package main

import (
	"crypto/sha256"
	"encoding/hex"
	"math/rand"
	"regexp"
	"strconv"
	"strings"
	"time"

	"github.com/google/uuid"
)

var (
	usernamePattern = regexp.MustCompile(`^[0-9A-Za-z]+$`)
	passwordPattern = regexp.MustCompile(`^[!-~]+$`)
)

// IsValidUsername reports whether s is a valid username.
func IsValidUsername(s string) bool {
	return usernamePattern.MatchString(s)
}

// IsValidPassword reports whether s is a valid password.
func IsValidPassword(s string) bool {
	return passwordPattern.MatchString(s)
}

// ComputePasswordHash returns the lowercased SHA-256 hex digest of password.
func ComputePasswordHash(password string) string {
	sum := sha256.Sum256([]byte(password))
	return hex.EncodeToString(sum[:])
}

// GenerateUUID returns a new UUID string, preferring version 1 (mirroring
// python's uuid.uuid1()), falling back to a random version-4 UUID on error.
func GenerateUUID() string {
	u, err := uuid.NewUUID()
	if err != nil {
		u = uuid.New()
	}
	return u.String()
}

// GenerateToken returns a SHA-256 hex digest of the username concatenated with
// a fresh UUID.
func GenerateToken(username string) string {
	sum := sha256.Sum256([]byte(username + GenerateUUID()))
	return hex.EncodeToString(sum[:])
}

// GenerateSalt returns a random salt in the inclusive range [0, 6172748].
func GenerateSalt() int {
	return rand.Intn(6172749)
}

// ComputePasswordHashWithSalt returns the lowercased SHA-256 hex digest of
// passwordHashed concatenated with the decimal string form of salt.
func ComputePasswordHashWithSalt(passwordHashed string, salt int) string {
	sum := sha256.Sum256([]byte(passwordHashed + strconv.Itoa(salt)))
	return hex.EncodeToString(sum[:])
}

// GetCurrentTimestamp returns the current UNIX timestamp in seconds.
func GetCurrentTimestamp() int {
	return int(time.Now().Unix())
}

// GetTokenExpireOn returns the timestamp 2 days from now, used as the token
// expiry.
func GetTokenExpireOn() int {
	return GetCurrentTimestamp() + 60*60*24*2
}

// Str2Bool reports whether s, compared case-insensitively, equals "true".
func Str2Bool(s string) bool {
	return strings.ToLower(s) == "true"
}

// GCD returns the greatest common divisor of a and b via the Euclidean
// algorithm.
func GCD(a, b int) int {
	for b != 0 {
		a, b = b, a%b
	}
	return a
}

// LCM returns the least common multiple of a and b.
func LCM(a, b int) int {
	return (a * b) / GCD(a, b)
}