// A Go web‑server that converts IPv4 addresses into IPv6 addresses
// inside a user‑defined /96 ULA prefix and outputs Windows 11‑ready fields.
//
// Features
// --------
// 1. Single conversion page  (http://localhost:8080/)      – converts one IPv4.
// 2. Range conversion page   (http://localhost:8080/range) – converts a span of IPv4s into a table.
//
// Environment variables (all optional):
//
//	ULA_PREFIX   – /96 prefix to embed into, default "fd09:cafe:affe:4010::"
//	DNS1         – preferred DNS IPv6 address (default: <prefix>::53)
//	DNS2         – alternate DNS  IPv6 address (default: <prefix>::54)
//	FORM_DEFAULT_IP – placeholder for the single‑convert page, default 172.16.0.0
//	RANGE_LIMIT  – max number of addresses allowed in /range (default 1024)
//
// Build & run:
//
//	go run main.go
package main

import (
	"fmt"
	"html/template"
	"log"
	"net"
	"net/http"
	"os"
	"strconv"
	"strings"
)

// ---------------------------------------------------------------------------
// Configuration constants & globals
// ---------------------------------------------------------------------------
const (
	listenAddr    = ":8080"
	defaultPrefix = "fd09:cafe:affe:4010::" // fallback /96
	defaultIP     = "172.16.0.0"
	defaultLimit  = 1024 // max. rows for range conversion
)

var (
	ulaPrefix      string
	dns1, dns2     string
	pageIP         string
	rangeLimit     int
	singleTemplate *template.Template
	rangeTemplate  *template.Template
	dhcpServer     string
	dhcpScope      string
)

// ---------------------------------------------------------------------------
// Template data structures
// ---------------------------------------------------------------------------
type singleData struct {
	IPv4, IPv6, Gateway, DNS1, DNS2, Error string
	HaveResult                             bool
}

type addrPair struct{ Name, IPv4, IPv6 string }

type rangeData struct {
	Start, End            string
	DhcpServer, DhcpScope string
	Rows                  []addrPair
	Error                 string
	HaveResult            bool
}

// ---------------------------------------------------------------------------
// main – HTTP routing
// ---------------------------------------------------------------------------
func main() {
	initConfigAndTemplates()

	http.HandleFunc("/", handleSingle)
	http.HandleFunc("/convert", handleSingleConvert)
	http.HandleFunc("/range", handleRange)

	log.Printf("Server läuft auf http://localhost%s  (Präfix %s, DNS1 %s, DNS2 %s, Limit %d)", listenAddr, ulaPrefix, dns1, dns2, rangeLimit)
	log.Fatal(http.ListenAndServe(listenAddr, nil))
}

// ---------------------------------------------------------------------------
// Handlers – single conversion
// ---------------------------------------------------------------------------
func handleSingle(w http.ResponseWriter, r *http.Request) {
	renderSingle(w, singleData{})
}

func handleSingleConvert(w http.ResponseWriter, r *http.Request) {
	if err := r.ParseForm(); err != nil {
		renderSingle(w, singleData{Error: "Ungültiges Formular"})
		return
	}
	ipv4Str := r.FormValue("ipv4")
	ipv6, err := embedIPv4(ipv4Str)
	d := singleData{IPv4: ipv4Str}
	if err != nil {
		d.Error = err.Error()
	} else {
		d.HaveResult = true
		d.IPv6 = ipv6
		d.Gateway = ulaPrefix + "1"
		d.DNS1, d.DNS2 = dns1, dns2
	}
	renderSingle(w, d)
}

// ---------------------------------------------------------------------------
// Handlers – range conversion
// ---------------------------------------------------------------------------
func handleRange(w http.ResponseWriter, r *http.Request) {
	switch r.Method {
	case http.MethodGet:
		renderRange(w, rangeData{})
		return
	case http.MethodPost:
		if err := r.ParseForm(); err != nil {
			renderRange(w, rangeData{Error: "Ungültiges Formular"})
			return
		}
		startStr := r.FormValue("start")
		endStr := r.FormValue("end")
		rows, err := convertRange(startStr, endStr)
		d := rangeData{Start: startStr, End: endStr, DhcpServer: dhcpServer, DhcpScope: dhcpScope}
		if err != nil {
			d.Error = err.Error()
		} else {
			d.HaveResult = true
			for a, b := range rows {
				fmt.Println(a, b)
				octets := strings.Split(b.IPv4, ".")
				if len(octets) != 4 {
					fmt.Println("Ungültige IP-Adresse!")
					return
				}
				O3 := ""
				O4 := ""
				if len(octets[2]) < 2 {
					O3 = "0" + octets[2]
				}
				if len(octets[3]) < 2 {
					O3 = "0" + octets[3]
				}
				N := "PC" + O3 + O4
				d.Rows = append(d.Rows, addrPair{IPv4: b.IPv4, IPv6: b.IPv6, Name: N})
			}
		}

		renderRange(w, d)
	default:
		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
	}
}

// convertRange returns slice of addrPairs for every IPv4 in [start,end].
func convertRange(start, end string) ([]addrPair, error) {
	sip := net.ParseIP(start).To4()
	eip := net.ParseIP(end).To4()
	if sip == nil || eip == nil {
		return nil, fmt.Errorf("start oder ende ist keine gültige ipv4-adresse")
	}
	su := ipToUint32(sip)
	eu := ipToUint32(eip)
	if su > eu {
		return nil, fmt.Errorf("start darf nicht größer als ende sein")
	}
	count := int(eu - su + 1)
	if count > rangeLimit {
		return nil, fmt.Errorf("bereich zu groß (%d > %d)", count, rangeLimit)
	}
	rows := make([]addrPair, 0, count)
	for cur := su; cur <= eu; cur++ {
		ipv4 := uint32ToIP(cur).String()
		ipv6, _ := embedIPv4(ipv4) // embedIPv4 kann hier nicht mehr fehlschlagen
		rows = append(rows, addrPair{IPv4: ipv4, IPv6: ipv6})
	}
	return rows, nil
}

// ---------------------------------------------------------------------------
// Helper functions
// ---------------------------------------------------------------------------
func embedIPv4(ipv4 string) (string, error) {
	ip := net.ParseIP(ipv4).To4()
	if ip == nil {
		return "", fmt.Errorf("%s ist keine gültige IPv4-Adresse", ipv4)
	}
	hi := uint16(ip[0])<<8 | uint16(ip[1])
	lo := uint16(ip[2])<<8 | uint16(ip[3])
	return fmt.Sprintf("%s%x:%x", ulaPrefix, hi, lo), nil
}

func ipToUint32(ip net.IP) uint32 {
	return uint32(ip[0])<<24 | uint32(ip[1])<<16 | uint32(ip[2])<<8 | uint32(ip[3])
}

func uint32ToIP(u uint32) net.IP {
	return net.IPv4(byte(u>>24), byte(u>>16), byte(u>>8), byte(u))
}

// ---------------------------------------------------------------------------
// Config & templates
// ---------------------------------------------------------------------------
func initConfigAndTemplates() {
	// ULA prefix ---------------------------------
	ulaPrefix = os.Getenv("ULA_PREFIX")
	if ulaPrefix == "" {
		ulaPrefix = defaultPrefix
	}
	if !strings.HasSuffix(ulaPrefix, "::") {
		if strings.HasSuffix(ulaPrefix, ":") {
			ulaPrefix += ":"
		} else {
			ulaPrefix += "::"
		}
	}

	// DNS defaults --------------------------------
	dns1 = os.Getenv("DNS1")
	dns2 = os.Getenv("DNS2")
	if dns1 == "" {
		dns1 = strings.Replace(ulaPrefix, "::", "::53", 1)
	}
	if dns2 == "" {
		dns2 = strings.Replace(ulaPrefix, "::", "::54", 1)
	}

	// Placeholder IP ------------------------------
	pageIP = os.Getenv("FORM_DEFAULT_IP")
	if pageIP == "" {
		pageIP = defaultIP
	}

	dhcpScope = os.Getenv("DHCP_SCOPE")
	dhcpServer = os.Getenv("DHCP_SERVER")

	// Range limit ---------------------------------
	if limStr := os.Getenv("RANGE_LIMIT"); limStr != "" {
		if v, err := strconv.Atoi(limStr); err == nil && v > 0 {
			rangeLimit = v
		} else {
			rangeLimit = defaultLimit
		}
	} else {
		rangeLimit = defaultLimit
	}

	// Templates -----------------------------------
	singleHTML := fmt.Sprintf(singlePageHTML, pageIP, ulaPrefix)
	rangeHTML := rangePageHTML
	singleTemplate = template.Must(template.New("single").Parse(singleHTML))
	rangeTemplate = template.Must(template.New("range").Parse(rangeHTML))

	fmt.Println(ulaPrefix, dns1, dns2, pageIP, dhcpServer, dhcpScope, rangeLimit)
	fmt.Println(singleHTML)
	fmt.Println(rangeHTML)
}

// ---------------------------------------------------------------------------
// Template rendering helpers
// ---------------------------------------------------------------------------
func renderSingle(w http.ResponseWriter, d singleData) {
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	_ = singleTemplate.Execute(w, d)
}

func renderRange(w http.ResponseWriter, d rangeData) {
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	_ = rangeTemplate.Execute(w, d)
}

// ---------------------------------------------------------------------------
// HTML templates as raw strings with %s placeholders (prefix, etc.)
// ---------------------------------------------------------------------------
var singlePageHTML = `<!DOCTYPE html>
<html lang="de">
<head>
<meta charset="utf-8" />
<title>IPv4 → IPv6-Mapper</title>
<style>
 body{font-family:system-ui,sans-serif;margin:2rem}
 form{display:flex;gap:.5rem;flex-wrap:wrap;margin-bottom:1rem}
 input[type=text],input[readonly]{padding:.4rem;font-size:1rem;border:1px solid #ccc;border-radius:4px;flex:1}
 button{padding:.5rem 1rem;font-size:1rem;cursor:pointer;border-radius:4px;border:1px solid #666;background:#eee}
 button.copy{padding:.3rem .6rem;font-size:.9rem;margin-left:.3rem;background:#def}
 .row{display:flex;align-items:center;margin-bottom:.4rem}
 .row label{width:14rem}
</style>
<script>async function copy(id){const v=document.getElementById(id).value;await navigator.clipboard.writeText(v);const b=document.getElementById(id+'Btn');const o=b.textContent;b.textContent='✔';setTimeout(()=>b.textContent=o,1200);}</script>
</head><body>
<h1>IPv4 → IPv6 (einzeln)</h1>
<form action="/convert" method="post">
 <input name="ipv4" type="text" placeholder="%s" value="{{.IPv4}}" required />
 <button>Umrechnen</button>
 <a href="/range" style="margin-left:1rem">&raquo; Bereich&nbsp;konvertieren</a>
</form>
{{if .HaveResult}}
<div>
 <h2>Windows 11-Eingaben</h2>
 <div class="row"><label>IP-Adresse</label><input readonly id="ip" value="{{.IPv6}}"><button type="button" class="copy" id="ipBtn" onclick="copy('ip')">Copy</button></div>
 <div class="row"><label>Subnetzpräfix</label><input readonly id="pl" value="96"><button type="button" class="copy" id="plBtn" onclick="copy('pl')">Copy</button></div>
 <div class="row"><label>Gateway</label><input readonly id="gw" value="{{.Gateway}}"><button type="button" class="copy" id="gwBtn" onclick="copy('gw')">Copy</button></div>
 <div class="row"><label>DNS bevorzugt</label><input readonly id="dns1" value="{{.DNS1}}"><button type="button" class="copy" id="dns1Btn" onclick="copy('dns1')">Copy</button></div>
 <div class="row"><label>DNS alternativ</label><input readonly id="dns2" value="{{.DNS2}}"><button type="button" class="copy" id="dns2Btn" onclick="copy('dns2')">Copy</button></div>
</div>
{{end}}
{{if .Error}}<p style="color:#b00">Fehler: {{.Error}}</p>{{end}}
<p style="margin-top:1rem">Aktives Präfix: <code>%s</code> (/96)</p>
</body></html>`

var rangePageHTML = `<!DOCTYPE html>
<html lang="de">
<head>
<meta charset="utf-8" />
<title>IPv4-Range → IPv6</title>
<style>
 body{font-family:system-ui,sans-serif;margin:2rem}
 table{border-collapse:collapse;width:100%}
 th,td{border:1px solid #ccc;padding:.4rem;text-align:left;font-family:monospace}
 form{display:flex;gap:.5rem;flex-wrap:wrap;margin-bottom:1rem}
 input[type=text]{padding:.4rem;font-size:1rem;border:1px solid #ccc;border-radius:4px;flex:1}
 button{padding:.5rem 1rem;font-size:1rem;cursor:pointer;border-radius:4px;border:1px solid #666;background:#eee}
</style>
</head><body>
<h1>IPv4-Bereich → IPv6-Tabelle</h1>
<form action="/range" method="post">
 <input name="start" type="text" placeholder="Start-IPv4" value="{{.Start}}" required />
 <input name="end"   type="text" placeholder="End-IPv4"   value="{{.End}}"   required />
 <button>Konvertieren</button>
 <a href="/" style="margin-left:1rem">&laquo; Einzelkonverter</a>
</form>
{{if .HaveResult}}
<table>
 <tr><th>IPv4</th><th>IPv6</th><th>DHCP-IPv4</th><th>DHCP-IPv6</th></tr>
 {{range .Rows}}<tr><td>{{.IPv4}}</td><td>{{.IPv6}}</td><td>netsh DHCP Server {{$.DhcpServer}} Scope {{$.DhcpScope}} Add reservedip {{.IPv4}} "{{.Name}}.stadt-hilden.de" "" "DHCP"</td><td>---</td></tr>{{end}}
</table>
{{end}}
{{if .Error}}<p style="color:#b00">Fehler: {{.Error}}</p>{{end}}
</body></html>`