//go:build linux || freebsd
// +build linux freebsd

package transport

import (
	"context"
	"fmt"
	"net"
	"net/url"
	"sort"
	"strings"
	"time"
)

// DialerConfig configures the dialer
type DialerConfig struct {
	// Timeout for total connection
	Timeout time.Duration

	// Timeout for IPv6 attempt before fallback to IPv4
	IPv6Timeout time.Duration

	// Enable IPv4 fallback
	IPv4Fallback bool

	// Debug mode for logging
	Debug bool

	// BindInterface specifies the network interface to bind to
	BindInterface string

	// BlockPrivateIPs blocks connections to private/internal IP ranges
	BlockPrivateIPs bool

	// Callback for reporting used IP version
	OnConnect func(ip string, version int)
}

// DefaultDialerConfig returns the default configuration
func DefaultDialerConfig() *DialerConfig {
	return &DialerConfig{
		Timeout:         30 * time.Second,
		IPv6Timeout:     5 * time.Second,
		IPv4Fallback:    true,
		Debug:           false,
		BlockPrivateIPs: true, // SSRF protection on by default
	}
}

// Dialer is an IPv6-first dialer with fallback
type Dialer struct {
	config   *DialerConfig
	resolver *net.Resolver
	dialer   *net.Dialer
}

// NewDialer creates a new dialer
func NewDialer(cfg *DialerConfig) *Dialer {
	if cfg == nil {
		cfg = DefaultDialerConfig()
	}

	d := &Dialer{
		config: cfg,
		resolver: &net.Resolver{
			PreferGo: true,
			Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
				dial := net.Dialer{
					Timeout:   5 * time.Second,
					DualStack: true,
				}
				return dial.DialContext(ctx, network, address)
			},
		},
	}

	// Configure bind interface if specified
	if cfg.BindInterface != "" {
		d.dialer = &net.Dialer{
			Timeout:   cfg.Timeout,
			DualStack: false,
			Control:   bindToInterface(cfg.BindInterface),
		}
	}

	return d
}

// DialContext connects to an address with IPv6-first strategy
func (d *Dialer) DialContext(ctx context.Context, network, addr string) (net.Conn, error) {
	host, port, err := net.SplitHostPort(addr)
	if err != nil {
		return nil, fmt.Errorf("invalid address: %w", err)
	}

	// Resolve all IP addresses
	ips, err := d.resolver.LookupIPAddr(ctx, host)
	if err != nil {
		return nil, fmt.Errorf("dns lookup failed: %w", err)
	}

	// Split into IPv6 and IPv4
	var ipv6Addrs, ipv4Addrs []net.IPAddr
	for _, ip := range ips {
		if ip.IP.To4() == nil {
			ipv6Addrs = append(ipv6Addrs, ip)
		} else {
			ipv4Addrs = append(ipv4Addrs, ip)
		}
	}

	// Sort: IPv6 first, then IPv4
	var orderedAddrs []net.IPAddr
	orderedAddrs = append(orderedAddrs, ipv6Addrs...)
	if d.config.IPv4Fallback {
		orderedAddrs = append(orderedAddrs, ipv4Addrs...)
	}

	if len(orderedAddrs) == 0 {
		return nil, fmt.Errorf("no ip addresses found for %s", host)
	}

	// SSRF protection: block private/internal IPs
	if d.config.BlockPrivateIPs {
		for _, addr := range orderedAddrs {
			if isPrivateIP(addr.IP) {
				return nil, fmt.Errorf("ssrf protection: blocked private ip %s for host %s", addr.IP, host)
			}
		}
	}

	// Try to connect in order (IPv6 first)
	var lastErr error
	for _, ipAddr := range orderedAddrs {
		ip := ipAddr.IP.String()
		address := net.JoinHostPort(ip, port)

		// Determine timeout based on IP version
		dialTimeout := d.config.Timeout
		if ipAddr.IP.To4() == nil && d.config.IPv4Fallback {
			// IPv6 with fallback: shorter timeout
			dialTimeout = d.config.IPv6Timeout
		}

		dialCtx, cancel := context.WithTimeout(ctx, dialTimeout)

		var conn net.Conn
		var err error
		if d.dialer != nil {
			// Use pre-configured dialer (e.g., with bind interface)
			dialer := *d.dialer
			dialer.Timeout = dialTimeout
			conn, err = dialer.DialContext(dialCtx, network, address)
		} else {
			conn, err = (&net.Dialer{
				Timeout:   dialTimeout,
				DualStack: false, // Explicitly disabled for deterministic behavior
			}).DialContext(dialCtx, network, address)
		}

		cancel()

		if err == nil {
			// Connection successful
			ipVersion := 6
			if ipAddr.IP.To4() != nil {
				ipVersion = 4
			}

			if d.config.Debug {
				fmt.Printf("[DEBUG] Connected to %s (IPv%d)\n", ip, ipVersion)
			}

			if d.config.OnConnect != nil {
				d.config.OnConnect(ip, ipVersion)
			}

			return conn, nil
		}

		lastErr = err

		if d.config.Debug {
			fmt.Printf("[DEBUG] Failed to connect to %s: %v\n", ip, err)
		}
	}

	return nil, fmt.Errorf("all connection attempts failed: %w", lastErr)
}

// LookupIPAddr performs DNS lookup with IPv6 preference
func (d *Dialer) LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error) {
	ips, err := d.resolver.LookupIPAddr(ctx, host)
	if err != nil {
		return nil, err
	}

	// Sort: IPv6 first
	sort.Slice(ips, func(i, j int) bool {
		iIsV6 := ips[i].IP.To4() == nil
		jIsV6 := ips[j].IP.To4() == nil
		if iIsV6 && !jIsV6 {
			return true
		}
		if !iIsV6 && jIsV6 {
			return false
		}
		return false
	})

	return ips, nil
}

// ParseProxyURL parses the proxy URL and returns host:port
func ParseProxyURL(proxyURL string) (*url.URL, error) {
	if proxyURL == "" {
		return nil, nil
	}

	u, err := url.Parse(proxyURL)
	if err != nil {
		return nil, fmt.Errorf("invalid proxy url: %w", err)
	}

	if u.Host == "" {
		return nil, fmt.Errorf("proxy url missing host")
	}

	return u, nil
}

// SetCustomDNS configures custom DNS servers for the dialer
func (d *Dialer) SetCustomDNS(servers []string) {
	if d == nil || len(servers) == 0 {
		return
	}

	d.resolver = &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			dialer := &net.Dialer{Timeout: d.config.Timeout}
			server := servers[0]
			if !strings.Contains(server, ":") {
				server = net.JoinHostPort(server, "53")
			}
			return dialer.DialContext(ctx, "udp", server)
		},
	}
}

// ParseDNSServers parses a comma-separated list of DNS servers
func ParseDNSServers(s string) []string {
	if s == "" {
		return nil
	}
	parts := strings.Split(s, ",")
	var servers []string
	for _, p := range parts {
		p = strings.TrimSpace(p)
		if p != "" {
			servers = append(servers, p)
		}
	}
	return servers
}

// isPrivateIP checks if an IP address is in a private/internal range.
// Always allows localhost (127.0.0.0/8, ::1).
// IPv4-mapped IPv6 addresses (e.g. ::ffff:10.0.0.1) are normalized to IPv4
// first, otherwise net.IP.IsPrivate returns false for them and an attacker
// controlling DNS could bypass SSRF protection.
func isPrivateIP(ip net.IP) bool {
	if ip.IsLoopback() {
		return false // always allow localhost
	}
	if v4 := ip.To4(); v4 != nil {
		ip = v4
	}
	if ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast() ||
		ip.IsPrivate() || ip.IsUnspecified() {
		return true
	}
	return false
}