package auth

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
)

func deriveKey(secret string) []byte {
	h := sha256.Sum256([]byte(secret))
	return h[:]
}

func Encrypt(plaintext, secret string) (string, error) {
	key := deriveKey(secret)
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

func Decrypt(encoded, secret string) (string, error) {
	data, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", err
	}
	key := deriveKey(secret)
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonceSize := gcm.NonceSize()
	if len(data) < nonceSize {
		return "", errors.New("ciphertext too short")
	}
	nonce, ciphertext := data[:nonceSize], data[nonceSize:]
	plain, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}
	return string(plain), nil
}

func HashToken(token string) string {
	h := sha256.Sum256([]byte(token))
	return fmt.Sprintf("%x", h)
}

func GenerateToken(n int) (string, error) {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.URLEncoding.EncodeToString(b)[:n], nil
}