package webutility

  
  import (

  	"errors"

  	"time"
  	"crypto/sha256"
  	"crypto/rand"
  	"encoding/hex"
  	"strings"
  	"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"

  // TokenClaims are JWT token claims.

  type TokenClaims struct {
  	Username string `json:"username"`

  	Role     string `json:"role"`

  	jwt.StandardClaims
  }

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

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

  // generateSalt returns a string of random characters of 'saltSize' length.

  func generateSalt() (salt string, err error) {

  	rawsalt := make([]byte, saltSize)

  	_, err = rand.Read(rawsalt)

  	if err != nil {
  		return "", err
  	}

  	salt = hex.EncodeToString(rawsalt)
  	return salt, nil
  }

  // HashString hashes input string with SHA256 algorithm.
  // If the presalt parameter is not provided HashString will generate new salt string.

  // Returns hash and salt string or an error if it fails.

  func HashString(str string, presalt string) (hash, salt string, err error) {

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

  		salt, err = generateSalt()

  		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
  }

  // CreateAPIToken returns JWT token with encoded username, role, expiration date and issuer claims.
  // It returns an error if it fails.

  func CreateAPIToken(username, role string) (string, error) {

  	var apiToken string

  	var err error
  
  	if err != nil {

  		return "", err

  	}
  
  	claims := TokenClaims{
  		username,
  		role,
  		jwt.StandardClaims{
  			ExpiresAt: (time.Now().Add(OneWeek)).Unix(),
  			Issuer:    appName,
  		},
  	}
  
  	jwtToken := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)

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

  	if err != nil {

  		return "", err

  	}
  	return apiToken, nil
  }

  // RefreshAPIToken prolongs JWT token's expiration date for one week.
  // It returns new JWT token or an error if it fails.

  func RefreshAPIToken(tokenString string) (string, error) {

  	var newToken string

  	tokenString = strings.TrimPrefix(tokenString, "Bearer ")

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

  	if err != nil {

  		return "", err

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

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

  	}
  
  	claims.ExpiresAt = (time.Now().Add(OneWeek)).Unix()
  	jwtToken := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)

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

  	if err != nil {

  		return "", err

  	}
  
  	return newToken, nil
  }

  // ParseAPIToken parses JWT token claims.

  // It returns a pointer to TokenClaims struct or an error if it fails.

  func ParseAPIToken(tokenString string) (*TokenClaims, error) {

  	if ok := strings.HasPrefix(tokenString, "Bearer "); ok {

  		tokenString = strings.TrimPrefix(tokenString, "Bearer ")
  	} else {
  		return &TokenClaims{}, errors.New("Authorization header is incomplete")
  	}

  	token, err := jwt.ParseWithClaims(tokenString, &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
  }

  // secretFunc returns byte slice of API secret keyword.

  func secretFunc(token *jwt.Token) (interface{}, error) {
  	return []byte(secret), nil

  }