package service

import (
	"crypto/sha1"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"regexp"
	"strings"
	"unicode/utf8"

	passwordvalidator "github.com/wagslane/go-password-validator"
)

const (
	minPasswordLength  = 12 // Entropy checks prevent short passwords anyway
	maxPasswordLength  = 72 // Limitation of bcrypt
	minPasswordEntropy = 60.0

	minUsernameLength = 3
	maxUsernameLength = 20

	hibpAPI = "https://api.pwnedpasswords.com/range" // Doesn't require an API key
)

var (
	usernameRegex = regexp.MustCompile("^[a-z0-9_]+$")
)

func respondJSON(w http.ResponseWriter, status int, data interface{}) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	json.NewEncoder(w).Encode(data)
}

func respondError(w http.ResponseWriter, status int, message string) {
	respondJSON(w, status, map[string]string{"error": message})
}

/*
	Client-side check:

	```
	function estimateEntropy(password: string): number {
	    const pool: number = getCharsetSize(password); // Character diversity (R)
	    const entropy: number = password.length * Math.log2(pool); // E = L * log_2(R)
	    return entropy; // Value (E) that can be compared against a hardcoded threshold (e.g. 60)
	}
	```
*/

// Validate the given password using a hybrid approach: length (max. set due to bcrypt's input
// upper limit of 72 bytes), entropy, and HIBP API.
func validatePassword(password string) error {
	if len(password) < minPasswordLength {
		return fmt.Errorf("password must be at least %d characters", minPasswordLength)
	}
	if len(password) > maxPasswordLength {
		return fmt.Errorf("password cannot be longer than %d characters", maxPasswordLength)
	}

	// Formatted error message will contain tips to increase the password strength (safe to show)
	err := passwordvalidator.Validate(password, minPasswordEntropy)
	if err != nil {
		return err
	}

	if compromised, _ := isPasswordCompromised(password); compromised {
		return errors.New("password is compromised")
	}

	return nil
}

// Send the first five bytes of the password's SHA-1 hash to HIBP API, then check if the rest of
// the hash is present in the APi's response data (k-Anonymity model).
func isPasswordCompromised(password string) (bool, error) {
	hash := sha1.Sum([]byte(password))
	hashStr := strings.ToUpper(hex.EncodeToString(hash[:]))
	prefix, suffix := hashStr[:5], hashStr[5:]

	resp, err := http.Get(fmt.Sprintf("%s/%s", hibpAPI, prefix))
	if err != nil {
		return false, err
	}
	defer resp.Body.Close()

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return false, err
	}

	return strings.Contains(strings.ToUpper(string(body)), suffix), nil
}

// Normalize the username by making it lowercase and trimming any leading or trailing whitespace.
func normalizeUsername(username string) string {
	return strings.ToLower(strings.TrimSpace(username))
}

/*
	Client-side check (additionally input should automatically perform the normalization steps):

	```
	function validateUsername(username: string): string {
	    const min: number = 3, max: number = 20;
	    if (username.length < min) return "Too short";
	    if (username.length > max) return "Too long";
	    if (!/^[a-zA-Z0-9_]+$/.test(username)) return "Invalid characters";
	    return "Valid";
	}
	```
*/

// Validate the given username by making sure it only contains alphanumeric characters or
// underscores and adheres the hardcoded minimum and maximum length rules.
func validateUsername(username string) error {
	if utf8.RuneCountInString(username) < minUsernameLength {
		return fmt.Errorf("username must be at least %d characters", minUsernameLength)
	}
	if utf8.RuneCountInString(username) > maxUsernameLength {
		return fmt.Errorf("username cannot be longer than %d characters", maxUsernameLength)
	}

	if !usernameRegex.MatchString(username) {
		return errors.New("username can only contain numbers, letters, and underscores")
	}

	return nil
}