package restutility
  
  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"

  type TokenClaims struct {
  	Username string `json:"username"`
  	Role string     `json:"role"`
  	jwt.StandardClaims
  }
  
  type CredentialsStruct struct {
  	Username string `json:"username"`
  	Password string `json:"password"`
  }

  func GenerateSalt() (string, error) {

  	salt := ""
  	
  	rawsalt := make([]byte, saltSize)
  	_, err := rand.Read(rawsalt)
  	if err != nil {
  		return "", err
  	}
  	salt = hex.EncodeToString(rawsalt)
  	return salt, nil
  }

  func HashMessage(message string, presalt string) (string, string, error) {

  	hash, salt := "", ""
  	var 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
  	rawmessage := []byte(message)
  	rawsalt, err := hex.DecodeString(salt)
  	if err != nil {
  		return "", "", err
  	}
  	rawdata := make([]byte, len(rawmessage) + len(rawsalt))
  	rawdata = append(rawdata, rawmessage...)
  	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
  }

  func IssueAPIToken(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
  }

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

  	tokenString = strings.TrimPrefix(tokenString, "Bearer ")
  	token, err := parseTokenFunc(tokenString)
  	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
  }

  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 := parseTokenFunc(tokenString)
  	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
  }
  
  func parseTokenFunc(tokenString string) (*jwt.Token, error) {
  	token, err := jwt.ParseWithClaims(tokenString,
  		&TokenClaims{},
  		func(token *jwt.Token) (interface{}, error) {
  			return []byte(secret), nil
  		},
  	)
  	return token, err
  }