package crypto

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

	"golang.org/x/crypto/pbkdf2"
)

const (
	KeyLength   = 32 // AES-256
	SaltLength  = 16
	Iterations  = 100000
)

var (
	ErrInvalidPassword = errors.New("invalid password")
	ErrDecryptionFailed = errors.New("decryption failed — wrong password or corrupted data")
)

// DeriveKey derives an AES-256 key from a password using PBKDF2
func DeriveKey(password string, salt []byte) []byte {
	return pbkdf2.Key([]byte(password), salt, Iterations, KeyLength, sha256.New)
}

// Encrypt encrypts plaintext using AES-256-GCM with a password
func Encrypt(plaintext []byte, password string) (string, error) {
	salt := make([]byte, SaltLength)
	if _, err := io.ReadFull(rand.Reader, salt); err != nil {
		return "", err
	}

	key := DeriveKey(password, salt)

	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}

	aesGCM, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}

	nonce := make([]byte, aesGCM.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}

	ciphertext := aesGCM.Seal(nil, nonce, plaintext, nil)

	// Format: base64(salt + nonce + ciphertext)
	result := make([]byte, 0, len(salt)+len(nonce)+len(ciphertext))
	result = append(result, salt...)
	result = append(result, nonce...)
	result = append(result, ciphertext...)

	return base64.StdEncoding.EncodeToString(result), nil
}

// Decrypt decrypts ciphertext using AES-256-GCM with a password
func Decrypt(encoded string, password string) ([]byte, error) {
	data, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return nil, ErrDecryptionFailed
	}

	if len(data) < SaltLength+12 { // 12 = minimum nonce size for GCM
		return nil, ErrDecryptionFailed
	}

	salt := data[:SaltLength]
	data = data[SaltLength:]

	key := DeriveKey(password, salt)

	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, ErrDecryptionFailed
	}

	aesGCM, err := cipher.NewGCM(block)
	if err != nil {
		return nil, ErrDecryptionFailed
	}

	nonceSize := aesGCM.NonceSize()
	if len(data) < nonceSize {
		return nil, ErrDecryptionFailed
	}

	nonce := data[:nonceSize]
	ciphertext := data[nonceSize:]

	plaintext, err := aesGCM.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return nil, ErrInvalidPassword
	}

	return plaintext, nil
}

// IsEncrypted checks if a string looks like base64-encoded encrypted data
func IsEncrypted(data string) bool {
	decoded, err := base64.StdEncoding.DecodeString(data)
	if err != nil {
		return false
	}
	// Minimum: 16 (salt) + 12 (nonce) + 16 (min ciphertext) = 44 bytes
	return len(decoded) >= 44
}

// HashPassword creates a SHA-256 hash of a password for verification
func HashPassword(password string) string {
	h := sha256.Sum256([]byte(password))
	return base64.StdEncoding.EncodeToString(h[:])
}