards.get(shard.nodeId)
      if (oldNodeShards) {
        oldNodeShards.delete(shardId)
      }
    }
    
    // Add to new node
    shard.nodeId = newNodeId
    const newNodeShards = this.nodeToShards.get(newNodeId)
    if (newNodeShards) {
      newNodeShards.add(shardId)
    } else {
      this.nodeToShards.set(newNodeId, new Set([shardId]))
    }
    
    this.emit('shardReassigned', { shardId, newNodeId })
  }

  /**
   * Get shard ID for a key
   */
  private getShardId(key: string): string {
    const hash = createHash('md5').update(key).digest()
    const shardIndex = hash.readUInt16BE(0) % this.shardCount
    return `shard-${shardIndex.toString().padStart(3, '0')}`
  }

  /**
   * Rebalance shards across nodes
   */
  private rebalanceShards(): void {
    if (!this.autoRebalance) return
    
    const nodes = this.hashRing.getAllNodes()
    if (nodes.length === 0) return
    
    // Clear current assignments
    for (const nodeSet of this.nodeToShards.values()) {
      nodeSet.clear()
    }
    
    // Reassign each shard
    for (const [shardId, shard] of this.shards) {
      const assignedNodes = this.hashRing.getNodes(shardId, 1)
      if (assignedNodes.length > 0) {
        shard.nodeId = assignedNodes[0]
        shard.status = 'active'
        
        const nodeShards = this.nodeToShards.get(assignedNodes[0])
        if (nodeShards) {
          nodeShards.add(shardId)
        }
      } else {
        shard.status = 'offline'
      }
    }
    
    this.emit('rebalanced', { nodes, shardCount: this.shardCount })
  }

  /**
   * Get shard assignment for all shards
   */
  getShardAssignments(): ShardAssignment[] {
    const assignments: ShardAssignment[] = []
    
    for (const [shardId, shard] of this.shards) {
      if (shard.nodeId) {
        assignments.push({
          shardId,
          nodeId: shard.nodeId,
          replicas: this.hashRing.getNodes(shardId, this.replicationFactor).slice(1)
        })
      }
    }
    
    return assignments
  }
  
  /**
   * Get shard statistics
   */
  getShardStats(): {
    totalShards: number
    activeShards: number
    rebalancingShards: number
    offlineShards: number
    averageItemsPerShard: number
  } {
    let activeShards = 0
    let rebalancingShards = 0
    let offlineShards = 0
    let totalItems = 0
    
    for (const shard of this.shards.values()) {
      switch (shard.status) {
        case 'active':
          activeShards++
          break
        case 'rebalancing':
          rebalancingShards++
          break
        case 'offline':
          offlineShards++
          break
      }
      totalItems += shard.itemCount
    }
    
    return {
      totalShards: this.shardCount,
      activeShards,
      rebalancingShards,
      offlineShards,
      averageItemsPerShard: this.shardCount > 0 ? Math.floor(totalItems / this.shardCount) : 0
    }
  }

  /**
   * Update shard metrics
   */
  updateShardMetrics(shardId: string, itemCount: number, sizeBytes: number): void {
    const shard = this.shards.get(shardId)
    if (shard) {
      shard.itemCount = itemCount
      shard.sizeBytes = sizeBytes
    }
  }

  /**
   * Get replication nodes for a shard
   */
  getReplicationNodes(shardId: string): string[] {
    return this.hashRing.getNodes(shardId, this.replicationFactor)
  }

  /**
   * Check if rebalancing is needed
   */
  needsRebalancing(): boolean {
    const stats = this.getShardStats()
    return stats.offlineShards > 0 || stats.rebalancingShards > 0
  }

  /**
   * Get cluster health
   */
  getHealth(): {
    healthy: boolean
    nodes: number
    shards: {
      total: number
      active: number
      inactive: number
    }
  } {
    const nodes = this.hashRing.getAllNodes()
    const stats = this.getShardStats()
    
    return {
      healthy: stats.activeShards >= this.shardCount * 0.9, // 90% shards active
      nodes: nodes.length,
      shards: {
        total: this.shardCount,
        active: stats.activeShards,
        inactive: stats.offlineShards + stats.rebalancingShards
      }
    }
  }
}

/**
 * Create a shard manager instance
 */
export function createShardManager(config?: ShardConfig): ShardManager {
  return new ShardManager(config)
}