package webutility

  
  import (

  	"crypto/rand"

  	"crypto/sha256"

  	"encoding/hex"

  	"errors"

  	"net/http"

  	"strings"
  	"time"

  	"github.com/dgrijalva/jwt-go"

  )

  const OneDay = time.Hour * 24
  const OneWeek = OneDay * 7

  const saltSize = 32

  const appName = "korisnicki-centar"
  const secret = "korisnicki-centar-api"

  type Role struct {

  	Name string `json:"name"`
  	ID   uint32 `json:"id"`

  }

  // TokenClaims are JWT token claims.

  type TokenClaims struct {

  	Token              string `json:"access_token"`
  	TokenType          string `json:"token_type"`
  	Username           string `json:"username"`
  	Role               string `json:"role"`
  	RoleID             uint32 `json:"role_id"`
  	ExpiresIn          int64  `json:"expires_in"`
  	jwt.StandardClaims        // extending a struct

  }

  // CredentialsStruct is an instace of username/password values.

  type CredentialsStruct struct {
  	Username string `json:"username"`
  	Password string `json:"password"`
  }

  // ValidateCredentials hashes pass and salt and returns comparison result with resultHash
  func ValidateCredentials(pass, salt, resultHash string) bool {
  	hash, _, err := CreateHash(pass, salt)

  	if err != nil {

  		return false

  	}

  	return hash == resultHash

  }

  // CreateHash hashes str using SHA256.
  // If the presalt parameter is not provided CreateHash will generate new salt string.
  // Returns hash and salt strings or an error if it fails.
  func CreateHash(str, presalt string) (hash, salt string, err error) {

  	// chech if message is presalted
  	if presalt == "" {

  		salt, err = randomSalt()

  		if err != nil {
  			return "", "", err
  		}
  	} else {
  		salt = presalt
  	}
  
  	// convert strings to raw byte slices

  	rawstr := []byte(str)

  	rawsalt, err := hex.DecodeString(salt)
  	if err != nil {
  		return "", "", err
  	}

  	rawdata := make([]byte, len(rawstr)+len(rawsalt))

  	rawdata = append(rawdata, rawstr...)

  	rawdata = append(rawdata, rawsalt...)
  
  	// hash message + salt
  	hasher := sha256.New()
  	hasher.Write(rawdata)
  	rawhash := hasher.Sum(nil)

  	hash = hex.EncodeToString(rawhash)
  	return hash, salt, nil
  }

  // CreateAuthToken returns JWT token with encoded username, role, expiration date and issuer claims.

  // It returns an error if it fails.

  func CreateAuthToken(username string, role Role) (TokenClaims, error) {
  	t0 := (time.Now()).Unix()
  	t1 := (time.Now().Add(OneWeek)).Unix()

  	claims := TokenClaims{

  		TokenType: "Bearer",
  		Username:  username,
  		Role:      role.Name,
  		RoleID:    role.ID,
  		ExpiresIn: t1 - t0,

  	}

  	// initialize jwt.StandardClaims fields (anonymous struct)
  	claims.IssuedAt = t0
  	claims.ExpiresAt = t1
  	claims.Issuer = appName

  
  	jwtToken := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)

  	token, err := jwtToken.SignedString([]byte(secret))

  	if err != nil {

  		return TokenClaims{}, err

  	}

  	claims.Token = token
  	return claims, nil

  }

  // RefreshAuthToken prolongs JWT token's expiration date for one week.

  // It returns new JWT token or an error if it fails.

  func RefreshAuthToken(req *http.Request) (TokenClaims, error) {
  	authHead := req.Header.Get("Authorization")
  	tokenstr := strings.TrimPrefix(authHead, "Bearer ")
  	token, err := jwt.ParseWithClaims(tokenstr, &TokenClaims{}, secretFunc)

  	if err != nil {

  		return TokenClaims{}, err

  	}
  
  	// type assertion
  	claims, ok := token.Claims.(*TokenClaims)
  	if !ok || !token.Valid {

  		return TokenClaims{}, errors.New("token is not valid")

  	}

  	// extend token expiration date
  	return CreateAuthToken(claims.Username, Role{claims.Role, claims.RoleID})

  }

  // RbacCheck returns true if role that made HTTP request is authorized to
  // access the resource it is targeting.
  // It exctracts user's role from the JWT token located in Authorization header of
  // http.Request and then compares it with the list of supplied roles and returns
  // true if there's a match, if "*" is provided or if the authRoles is nil.
  // Otherwise it returns false.
  func RbacCheck(req *http.Request, authRoles []string) (*TokenClaims, error) {
  	if authRoles == nil {
  		return &TokenClaims{}, nil

  	}

  	// validate token and check expiration date
  	claims, err := GetTokenClaims(req)

  	if err != nil {
  		return &TokenClaims{}, err
  	}

  	// check if token has expired
  	if claims.ExpiresAt < (time.Now()).Unix() {
  		return &TokenClaims{}, errors.New("token has expired")
  	}

  	// check if role extracted from token matches
  	// any of the provided (allowed) ones
  	for _, r := range authRoles {
  		if claims.Role == r || r == "*" {
  			return claims, nil
  		}

  	}

  
  	return &TokenClaims{}, errors.New("role is not authorized")

  }

  // GetTokenClaims extracts JWT claims from Authorization header of the request.
  // Returns token claims or an error.
  func GetTokenClaims(req *http.Request) (*TokenClaims, error) {
  	// check for and strip 'Bearer' prefix
  	var tokstr string
  	authHead := req.Header.Get("Authorization")
  	if ok := strings.HasPrefix(authHead, "Bearer "); ok {
  		tokstr = strings.TrimPrefix(tokstr, "Bearer ")

  	} else {

  		return &TokenClaims{}, errors.New("authorization header in incomplete")

  	}

  	token, err := jwt.ParseWithClaims(tokstr, &TokenClaims{}, secretFunc)

  	if err != nil {
  		return &TokenClaims{}, err
  	}
  
  	// type assertion

  	claims, ok := token.Claims.(*TokenClaims)
  	if !ok || !token.Valid {

  		return &TokenClaims{}, errors.New("token is not valid")
  	}

  	return claims, nil

  }

  // randomSalt returns a string of random characters of 'saltSize' length.
  func randomSalt() (s string, err error) {
  	rawsalt := make([]byte, saltSize)

  	_, err = rand.Read(rawsalt)

  	if err != nil {

  		return "", err

  	}

  	s = hex.EncodeToString(rawsalt)
  	return s, nil

  }

  // secretFunc returns byte slice of API secret keyword.
  func secretFunc(token *jwt.Token) (interface{}, error) {
  	return []byte(secret), nil

  }