package pool

import (
	"context"
	"crypto/tls"
	"net"
	"net/http"
	"sync"
	"sync/atomic"
	"time"
)

// TransportPool manages a pool of shared HTTP transports to prevent connection exhaustion
// and resource leaks. It provides centralized management of HTTP client transports with
// proper lifecycle management and security controls.
type TransportPool struct {
	mu          sync.RWMutex
	transports  map[string]*sharedTransport
	maxConns    int
	ctx         context.Context
	cancel      context.CancelFunc
	clientCount int32 // Track total HTTP clients
	maxClients  int32 // Limit total clients
}

// sharedTransport wraps an HTTP transport with reference counting
type sharedTransport struct {
	transport *http.Transport
	refCount  int32
	lastUsed  time.Time
	config    TransportConfig
}

// TransportConfig defines configuration for HTTP transports
type TransportConfig struct {
	// Timeouts
	DialTimeout           time.Duration
	TLSHandshakeTimeout   time.Duration
	ResponseHeaderTimeout time.Duration
	ExpectContinueTimeout time.Duration
	IdleConnTimeout       time.Duration
	KeepAlive             time.Duration

	// Connection limits
	MaxIdleConns        int
	MaxIdleConnsPerHost int
	MaxConnsPerHost     int

	// Features
	ForceHTTP2         bool
	DisableKeepAlives  bool
	DisableCompression bool

	// Buffer sizes
	WriteBufferSize int
	ReadBufferSize  int

	// TLS
	InsecureSkipVerify bool
	MinTLSVersion      uint16
}

var (
	// globalTransportPool is the singleton transport pool instance
	globalTransportPool *TransportPool
	// transportPoolOnce ensures single initialization
	transportPoolOnce sync.Once
)

// GetTransportPool returns the global transport pool instance
func GetTransportPool() *TransportPool {
	transportPoolOnce.Do(func() {
		ctx, cancel := context.WithCancel(context.Background())
		globalTransportPool = &TransportPool{
			transports:  make(map[string]*sharedTransport),
			maxConns:    20,
			ctx:         ctx,
			cancel:      cancel,
			clientCount: 0,
			maxClients:  5,
		}
		go globalTransportPool.cleanupRoutine(ctx)
	})
	return globalTransportPool
}

// DefaultTransportConfig returns a secure default configuration
func DefaultTransportConfig() TransportConfig {
	return TransportConfig{
		DialTimeout:           30 * time.Second,
		TLSHandshakeTimeout:   10 * time.Second,
		ResponseHeaderTimeout: 10 * time.Second,
		ExpectContinueTimeout: 1 * time.Second,
		IdleConnTimeout:       30 * time.Second,
		KeepAlive:             30 * time.Second,
		MaxIdleConns:          10,
		MaxIdleConnsPerHost:   2,
		MaxConnsPerHost:       5,
		ForceHTTP2:            true,
		DisableKeepAlives:     false,
		DisableCompression:    false,
		WriteBufferSize:       4096,
		ReadBufferSize:        4096,
		InsecureSkipVerify:    false,
		MinTLSVersion:         tls.VersionTLS12,
	}
}

// GetTransport gets or creates a shared transport with the given config
func (p *TransportPool) GetTransport(config TransportConfig) *http.Transport {
	// Check client limit
	if atomic.LoadInt32(&p.clientCount) >= p.maxClients {
		return p.getExistingTransport()
	}

	key := p.configKey(config)

	// Fast path: check with read lock
	p.mu.RLock()
	if shared, exists := p.transports[key]; exists {
		atomic.AddInt32(&shared.refCount, 1)
		shared.lastUsed = time.Now()
		p.mu.RUnlock()
		return shared.transport
	}
	p.mu.RUnlock()

	// Slow path: create new transport
	p.mu.Lock()
	defer p.mu.Unlock()

	// Double-check after acquiring write lock
	if shared, exists := p.transports[key]; exists {
		atomic.AddInt32(&shared.refCount, 1)
		shared.lastUsed = time.Now()
		return shared.transport
	}

	// Create new transport
	transport := p.createTransport(config)
	shared := &sharedTransport{
		transport: transport,
		refCount:  1,
		lastUsed:  time.Now(),
		config:    config,
	}

	p.transports[key] = shared
	atomic.AddInt32(&p.clientCount, 1)

	return transport
}

// ReleaseTransport decrements the reference count for a transport
func (p *TransportPool) ReleaseTransport(transport *http.Transport) {
	if transport == nil {
		return
	}

	p.mu.RLock()
	defer p.mu.RUnlock()

	for _, shared := range p.transports {
		if shared.transport == transport {
			count := atomic.AddInt32(&shared.refCount, -1)
			if count <= 0 {
				shared.lastUsed = time.Now()
			}
			return
		}
	}
}

// getExistingTransport returns any available transport when limit is reached
func (p *TransportPool) getExistingTransport() *http.Transport {
	p.mu.RLock()
	defer p.mu.RUnlock()

	for _, shared := range p.transports {
		if shared != nil && shared.transport != nil {
			atomic.AddInt32(&shared.refCount, 1)
			shared.lastUsed = time.Now()
			return shared.transport
		}
	}
	return nil
}

// createTransport creates a new HTTP transport with the given config
func (p *TransportPool) createTransport(config TransportConfig) *http.Transport {
	// Set secure defaults
	if config.MinTLSVersion == 0 {
		config.MinTLSVersion = tls.VersionTLS12
	}

	tlsConfig := &tls.Config{
		MinVersion: config.MinTLSVersion,
		MaxVersion: tls.VersionTLS13,
		CipherSuites: []uint16{
			tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
			tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
			tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
			tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
		},
		PreferServerCipherSuites: true,
		InsecureSkipVerify:       config.InsecureSkipVerify,
	}

	return &http.Transport{
		Proxy: http.ProxyFromEnvironment,
		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
			dialer := &net.Dialer{
				Timeout:   config.DialTimeout,
				KeepAlive: config.KeepAlive,
			}
			return dialer.DialContext(ctx, network, addr)
		},
		TLSClientConfig:       tlsConfig,
		ForceAttemptHTTP2:     config.ForceHTTP2,
		TLSHandshakeTimeout:   config.TLSHandshakeTimeout,
		ExpectContinueTimeout: config.ExpectContinueTimeout,
		MaxIdleConns:          config.MaxIdleConns,
		MaxIdleConnsPerHost:   config.MaxIdleConnsPerHost,
		IdleConnTimeout:       config.IdleConnTimeout,
		DisableKeepAlives:     config.DisableKeepAlives,
		MaxConnsPerHost:       config.MaxConnsPerHost,
		ResponseHeaderTimeout: config.ResponseHeaderTimeout,
		DisableCompression:    config.DisableCompression,
		WriteBufferSize:       config.WriteBufferSize,
		ReadBufferSize:        config.ReadBufferSize,
	}
}

// configKey generates a unique key for a transport config
func (p *TransportPool) configKey(config TransportConfig) string {
	// Create a simple key based on critical parameters
	sb := Get().GetStringBuilder()
	defer Get().PutStringBuilder(sb)

	sb.WriteByte(byte(config.MaxConnsPerHost))
	sb.WriteByte(byte(config.MaxIdleConnsPerHost))
	sb.WriteByte(byte(config.MaxIdleConns))
	if config.ForceHTTP2 {
		sb.WriteByte(1)
	} else {
		sb.WriteByte(0)
	}
	if config.DisableKeepAlives {
		sb.WriteByte(1)
	} else {
		sb.WriteByte(0)
	}
	if config.DisableCompression {
		sb.WriteByte(1)
	} else {
		sb.WriteByte(0)
	}
	if config.InsecureSkipVerify {
		sb.WriteByte(1)
	} else {
		sb.WriteByte(0)
	}

	return sb.String()
}

// cleanupRoutine periodically cleans up unused transports
func (p *TransportPool) cleanupRoutine(ctx context.Context) {
	ticker := time.NewTicker(1 * time.Minute)
	defer ticker.Stop()

	for {
		select {
		case <-ctx.Done():
			p.cleanup()
			return
		case <-ticker.C:
			p.cleanupIdle()
		}
	}
}

// cleanupIdle removes idle transports
func (p *TransportPool) cleanupIdle() {
	p.mu.Lock()
	defer p.mu.Unlock()

	now := time.Now()
	for key, shared := range p.transports {
		refCount := atomic.LoadInt32(&shared.refCount)
		if refCount <= 0 && now.Sub(shared.lastUsed) > 2*time.Minute {
			shared.transport.CloseIdleConnections()
			delete(p.transports, key)
			atomic.AddInt32(&p.clientCount, -1)
		}
	}
}

// cleanup closes all transports
func (p *TransportPool) cleanup() {
	p.mu.Lock()
	defer p.mu.Unlock()

	for _, shared := range p.transports {
		shared.transport.CloseIdleConnections()
	}
	p.transports = make(map[string]*sharedTransport)
	atomic.StoreInt32(&p.clientCount, 0)
}

// Shutdown gracefully shuts down the transport pool
func (p *TransportPool) Shutdown() {
	if p.cancel != nil {
		p.cancel()
	}
}

// Stats returns transport pool statistics
type TransportPoolStats struct {
	ActiveTransports int
	TotalClients     int32
	MaxClients       int32
}

// GetStats returns current pool statistics
func (p *TransportPool) GetStats() TransportPoolStats {
	p.mu.RLock()
	defer p.mu.RUnlock()

	activeCount := 0
	for _, shared := range p.transports {
		if atomic.LoadInt32(&shared.refCount) > 0 {
			activeCount++
		}
	}

	return TransportPoolStats{
		ActiveTransports: activeCount,
		TotalClients:     atomic.LoadInt32(&p.clientCount),
		MaxClients:       p.maxClients,
	}
}

// CreateHTTPClient creates an HTTP client using the transport pool
func CreateHTTPClient(config TransportConfig, timeout time.Duration) *http.Client {
	pool := GetTransportPool()
	transport := pool.GetTransport(config)

	if transport == nil {
		// Fallback to a basic client if pool is exhausted
		return &http.Client{
			Timeout: timeout,
		}
	}

	client := &http.Client{
		Transport: transport,
		Timeout:   timeout,
	}

	// Configure redirect policy
	client.CheckRedirect = func(req *http.Request, via []*http.Request) error {
		if len(via) >= 10 {
			return http.ErrUseLastResponse
		}
		return nil
	}

	return client
}