package main

import (
	"context"
	"io"
	"net"
	"net/http"
	"sort"
	"strings"
	"sync"
	"time"
)

func validIP(raw string) string {
	ip := net.ParseIP(strings.TrimSpace(raw))
	if ip == nil || ip.IsUnspecified() || ip.IsLoopback() || ip.IsMulticast() || ip.IsLinkLocalUnicast() {
		return ""
	}
	return ip.String()
}

func publicIPFromURL(ctx context.Context, rawURL string) string {
	if rawURL == "" {
		return ""
	}
	client := &http.Client{Timeout: 2 * time.Second}
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, rawURL, nil)
	if err != nil {
		return ""
	}
	resp, err := client.Do(req)
	if err != nil {
		return ""
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return ""
	}
	body, err := io.ReadAll(io.LimitReader(resp.Body, 128))
	if err != nil {
		return ""
	}
	return validIP(string(body))
}

func routeIP() string {
	conn, err := net.DialTimeout("udp", "1.1.1.1:53", time.Second)
	if err != nil {
		return ""
	}
	defer conn.Close()
	if addr, ok := conn.LocalAddr().(*net.UDPAddr); ok {
		return validIP(addr.IP.String())
	}
	return ""
}

func ipScope(ip net.IP) string {
	switch {
	case ip.IsPrivate():
		return "private"
	case ip.IsLinkLocalUnicast():
		return "link_local"
	default:
		return "public"
	}
}

func skipInterface(name string) bool {
	name = strings.ToLower(strings.TrimSpace(name))
	return name == "docker0" || strings.HasPrefix(name, "docker") || strings.HasPrefix(name, "veth") || strings.HasPrefix(name, "br-") || strings.HasPrefix(name, "virbr")
}

func localHostAddresses() []hostAddress {
	interfaces, err := net.Interfaces()
	if err != nil {
		return nil
	}
	out := make([]hostAddress, 0)
	for _, iface := range interfaces {
		if iface.Flags&net.FlagUp == 0 || iface.Flags&net.FlagLoopback != 0 || skipInterface(iface.Name) {
			continue
		}
		addrs, err := iface.Addrs()
		if err != nil {
			continue
		}
		for _, addr := range addrs {
			raw := addr.String()
			if host, _, err := net.ParseCIDR(raw); err == nil {
				raw = host.String()
			} else if host, _, err := net.SplitHostPort(raw); err == nil {
				raw = host
			}
			clean := validIP(raw)
			if clean == "" {
				continue
			}
			ip := net.ParseIP(clean)
			out = append(out, hostAddress{IP: clean, Source: "interface", Interface: iface.Name, Scope: ipScope(ip)})
		}
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].Interface != out[j].Interface {
			return out[i].Interface < out[j].Interface
		}
		return out[i].IP < out[j].IP
	})
	return out
}

type ipDetector struct {
	mu       sync.Mutex
	publicIP string
	expires  time.Time
}

func (d *ipDetector) public(ctx context.Context, rawURL string) string {
	if rawURL == "" {
		return ""
	}
	d.mu.Lock()
	defer d.mu.Unlock()
	if time.Now().Before(d.expires) {
		return d.publicIP
	}
	d.publicIP = publicIPFromURL(ctx, rawURL)
	d.expires = time.Now().Add(5 * time.Minute)
	if d.publicIP == "" {
		d.expires = time.Now().Add(30 * time.Second)
	}
	return d.publicIP
}

func mergeHostAddress(out *[]hostAddress, index map[string]int, candidate hostAddress) {
	candidate.IP = validIP(candidate.IP)
	if candidate.IP == "" {
		return
	}
	if candidate.Scope == "" {
		candidate.Scope = ipScope(net.ParseIP(candidate.IP))
	}
	if pos, ok := index[candidate.IP]; ok {
		existing := &(*out)[pos]
		if candidate.Interface != "" && existing.Interface == "" {
			existing.Interface = candidate.Interface
		}
		if candidate.Source != "" && !strings.Contains(existing.Source, candidate.Source) {
			if existing.Source == "" {
				existing.Source = candidate.Source
			} else {
				existing.Source += "," + candidate.Source
			}
		}
		return
	}
	index[candidate.IP] = len(*out)
	*out = append(*out, candidate)
}

func detectedAddresses(ctx context.Context, cfg config, detector *ipDetector) ([]hostAddress, string, string) {
	out := make([]hostAddress, 0, 8)
	index := make(map[string]int)
	for _, item := range localHostAddresses() {
		mergeHostAddress(&out, index, item)
	}
	routed := routeIP()
	if routed != "" {
		mergeHostAddress(&out, index, hostAddress{IP: routed, Source: "route"})
	}
	public := detector.public(ctx, cfg.PublicIPURL)
	if public != "" {
		mergeHostAddress(&out, index, hostAddress{IP: public, Source: "public_api"})
	}
	advertised := validIP(cfg.AdvertiseIP)
	if advertised != "" {
		mergeHostAddress(&out, index, hostAddress{IP: advertised, Source: "override"})
	}

	switch {
	case advertised != "":
		return out, advertised, "override"
	case routed != "":
		return out, routed, "route"
	case public != "":
		return out, public, "public_api"
	case len(out) > 0:
		return out, out[0].IP, out[0].Source
	default:
		return out, "", "unavailable"
	}
}