package handlers

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"log"
	"net/http"
	"net/url"
	"strings"
	"time"

	"github.com/gorilla/mux"
	"github.com/pkg/errors"
	bolt "go.etcd.io/bbolt"

	"gitea.hashru.nl/dsprenkels/rushlink/db"
	"gitea.hashru.nl/dsprenkels/rushlink/gobmarsh"
)

type pasteType int
type pasteState int

type storedPaste struct {
	Type        pasteType
	State       pasteState
	Content     []byte
	Key         string
	DeleteToken [16]byte
	TimeCreated time.Time
}

const (
	typeUndef    pasteType = 0
	typePaste              = 1
	typeRedirect           = 2
)

const (
	stateUndef   pasteState = 0
	statePresent            = 1
	stateDeleted            = 2
)

type viewPaste uint

const (
	_ viewPaste = 1 << iota
	viewNoRedirect
	viewShowMeta
)

const CookieDeleteToken = "owner_token"

// These keys are designated reserved, and will not be randomly chosen
var ReservedPasteKeys = []string{"xd42", "example"}

// Base64 encoding and decoding
var base64Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
var base64Encoder = base64.RawURLEncoding.WithPadding(base64.NoPadding)

func (t pasteType) String() string {
	switch t {
	case typeUndef:
		return "unknown"
	case typePaste:
		return "paste"
	case typeRedirect:
		return "redirect"
	default:
		return "invalid"
	}
}

func (t pasteState) String() string {
	switch t {
	case stateUndef:
		return "unknown"
	case statePresent:
		return "present"
	case stateDeleted:
		return "deleted"
	default:
		return "invalid"
	}
}

func indexGetHandler(w http.ResponseWriter, r *http.Request) {
	render(w, r, "index", map[string]interface{}{})
}

func viewPasteHandler(w http.ResponseWriter, r *http.Request) {
	viewPasteHandlerInner(w, r, 0)
}

func viewPasteHandlerNoRedirect(w http.ResponseWriter, r *http.Request) {
	viewPasteHandlerInner(w, r, viewNoRedirect)
}

func viewPasteHandlerMeta(w http.ResponseWriter, r *http.Request) {
	viewPasteHandlerInner(w, r, viewShowMeta)
}

func viewPasteHandlerInner(w http.ResponseWriter, r *http.Request, flags viewPaste) {
	vars := mux.Vars(r)
	key := vars["key"]
	var storedPaste *storedPaste
	if err := db.DB.View(func(tx *bolt.Tx) error {
		var err error
		storedPaste, err = getURL(tx, key)
		return err
	}); err != nil {
		log.Printf("error: %v\n", err)
		renderInternalServerError(w, r, err)
		return
	}
	if storedPaste == nil {
		renderError(w, r, http.StatusNotFound, "url key not found in the database")
		return
	}

	if flags&viewShowMeta != 0 {
		canDelete := struct {
			Bool   bool
			String string
		}{Bool: false}
		deleteToken, err := getDeleteTokenFromRequest(r)
		if err != nil {
			canDelete.String = "invalid"
		} else if deleteToken == nil {
			canDelete.String = "undefined"
		} else {
			if subtle.ConstantTimeCompare(deleteToken[:], storedPaste.DeleteToken[:]) == 1 {
				canDelete.Bool = true
				canDelete.String = "correct"
			} else {
				canDelete.String = "invalid"
			}
		}

		data := map[string]interface{}{
			"Paste":     storedPaste,
			"CanDelete": canDelete,
		}
		render(w, r, "pasteMeta", data)
		return
	}

	switch storedPaste.State {
	case statePresent:
		if flags&viewNoRedirect == 0 {
			rawurl := string(storedPaste.Content)
			urlParse, err := url.Parse(rawurl)
			if err != nil {
				log.Printf("error: invalid URL ('%v') in database for key '%v': %v\n", rawurl, storedPaste.Key, err)
				renderInternalServerError(w, r, "invalid url in database")
				return
			}
			http.Redirect(w, r, urlParse.String(), http.StatusSeeOther)
		}
		w.Write(storedPaste.Content)
	case stateDeleted:
		renderError(w, r, http.StatusGone, "key has been deleted")
	default:
		log.Printf("error: invalid storedPaste.State (%v) for key '%v'\n", storedPaste.State, storedPaste.Key)
		msg := fmt.Sprintf("internal server error: invalid storedPaste.State (%v\n)", storedPaste.State)
		renderInternalServerError(w, r, msg)
	}
}

func newPasteHandler(w http.ResponseWriter, r *http.Request) {
	if err := r.ParseMultipartForm(50 * 1000 * 1000); err != nil {
		log.Printf("error: %v\n", err)
		renderInternalServerError(w, r, err)
		return
	}

	// Determine what kind of post this is, currently only `shorten=...`
	if len(r.PostForm) == 0 {
		renderError(w, r, http.StatusBadRequest, "empty body in POST request\n")
		return
	}
	shorten_values, prs := r.PostForm["shorten"]
	if !prs {
		renderError(w, r, http.StatusBadRequest, "no 'shorten' param given\n")
		return
	}
	if len(shorten_values) != 1 {
		renderError(w, r, http.StatusBadRequest, "only one 'shorten' param is allowed per request\n")
		return
	}

	newRedirectPasteHandler(w, r)
}

func newRedirectPasteHandler(w http.ResponseWriter, r *http.Request) {
	rawurl := r.PostForm.Get("shorten")
	userURL, err := url.ParseRequestURI(rawurl)
	if err != nil {
		msg := fmt.Sprintf("invalid url (%v): %v", err, rawurl)
		renderError(w, r, http.StatusBadRequest, msg)
		return
	}
	if userURL.Scheme == "" {
		renderError(w, r, http.StatusBadRequest, "invalid url (unspecified scheme)")
		return
	}
	if userURL.Host == "" {
		renderError(w, r, http.StatusBadRequest, "invalid url (unspecified host)")
		return
	}

	var storedPaste *storedPaste
	if err := db.DB.Update(func(tx *bolt.Tx) error {
		// Generate a new delete token for this paste
		deleteToken, err := generateDeleteToken()
		if err != nil {
			return errors.Wrap(err, "generating delete token")
		}

		sp, err := shortenURL(tx, userURL, deleteToken)
		storedPaste = sp
		return err
	}); err != nil {
		log.Printf("error: %v\n", err)
		renderInternalServerError(w, r, err)
		return
	}

	deleteToken := hex.EncodeToString(storedPaste.DeleteToken[:])
	saveRawurl := fmt.Sprintf("%v/%v?deleteToken=%v", r.Host, string(storedPaste.Key), deleteToken)
	saveURL, err := r.URL.Parse(saveRawurl)
	if err != nil {
		err = errors.Wrap(err, "parsing url")
		log.Printf("error: %v\n", err)
		renderInternalServerError(w, r, err)
		return
	}

	// TODO(dsprenkels) Put this into a template
	w.WriteHeader(http.StatusOK)
	fmt.Fprintf(w, "%v\n", saveURL)
}

// Delete a URL from the database
func deletePasteHandler(w http.ResponseWriter, r *http.Request) {
	// TODO(dsprenkels) LEFT HERE; this functionality still untested
	vars := mux.Vars(r)
	key := vars["key"]

	deleteToken, err := getDeleteTokenFromRequest(r)
	if err != nil {
		renderError(w, r, http.StatusBadRequest, "invalid delete token")
		return
	} else if deleteToken == nil {
		renderError(w, r, http.StatusBadRequest, "no delete token provided")
		return
	}

	var errorCode int
	if err := db.DB.Update(func(tx *bolt.Tx) error {
		paste, err := getURL(tx, key)
		if err != nil {
			errorCode = http.StatusNotFound
			return err
		}
		if subtle.ConstantTimeCompare(deleteToken[:], paste.DeleteToken[:]) == 1 {
			// Replace the old paste with a new empty paste
			return savePaste(tx, key, storedPaste{
				Key:         paste.Key,
				State:       stateDeleted,
				DeleteToken: paste.DeleteToken,
			})
		}
		errorCode = http.StatusForbidden
		return errors.New("invalid delete token")
	}); err != nil {
		log.Printf("error: %v\n", err)
		renderError(w, r, errorCode, fmt.Sprintf("error: %v", err))
		return
	}
}

// Retrieve a URL from the database
func getURL(tx *bolt.Tx, key string) (*storedPaste, error) {
	pastesBucket := tx.Bucket([]byte(db.BUCKET_PASTES))
	if pastesBucket == nil {
		return nil, errors.Errorf("bucket %v does not exist", db.BUCKET_PASTES)
	}
	storedBytes := pastesBucket.Get([]byte(key))
	if storedBytes == nil {
		return nil, nil
	}
	storedPaste := &storedPaste{}
	err := gobmarsh.Unmarshal(storedBytes, storedPaste)
	return storedPaste, err
}

// Add a new URL to the database
//
// Returns the new ID if the url was successfully shortened
func shortenURL(tx *bolt.Tx, userURL *url.URL, deleteToken [16]byte) (*storedPaste, error) {
	pastesBucket := tx.Bucket([]byte(db.BUCKET_PASTES))
	if pastesBucket == nil {
		return nil, errors.Errorf("bucket %v does not exist", db.BUCKET_PASTES)
	}

	// Generate a key until it is not in the database, this occurs in O(log N),
	// where N is the amount of keys stored in the url-shorten database.
	epoch := 0
	var urlKey string
	for {
		var err error
		urlKey, err = generateURLKey(epoch)
		if err != nil {
			return nil, errors.Wrap(err, "url-key generation failed")
		}

		found := pastesBucket.Get([]byte(urlKey))
		if found == nil {
			break
		}

		isReserved := false
		for _, reservedKey := range ReservedPasteKeys {
			if strings.HasPrefix(urlKey, reservedKey) {
				isReserved = true
				break
			}
		}
		if !isReserved {
			break
		}

		epoch++
	}

	// Store the new key
	storedPaste := storedPaste{
		Type:        typeRedirect,
		State:       statePresent,
		Content:     []byte(userURL.String()),
		Key:         urlKey,
		DeleteToken: deleteToken,
		TimeCreated: time.Now().UTC(),
	}
	if err := savePaste(tx, urlKey, storedPaste); err != nil {
		return nil, err
	}
	return &storedPaste, nil
}

func savePaste(tx *bolt.Tx, key string, paste storedPaste) error {
	bucket := tx.Bucket([]byte(db.BUCKET_PASTES))
	if bucket == nil {
		return errors.Errorf("bucket %v does not exist", db.BUCKET_PASTES)
	}

	buf, err := gobmarsh.Marshal(paste)
	if err != nil {
		return errors.Wrap(err, "encoding for database failed")
	}
	if err := bucket.Put([]byte(key), buf); err != nil {
		return errors.Wrap(err, "database transaction failed")
	}
	return nil
}

func generateURLKey(epoch int) (string, error) {
	urlKey := make([]byte, 4+epoch)
	_, err := rand.Read(urlKey)
	if err != nil {
		return "", err
	}
	// Put all the values in the range 0..64 for easier base64-encoding
	for i := 0; i < len(urlKey); i++ {
		urlKey[i] &= 0x3F
	}
	// Implement truncate-resistance by forcing the prefix to
	//     0b111110xxxxxxxxxx
	//       ^----- {epoch} ones followed by a single 0
	//
	// Example when epoch is 1: prefix is 0b10.
	i := 0
	for i < epoch {
		// Set this bit to 1
		limb := i / 6
		bit := i % 6
		urlKey[limb] |= 1 << uint(5-bit)
		i++
	}
	// Finally set the next bit to 0
	limb := i / 6
	bit := i % 6
	urlKey[limb] &= ^(1 << uint(5-bit))

	// Convert this ID to a canonical base64 notation
	for i := range urlKey {
		urlKey[i] = base64Alphabet[urlKey[i]]
	}
	return string(urlKey), nil
}

func generateDeleteToken() ([16]byte, error) {
	var deleteToken [16]byte
	_, err := rand.Read(deleteToken[:])
	if err != nil {
		return deleteToken, err
	}
	return deleteToken, nil
}

func getDeleteTokenFromRequest(r *http.Request) (*[16]byte, error) {
	deleteTokenQuery := r.URL.Query().Get("deleteToken")
	if deleteTokenQuery == "" {
		return nil, nil
	}
	var deleteToken [16]byte
	n, err := hex.Decode(deleteToken[:], []byte(deleteTokenQuery))
	if err != nil {
		return nil, errors.Wrap(err, "decoding hex")
	} else if n != 16 {
		return nil, errors.Errorf("invalid deleteToken length (%v bytes)", n)
	}
	return &deleteToken, nil
}