// Package crypto provides AES-256-GCM encryption for sensitive data at rest.
package crypto

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

	"golang.org/x/crypto/bcrypt"
)

const (
	// BcryptCost is the bcrypt work factor for password hashing.
	BcryptCost = 12
)

// Encryptor wraps AES-256-GCM for field-level encryption.
type Encryptor struct {
	key []byte // 32 bytes
}

// New creates an Encryptor from a 32-byte key.
func New(key []byte) (*Encryptor, error) {
	if len(key) != 32 {
		return nil, errors.New("encryption key must be exactly 32 bytes")
	}
	k := make([]byte, 32)
	copy(k, key)
	return &Encryptor{key: k}, nil
}

// Encrypt encrypts plaintext using AES-256-GCM and returns a base64-encoded ciphertext.
// Format: base64(nonce || ciphertext || tag)
func (e *Encryptor) Encrypt(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}

	block, err := aes.NewCipher(e.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
	}

	// Seal appends ciphertext+tag to nonce
	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

// Decrypt decrypts a base64-encoded ciphertext produced by Encrypt.
func (e *Encryptor) Decrypt(encoded string) (string, error) {
	if encoded == "" {
		return "", nil
	}

	data, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", err
	}

	block, err := aes.NewCipher(e.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:]
	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", errors.New("decryption failed: data may be corrupted or key is wrong")
	}

	return string(plaintext), nil
}

// EncryptBytes encrypts raw bytes and returns base64 encoding.
func (e *Encryptor) EncryptBytes(data []byte) (string, error) {
	return e.Encrypt(string(data))
}

// DecryptBytes decrypts to raw bytes.
func (e *Encryptor) DecryptBytes(encoded string) ([]byte, error) {
	s, err := e.Decrypt(encoded)
	if err != nil {
		return nil, err
	}
	return []byte(s), nil
}

// ---- Password hashing ----

// HashPassword hashes a password using bcrypt.
func HashPassword(password string) (string, error) {
	hash, err := bcrypt.GenerateFromPassword([]byte(password), BcryptCost)
	if err != nil {
		return "", err
	}
	return string(hash), nil
}

// CheckPassword compares a plaintext password against a bcrypt hash.
func CheckPassword(password, hash string) error {
	return bcrypt.CompareHashAndPassword([]byte(hash), []byte(password))
}

// GenerateToken generates a cryptographically secure random token of n bytes, base64-encoded.
func GenerateToken(n int) (string, error) {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.URLEncoding.EncodeToString(b), nil
}