package password

import (
	"crypto/rand"
	"errors"
	"math/big"
)

const defaultLength = 16

// charset excludes visually ambiguous characters: 0/O, 1/l/I
const (
	upperChars   = "ABCDEFGHJKLMNPQRSTUVWXYZ"
	lowerChars   = "abcdefghjkmnpqrstuvwxyz"
	digitChars   = "23456789"
	specialChars = "!@#$%^&*-_=+"
	allChars     = upperChars + lowerChars + digitChars + specialChars
)

// GenerateSecure returns a cryptographically random password that satisfies
// uppercase, lowercase, digit, and special character requirements.
func GenerateSecure() (string, error) {
	return GenerateSecureWithLength(defaultLength)
}

// GenerateSecureWithLength generates a password of at least 12 characters.
func GenerateSecureWithLength(length int) (string, error) {
	if length < 12 {
		length = 12
	}

	password := make([]byte, length)
	requiredSets := []string{upperChars, lowerChars, digitChars, specialChars}

	for i, charset := range requiredSets {
		ch, err := randomChar(charset)
		if err != nil {
			return "", err
		}
		password[i] = ch
	}

	for i := len(requiredSets); i < length; i++ {
		ch, err := randomChar(allChars)
		if err != nil {
			return "", err
		}
		password[i] = ch
	}

	if err := shuffle(password); err != nil {
		return "", err
	}

	return string(password), nil
}

func randomChar(charset string) (byte, error) {
	n := big.NewInt(int64(len(charset)))
	idx, err := rand.Int(rand.Reader, n)
	if err != nil {
		return 0, err
	}
	return charset[idx.Int64()], nil
}

func shuffle(password []byte) error {
	for i := len(password) - 1; i > 0; i-- {
		jBig, err := rand.Int(rand.Reader, big.NewInt(int64(i+1)))
		if err != nil {
			return err
		}
		j := int(jBig.Int64())
		password[i], password[j] = password[j], password[i]
	}
	return nil
}

// ValidatePolicy checks that a password meets admin-creation policy.
func ValidatePolicy(password string) error {
	if len(password) < 8 {
		return errors.New("password too short")
	}

	hasUpper, hasLower, hasDigit, hasSpecial := false, false, false, false
	for _, char := range password {
		switch {
		case char >= 'A' && char <= 'Z':
			hasUpper = true
		case char >= 'a' && char <= 'z':
			hasLower = true
		case char >= '0' && char <= '9':
			hasDigit = true
		default:
			if isSpecialRune(char) {
				hasSpecial = true
			}
		}
	}

	if !hasUpper || !hasLower || !hasDigit || !hasSpecial {
		return errors.New("password does not meet policy")
	}
	return nil
}

func isSpecialRune(char rune) bool {
	for _, c := range specialChars {
		if char == c {
			return true
		}
	}
	return false
}
