package main

import (
	"bytes"
	"crypto/hmac"
	"crypto/sha512"
	"crypto/tls"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"os"
	"path"
	"strings"
	"text/template"
	"time"

	"gopkg.in/ldap.v2"
)

var loginSalt string

type loginChecker struct {
	students         []Student
	noAuth           bool
	ldapAddr         string
	ldapPort         int
	ldapIsTLS        bool
	ldapBase         string
	ldapBindUsername string
	ldapBindPassword string
}

type loginUpload struct {
	Username string
	Password string
}

func (l loginChecker) ldapAuth(username, password string) (res bool, err error) {
	tlsCnf := tls.Config{InsecureSkipVerify: true}

	var c *ldap.Conn

	if l.ldapIsTLS {
		c, err = ldap.DialTLS("tcp", fmt.Sprintf("%s:%d", l.ldapAddr, l.ldapPort), &tlsCnf)
		if err != nil {
			return false, err
		}
	} else {
		c, err = ldap.Dial("tcp", fmt.Sprintf("%s:%d", l.ldapAddr, l.ldapPort))
		if err != nil {
			return false, err
		}

		// Reconnect with TLS
		err = c.StartTLS(&tlsCnf)
		if err != nil {
			return false, err
		}
	}
	defer c.Close()

	if l.ldapBindUsername != "" {
		err = c.Bind(l.ldapBindUsername, l.ldapBindPassword)
		if err != nil {
			return false, err
		}
	}

	// Search for the given username
	searchRequest := ldap.NewSearchRequest(
		l.ldapBase,
		ldap.ScopeWholeSubtree, ldap.NeverDerefAliases, 0, 0, false,
		fmt.Sprintf("(&(objectClass=person)(uid=%s))", username),
		[]string{"dn"},
		nil,
	)

	sr, err := c.Search(searchRequest)
	if err != nil {
		return false, err
	}

	if len(sr.Entries) != 1 {
		return false, errors.New("User does not exist or too many entries returned")
	}

	userdn := sr.Entries[0].DN

	err = c.Bind(userdn, password)
	if err != nil {
		return false, err
	}

	return true, nil
}

func (l loginChecker) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	if addr := r.Header.Get("X-Forwarded-For"); addr != "" {
		r.RemoteAddr = addr
	}
	log.Printf("%s \"%s %s\" [%s]\n", r.RemoteAddr, r.Method, r.URL.Path, r.UserAgent())

	w.Header().Set("Content-Type", "text/plain")

	// Check request type and size
	if r.Method != "POST" {
		http.Error(w,
			"Invalid request",
			http.StatusBadRequest)
		return
	} else if r.ContentLength < 0 || r.ContentLength > 1023 {
		http.Error(w,
			"Request entity too large",
			http.StatusRequestEntityTooLarge)
		return
	}

	dec := json.NewDecoder(r.Body)
	var lu loginUpload
	if err := dec.Decode(&lu); err != nil {
		http.Error(w,
			err.Error(),
			http.StatusBadRequest)
		return
	}

	// Perform login check
	canContinue := false
	for _, std := range l.students {
		if std.Login == lu.Username {
			canContinue = true
		}
	}

	if !canContinue {
		log.Println("Login not found:", lu.Username, "at", r.RemoteAddr)
		http.Error(w, "Login not found in whitelist.", http.StatusUnauthorized)
		return
	}

	if ! l.noAuth {
		if ok, err := l.ldapAuth(lu.Username, lu.Password); err != nil {
			log.Println("Unable to perform authentication for", lu.Username, ":", err, "at", r.RemoteAddr)
			http.Error(w, err.Error(), http.StatusUnauthorized)
			return
		} else if !ok {
			log.Println("Login failed:", lu.Username, "at", r.RemoteAddr)
			http.Error(w, "Invalid password", http.StatusUnauthorized)
			return
		}
	}

	// Find corresponding MAC
	var fname string
	spl := strings.SplitN(r.RemoteAddr, ":", 2)
	if ip := net.ParseIP(spl[0]); ip == nil {
		http.Error(w, "Unable to parse given IPv4: " + spl[0], http.StatusInternalServerError)
		return
	} else if arptable, err := ARPAnalyze(); err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	} else if arpent := ARPContainsIP(arptable, ip); arpent == nil {
		http.Error(w, "Unable to find MAC in ARP table", http.StatusInternalServerError)
		return
	} else {
		fname = fmt.Sprintf("%02x-%02x-%02x-%02x-%02x-%02x-%02x", arpent.HWType, arpent.HWAddress[0], arpent.HWAddress[1], arpent.HWAddress[2], arpent.HWAddress[3], arpent.HWAddress[4], arpent.HWAddress[5])
	}

	// Register the user remotely
	if err := l.registerUser(lu.Username, r.RemoteAddr, fname); err != nil {
		log.Println("Error on remote registration:", err)
		http.Error(w, "Internal server error. Please retry in a few minutes", http.StatusInternalServerError)
		return
	}

	// Generate PXE file
	if err := l.lateLoginAction(lu.Username, r.RemoteAddr, fname); err != nil {
		log.Println("Error on late login action:", err)
		http.Error(w, "Internal server error. Please retry in a few minutes", http.StatusInternalServerError)
		return
	}

	log.Println("Successful login of", lu.Username, "at", r.RemoteAddr)
	http.Error(w, "Success", http.StatusOK)
}

func (l loginChecker) registerUser(username, remoteAddr, mac string) error {
	bts, err := json.Marshal(map[string]interface{}{"login": username, "ip": remoteAddr, "mac": mac})
	if err != nil {
		return nil
	}

	req, err := http.NewRequest("POST", "https://adlin.nemunai.re/api/students/", bytes.NewReader(bts))
	if err != nil {
		return err
	}
	req.Header.Add("X-ADLIN-Authentication", base64.StdEncoding.EncodeToString(hmac.New(sha512.New, []byte(loginSalt)).Sum([]byte(fmt.Sprintf("%d", time.Now().Unix()/10)))))
	req.Header.Set("Content-Type", "application/json")

	client := &http.Client{}
	resp, err := client.Do(req)
	if err != nil {
		return err
	}

	if resp.StatusCode != http.StatusOK {
		return errors.New(resp.Status)
	} else {
		return nil
	}
}

func (l loginChecker) lateLoginAction(username, remoteAddr, fname string) error {
	if tpl, err := ioutil.ReadFile(path.Join(tftpDir, "pxelinux.cfg", "tpl")); err != nil {
		log.Println("Unable to open tpl: ", err)
	} else if file, err := os.OpenFile(path.Join(tftpDir, "pxelinux.cfg", fname), os.O_CREATE|os.O_WRONLY|os.O_TRUNC, os.FileMode(0644)); err != nil {
		log.Println("Unable to open destination file: ", err)
	} else {
		defer file.Close()

		mac := hmac.New(sha512.New512_224, []byte(loginSalt))

		if configTmpl, err := template.New("pxelinux.cfg").Parse(string(tpl)); err != nil {
			log.Println("Cannot create template: ", err)
		} else if err := configTmpl.Execute(file, map[string]string{"username": username, "remoteAddr": remoteAddr, "pkey": fmt.Sprintf("%x", mac.Sum([]byte(username))), "fname": fname}); err != nil {
			log.Println("An error occurs during template execution: ", err)
		}
	}

	return nil
}