// Package utils 提供通用工具函数：客户端 IP 解析、随机字符串等。
package utils

import (
	"fmt"
	"net"
	"net/http"
	"strings"
	"sync"

	"github.com/gin-gonic/gin"
)

// clientIPHeaders 按优先级排列的单值客户端 IP 请求头；Forwarded 与 X-Forwarded-For 因需要链式解析单独处理。
var clientIPHeaders = []string{
	"CF-Connecting-IP",
	"True-Client-IP",
	"Ali-CDN-Real-IP",
	"X-Real-IP",
	"X-Client-IP",
	"Fastly-Client-IP",
}

var (
	trustedMu    sync.RWMutex
	trustedCIDRs []*net.IPNet
)

// SetTrustedProxies 设置可信代理/CDN 回源网段，支持 IP 与 CIDR，启动时调用一次；并发安全。
func SetTrustedProxies(cidrs []string) error {
	parsed := make([]*net.IPNet, 0, len(cidrs))
	for _, raw := range cidrs {
		value := strings.TrimSpace(raw)
		if value == "" {
			continue
		}
		if ip := net.ParseIP(value); ip != nil {
			bits := 128
			if ip.To4() != nil {
				bits = 32
			}
			_, cidr, err := net.ParseCIDR(fmt.Sprintf("%s/%d", ip.String(), bits))
			if err != nil {
				return fmt.Errorf("invalid trusted proxy %q: %w", raw, err)
			}
			parsed = append(parsed, cidr)
			continue
		}
		if _, cidr, err := net.ParseCIDR(value); err == nil {
			parsed = append(parsed, cidr)
			continue
		}
		return fmt.Errorf("invalid trusted proxy %q", raw)
	}

	trustedMu.Lock()
	trustedCIDRs = parsed
	trustedMu.Unlock()
	return nil
}

// ClientIP 依次枚举 CDN/代理请求头，仅在直连地址可信时采信；否则返回直连 IP。
func ClientIP(c *gin.Context) string {
	direct := RemoteIP(c)
	if !isTrustedPeer(c, direct) {
		return direct
	}

	for _, header := range clientIPHeaders {
		if ip := normalizeIP(c.GetHeader(header)); ip != "" {
			return ip
		}
	}
	if ip := pickClientIP(parseForwarded(c.GetHeader("Forwarded"))); ip != "" {
		return ip
	}
	if ip := pickClientIP(parseIPList(c.GetHeader("X-Forwarded-For"))); ip != "" {
		return ip
	}
	return direct
}

// RemoteIP 返回直连地址（已去端口），不读取任何请求头。
func RemoteIP(c *gin.Context) string {
	return normalizeIP(c.Request.RemoteAddr)
}

// pickClientIP 从右往左取第一个非可信代理 IP；全部可信时取最左值。
func pickClientIP(chain []string) string {
	for i := len(chain) - 1; i >= 0; i-- {
		if !isTrustedStringIP(chain[i]) {
			return chain[i]
		}
	}
	if len(chain) > 0 {
		return chain[0]
	}
	return ""
}

func isTrustedPeer(c *gin.Context, direct string) bool {
	if isUnixSocket(c) {
		return true
	}
	ip := net.ParseIP(direct)
	return ip != nil && isTrustedIP(ip)
}

func isTrustedIP(ip net.IP) bool {
	trustedMu.RLock()
	defer trustedMu.RUnlock()
	for _, cidr := range trustedCIDRs {
		if cidr.Contains(ip) {
			return true
		}
	}
	return false
}

func isTrustedStringIP(value string) bool {
	ip := net.ParseIP(value)
	return ip != nil && isTrustedIP(ip)
}

func isUnixSocket(c *gin.Context) bool {
	addr, ok := c.Request.Context().Value(http.LocalAddrContextKey).(net.Addr)
	return ok && strings.HasPrefix(addr.Network(), "unix")
}

// normalizeIP 清理引号与端口后校验为合法 IP，非法返回空串。
func normalizeIP(raw string) string {
	value := strings.TrimSpace(raw)
	if value == "" {
		return ""
	}
	value = strings.Trim(value, `"`)
	if host, _, err := net.SplitHostPort(value); err == nil {
		value = host
	}
	value = strings.Trim(value, "[]")
	ip := net.ParseIP(value)
	if ip == nil {
		return ""
	}
	return ip.String()
}

func parseIPList(value string) []string {
	if strings.TrimSpace(value) == "" {
		return nil
	}
	parts := strings.Split(value, ",")
	ips := make([]string, 0, len(parts))
	for _, part := range parts {
		if ip := normalizeIP(part); ip != "" {
			ips = append(ips, ip)
		}
	}
	return ips
}

// parseForwarded 解析 RFC 7239 Forwarded 头中的 for= 参数，按出现顺序返回合法 IP。
func parseForwarded(value string) []string {
	if strings.TrimSpace(value) == "" {
		return nil
	}
	var ips []string
	for _, element := range strings.Split(value, ",") {
		for _, param := range strings.Split(element, ";") {
			key, val, ok := strings.Cut(strings.TrimSpace(param), "=")
			if !ok || !strings.EqualFold(strings.TrimSpace(key), "for") {
				continue
			}
			candidate := strings.TrimSpace(val)
			if candidate == "" || candidate == "_hidden" || strings.EqualFold(candidate, "unknown") {
				continue
			}
			if ip := normalizeIP(candidate); ip != "" {
				ips = append(ips, ip)
			}
		}
	}
	return ips
}