/*
 * Distributed Cache System with Consistent Hashing
 * Challenge: Design and implement a distributed caching system with fault tolerance
 * 
 * TODO for GitHub Copilot:
 * 1. Complete the DistributedCache implementation with consistent hashing
 * 2. Add replication and fault tolerance mechanisms
 * 3. Implement TTL support with multiple eviction policies
 * 4. Create comprehensive monitoring and metrics collection
 * 5. Add dynamic node management and rebalancing
 * 
 * Expected Copilot prompts:
 * - "Implement consistent hashing algorithm for distributed cache nodes"
 * - "Add replication strategy with fault tolerance and automatic failover"
 * - "Create TTL support with LRU, LFU, and time-based eviction policies"
 * - "Implement comprehensive monitoring with performance metrics"
 * - "Add dynamic node addition/removal with automatic rebalancing"
 */

package main

import (
    "context"
    "crypto/md5"
    "encoding/binary"
    "fmt"
    "hash/crc32"
    "log"
    "sort"
    "sync"
    "time"
)

// Node represents a cache node in the distributed system
type Node struct {
    ID          string
    Address     string
    IsAlive     bool
    LastSeen    time.Time
    LoadFactor  float64
    Connections int32
}

// CacheItem represents an item stored in the cache
type CacheItem struct {
    Key        string
    Value      []byte
    TTL        time.Duration
    CreatedAt  time.Time
    AccessedAt time.Time
    AccessCount int64
    Size       int64
}

// EvictionPolicy defines the cache eviction strategy
type EvictionPolicy int

const (
    LRU EvictionPolicy = iota
    LFU
    TTL
    FIFO
)

// ReplicationStrategy defines how data is replicated
type ReplicationStrategy struct {
    ReplicationFactor int
    ConsistencyLevel  ConsistencyLevel
    ReadQuorum       int
    WriteQuorum      int
}

type ConsistencyLevel int

const (
    Eventual ConsistencyLevel = iota
    Strong
    Quorum
)

// CacheMetrics holds performance and health metrics
type CacheMetrics struct {
    TotalRequests    int64
    CacheHits        int64
    CacheMisses      int64
    Evictions        int64
    NetworkErrors    int64
    AverageLatency   time.Duration
    TotalMemoryUsage int64
    NodeCount        int
    ReplicationLag   time.Duration
}

// DistributedCache main cache implementation
// TODO: Implement complete distributed cache with Copilot assistance
type DistributedCache struct {
    nodes           []Node
    ring            []uint32
    nodeMap         map[uint32]string
    virtualNodes    int
    data            map[string]CacheItem
    replicas        ReplicationStrategy
    evictionPolicy  EvictionPolicy
    maxMemory       int64
    currentMemory   int64
    metrics         *CacheMetrics
    mutex           sync.RWMutex
    healthChecker   *HealthChecker
    rebalancer      *Rebalancer
    monitoring      *MonitoringService
}

// ConsistentHashRing manages the consistent hashing ring
// TODO: Implement consistent hashing with virtual nodes
type ConsistentHashRing struct {
    ring         []uint32
    nodeMap      map[uint32]string
    virtualNodes int
    mutex        sync.RWMutex
}

// HealthChecker monitors node health and handles failures
// TODO: Implement health checking and failure detection
type HealthChecker struct {
    checkInterval time.Duration
    timeout       time.Duration
    failureThreshold int
    nodes         map[string]*NodeHealth
    mutex         sync.RWMutex
}

type NodeHealth struct {
    Node           *Node
    FailureCount   int
    LastCheck      time.Time
    ResponseTimes  []time.Duration
}

// Rebalancer handles data redistribution when nodes change
// TODO: Implement automatic rebalancing with Copilot assistance
type Rebalancer struct {
    cache          *DistributedCache
    rebalancing    bool
    mutex          sync.Mutex
}

// MonitoringService collects and reports metrics
// TODO: Implement comprehensive monitoring with Copilot assistance
type MonitoringService struct {
    metrics        *CacheMetrics
    alertThresholds map[string]float64
    mutex          sync.RWMutex
}

// NewDistributedCache creates a new distributed cache instance
// TODO: Initialize all cache components with Copilot assistance
func NewDistributedCache(config CacheConfig) *DistributedCache {
    // TODO: Implement with Copilot assistance
    // - Initialize consistent hash ring
    // - Set up replication strategy
    // - Configure eviction policies
    // - Initialize monitoring
    // - Start health checking
    cache := &DistributedCache{
        virtualNodes:   config.VirtualNodes,
        replicas:       config.Replication,
        evictionPolicy: config.EvictionPolicy,
        maxMemory:      config.MaxMemory,
        data:           make(map[string]CacheItem),
        metrics:        &CacheMetrics{},
    }
    
    // TODO: Initialize components
    
    return cache
}

// Get retrieves a value from the cache
// TODO: Implement distributed get with replication and consistency
func (dc *DistributedCache) Get(ctx context.Context, key string) ([]byte, error) {
    // TODO: Implement with Copilot assistance
    // - Hash key to find responsible nodes
    // - Handle replication reads
    // - Apply consistency level requirements
    // - Update access statistics
    // - Handle node failures gracefully
    // - Update metrics
    return nil, nil
}

// Put stores a value in the cache
// TODO: Implement distributed put with replication
func (dc *DistributedCache) Put(ctx context.Context, key string, value []byte, ttl time.Duration) error {
    // TODO: Implement with Copilot assistance
    // - Hash key to find responsible nodes
    // - Replicate to multiple nodes
    // - Handle write quorum requirements
    // - Check memory limits and evict if needed
    // - Update metrics
    // - Handle failures and retries
    return nil
}

// Delete removes a value from the cache
// TODO: Implement distributed delete with replication
func (dc *DistributedCache) Delete(ctx context.Context, key string) error {
    // TODO: Implement with Copilot assistance
    // - Find all replicas of the key
    // - Delete from all replica nodes
    // - Handle partial failures
    // - Update metrics
    return nil
}

// AddNode adds a new node to the cache cluster
// TODO: Implement dynamic node addition with rebalancing
func (dc *DistributedCache) AddNode(node Node) error {
    // TODO: Implement with Copilot assistance
    // - Add node to consistent hash ring
    // - Trigger data rebalancing
    // - Update replication mappings
    // - Start health monitoring for new node
    // - Update metrics
    return nil
}

// RemoveNode removes a node from the cache cluster
// TODO: Implement node removal with data migration
func (dc *DistributedCache) RemoveNode(nodeID string) error {
    // TODO: Implement with Copilot assistance
    // - Remove node from hash ring
    // - Migrate data to other nodes
    // - Update replication mappings
    // - Stop health monitoring
    // - Update metrics
    return nil
}

// NewConsistentHashRing creates a new consistent hash ring
// TODO: Implement consistent hashing algorithm
func NewConsistentHashRing(virtualNodes int) *ConsistentHashRing {
    // TODO: Implement with Copilot assistance
    return &ConsistentHashRing{
        virtualNodes: virtualNodes,
        nodeMap:      make(map[uint32]string),
    }
}

// AddNode adds a node to the hash ring
// TODO: Implement node addition to hash ring
func (chr *ConsistentHashRing) AddNode(nodeID string) {
    // TODO: Implement with Copilot assistance
    // - Create virtual nodes for the physical node
    // - Hash virtual node identifiers
    // - Add to ring and sort
    // - Update node mapping
}

// RemoveNode removes a node from the hash ring
// TODO: Implement node removal from hash ring
func (chr *ConsistentHashRing) RemoveNode(nodeID string) {
    // TODO: Implement with Copilot assistance
    // - Find all virtual nodes for the physical node
    // - Remove from ring
    // - Update mappings
    // - Resort ring
}

// GetNodes returns the nodes responsible for a key
// TODO: Implement key-to-node mapping with replication
func (chr *ConsistentHashRing) GetNodes(key string, replicationFactor int) []string {
    // TODO: Implement with Copilot assistance
    // - Hash the key
    // - Find position on ring
    // - Get next N nodes for replication
    // - Handle ring wraparound
    // - Return node list
    return nil
}

// hash computes hash value for consistent hashing
// TODO: Implement consistent hash function
func (chr *ConsistentHashRing) hash(key string) uint32 {
    // TODO: Implement with Copilot assistance
    // - Use consistent hash function (CRC32, MD5, etc.)
    // - Ensure uniform distribution
    return crc32.ChecksumIEEE([]byte(key))
}

// NewHealthChecker creates a new health checker
// TODO: Initialize health monitoring system
func NewHealthChecker(cache *DistributedCache) *HealthChecker {
    // TODO: Implement with Copilot assistance
    hc := &HealthChecker{
        checkInterval:    30 * time.Second,
        timeout:         5 * time.Second,
        failureThreshold: 3,
        nodes:           make(map[string]*NodeHealth),
    }
    
    // TODO: Start health checking goroutine
    
    return hc
}

// CheckNodeHealth performs health check on a node
// TODO: Implement node health checking
func (hc *HealthChecker) CheckNodeHealth(node *Node) bool {
    // TODO: Implement with Copilot assistance
    // - Send health check request
    // - Measure response time
    // - Update failure count
    // - Mark node as alive/dead
    // - Trigger failover if needed
    return true
}

// HandleNodeFailure handles when a node fails
// TODO: Implement failure handling and recovery
func (hc *HealthChecker) HandleNodeFailure(nodeID string) {
    // TODO: Implement with Copilot assistance
    // - Mark node as failed
    // - Trigger data migration
    // - Update hash ring
    // - Alert monitoring system
}

// NewRebalancer creates a new data rebalancer
// TODO: Initialize rebalancing system
func NewRebalancer(cache *DistributedCache) *Rebalancer {
    // TODO: Implement with Copilot assistance
    return &Rebalancer{
        cache: cache,
    }
}

// RebalanceData redistributes data after cluster changes
// TODO: Implement data rebalancing algorithm
func (r *Rebalancer) RebalanceData() error {
    // TODO: Implement with Copilot assistance
    // - Calculate new data distribution
    // - Move data between nodes
    // - Handle replication during move
    // - Ensure data consistency
    // - Update metrics
    return nil
}

// NewMonitoringService creates a new monitoring service
// TODO: Initialize monitoring and metrics collection
func NewMonitoringService() *MonitoringService {
    // TODO: Implement with Copilot assistance
    return &MonitoringService{
        metrics:         &CacheMetrics{},
        alertThresholds: make(map[string]float64),
    }
}

// UpdateMetrics updates cache performance metrics
// TODO: Implement metrics collection and aggregation
func (ms *MonitoringService) UpdateMetrics(operation string, latency time.Duration, success bool) {
    // TODO: Implement with Copilot assistance
    // - Update operation counters
    // - Calculate rolling averages
    // - Track error rates
    // - Update latency histograms
    // - Check alert thresholds
}

// GetMetrics returns current cache metrics
// TODO: Implement metrics retrieval and formatting
func (ms *MonitoringService) GetMetrics() CacheMetrics {
    // TODO: Implement with Copilot assistance
    ms.mutex.RLock()
    defer ms.mutex.RUnlock()
    return *ms.metrics
}

// evictLRU evicts least recently used items
// TODO: Implement LRU eviction policy
func (dc *DistributedCache) evictLRU(bytesToFree int64) {
    // TODO: Implement with Copilot assistance
    // - Sort items by access time
    // - Remove oldest items
    // - Update memory usage
    // - Update metrics
}

// evictLFU evicts least frequently used items
// TODO: Implement LFU eviction policy
func (dc *DistributedCache) evictLFU(bytesToFree int64) {
    // TODO: Implement with Copilot assistance
    // - Sort items by access count
    // - Remove least accessed items
    // - Update memory usage
    // - Update metrics
}

// evictTTL evicts expired items
// TODO: Implement TTL-based eviction
func (dc *DistributedCache) evictTTL() {
    // TODO: Implement with Copilot assistance
    // - Find expired items
    // - Remove from cache
    // - Update memory usage
    // - Update metrics
}

// CacheConfig holds cache configuration
type CacheConfig struct {
    VirtualNodes     int
    Replication      ReplicationStrategy
    EvictionPolicy   EvictionPolicy
    MaxMemory        int64
    HealthCheckInterval time.Duration
}

// TODO: Add comprehensive test suite and benchmarks
func TestDistributedCache() {
    // TODO: Implement comprehensive tests with Copilot assistance
    // - Test consistent hashing distribution
    // - Test replication and consistency
    // - Test node failure scenarios
    // - Test eviction policies
    // - Performance benchmarks
    // - Stress testing
}

// TODO: Add usage examples and integration guides
func main() {
    // TODO: Implement example usage with Copilot assistance
    // - Create cache configuration
    // - Initialize distributed cache
    // - Add nodes to cluster
    // - Perform cache operations
    // - Monitor metrics and health
    
    config := CacheConfig{
        VirtualNodes: 150,
        Replication: ReplicationStrategy{
            ReplicationFactor: 3,
            ConsistencyLevel:  Quorum,
            ReadQuorum:       2,
            WriteQuorum:      2,
        },
        EvictionPolicy: LRU,
        MaxMemory:      1024 * 1024 * 1024, // 1GB
    }
    
    cache := NewDistributedCache(config)
    
    // TODO: Add example operations
    
    log.Println("Distributed cache started successfully")
}
