package crypto

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"io"
	"math/big"

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

var (
	ErrInvalidKeySize     = errors.New("invalid key size")
	ErrInvalidCipherText  = errors.New("invalid ciphertext")
	ErrEncryptionFailed   = errors.New("encryption failed")
	ErrDecryptionFailed   = errors.New("decryption failed")
	ErrInvalidPublicKey   = errors.New("invalid public key")
	ErrInvalidSignature   = errors.New("invalid signature")
)

// CryptoService provides cryptographic operations
type CryptoService struct {
	masterKey []byte
}

// NewCryptoService creates a new crypto service
func NewCryptoService(masterKey []byte) (*CryptoService, error) {
	if len(masterKey) != 32 {
		return nil, ErrInvalidKeySize
	}
	return &CryptoService{masterKey: masterKey}, nil
}

// GenerateRandomBytes generates random bytes
func GenerateRandomBytes(n int) ([]byte, error) {
	b := make([]byte, n)
	_, err := rand.Read(b)
	if err != nil {
		return nil, err
	}
	return b, nil
}

// GenerateRandomHex generates a random hex string
func GenerateRandomHex(n int) (string, error) {
	bytes, err := GenerateRandomBytes(n)
	if err != nil {
		return "", err
	}
	return hex.EncodeToString(bytes), nil
}

// DeriveKey derives a key from the master key using HKDF
func (c *CryptoService) DeriveKey(context string, length int) ([]byte, error) {
	hkdfReader := hkdf.New(sha256.New, c.masterKey, nil, []byte(context))
	key := make([]byte, length)
	if _, err := io.ReadFull(hkdfReader, key); err != nil {
		return nil, err
	}
	return key, nil
}

// EncryptShare encrypts a key share using AES-256-GCM
func (c *CryptoService) EncryptShare(shareData []byte, partyID string) ([]byte, error) {
	// Derive a unique key for this party
	key, err := c.DeriveKey("share_encryption:"+partyID, 32)
	if err != nil {
		return nil, err
	}

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

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

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

	// Encrypt and prepend nonce
	ciphertext := aesGCM.Seal(nonce, nonce, shareData, []byte(partyID))
	return ciphertext, nil
}

// DecryptShare decrypts a key share
func (c *CryptoService) DecryptShare(encryptedData []byte, partyID string) ([]byte, error) {
	// Derive the same key used for encryption
	key, err := c.DeriveKey("share_encryption:"+partyID, 32)
	if err != nil {
		return nil, err
	}

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

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

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

	nonce, ciphertext := encryptedData[:nonceSize], encryptedData[nonceSize:]
	plaintext, err := aesGCM.Open(nil, nonce, ciphertext, []byte(partyID))
	if err != nil {
		return nil, ErrDecryptionFailed
	}

	return plaintext, nil
}

// EncryptMessage encrypts a message using AES-256-GCM
func (c *CryptoService) EncryptMessage(plaintext []byte) ([]byte, error) {
	block, err := aes.NewCipher(c.masterKey)
	if err != nil {
		return nil, err
	}

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

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

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

// DecryptMessage decrypts a message
func (c *CryptoService) DecryptMessage(ciphertext []byte) ([]byte, error) {
	block, err := aes.NewCipher(c.masterKey)
	if err != nil {
		return nil, err
	}

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

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

	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
	plaintext, err := aesGCM.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return nil, ErrDecryptionFailed
	}

	return plaintext, nil
}

// Hash256 computes SHA-256 hash
func Hash256(data []byte) []byte {
	hash := sha256.Sum256(data)
	return hash[:]
}

// VerifyECDSASignature verifies an ECDSA signature
func VerifyECDSASignature(messageHash, signature, publicKey []byte) (bool, error) {
	// Parse public key (assuming secp256k1/P256 uncompressed format)
	curve := elliptic.P256()
	x, y := elliptic.Unmarshal(curve, publicKey)
	if x == nil {
		return false, ErrInvalidPublicKey
	}

	pubKey := &ecdsa.PublicKey{
		Curve: curve,
		X:     x,
		Y:     y,
	}

	// Parse signature (R || S, each 32 bytes)
	if len(signature) != 64 {
		return false, ErrInvalidSignature
	}

	r := new(big.Int).SetBytes(signature[:32])
	s := new(big.Int).SetBytes(signature[32:])

	// Verify signature
	valid := ecdsa.Verify(pubKey, messageHash, r, s)
	return valid, nil
}

// GenerateNonce generates a cryptographic nonce
func GenerateNonce() ([]byte, error) {
	return GenerateRandomBytes(32)
}

// SecureCompare performs constant-time comparison
func SecureCompare(a, b []byte) bool {
	if len(a) != len(b) {
		return false
	}

	var result byte
	for i := 0; i < len(a); i++ {
		result |= a[i] ^ b[i]
	}
	return result == 0
}

// ParsePublicKey parses a public key from bytes (P256 uncompressed format)
func ParsePublicKey(publicKeyBytes []byte) (*ecdsa.PublicKey, error) {
	curve := elliptic.P256()
	x, y := elliptic.Unmarshal(curve, publicKeyBytes)
	if x == nil {
		return nil, ErrInvalidPublicKey
	}

	return &ecdsa.PublicKey{
		Curve: curve,
		X:     x,
		Y:     y,
	}, nil
}

// VerifySignature verifies an ECDSA signature using a public key
func VerifySignature(pubKey *ecdsa.PublicKey, messageHash, signature []byte) bool {
	// Parse signature (R || S, each 32 bytes)
	if len(signature) != 64 {
		return false
	}

	r := new(big.Int).SetBytes(signature[:32])
	s := new(big.Int).SetBytes(signature[32:])

	return ecdsa.Verify(pubKey, messageHash, r, s)
}

// HashMessage computes SHA-256 hash of a message (alias for Hash256)
func HashMessage(message []byte) []byte {
	return Hash256(message)
}

// Encrypt encrypts data using AES-256-GCM with the provided key
func Encrypt(key, plaintext []byte) ([]byte, error) {
	if len(key) != 32 {
		return nil, ErrInvalidKeySize
	}

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

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

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

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

// Decrypt decrypts data using AES-256-GCM with the provided key
func Decrypt(key, ciphertext []byte) ([]byte, error) {
	if len(key) != 32 {
		return nil, ErrInvalidKeySize
	}

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

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

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

	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
	plaintext, err := aesGCM.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return nil, ErrDecryptionFailed
	}

	return plaintext, nil
}

// DeriveKey derives a key from secret and salt using HKDF (standalone function)
func DeriveKey(secret, salt []byte, length int) ([]byte, error) {
	hkdfReader := hkdf.New(sha256.New, secret, salt, nil)
	key := make([]byte, length)
	if _, err := io.ReadFull(hkdfReader, key); err != nil {
		return nil, err
	}
	return key, nil
}

// SignMessage signs a message using ECDSA private key
func SignMessage(privateKey *ecdsa.PrivateKey, message []byte) ([]byte, error) {
	hash := Hash256(message)
	r, s, err := ecdsa.Sign(rand.Reader, privateKey, hash)
	if err != nil {
		return nil, err
	}

	// Encode R and S as 32 bytes each (total 64 bytes)
	signature := make([]byte, 64)
	rBytes := r.Bytes()
	sBytes := s.Bytes()

	// Pad with zeros if necessary
	copy(signature[32-len(rBytes):32], rBytes)
	copy(signature[64-len(sBytes):64], sBytes)

	return signature, nil
}

// EncodeToHex encodes bytes to hex string
func EncodeToHex(data []byte) string {
	return hex.EncodeToString(data)
}

// DecodeFromHex decodes hex string to bytes
func DecodeFromHex(s string) ([]byte, error) {
	return hex.DecodeString(s)
}

// EncodeToBase64 encodes bytes to base64 string
func EncodeToBase64(data []byte) string {
	return hex.EncodeToString(data) // Using hex for simplicity, could use base64
}

// DecodeFromBase64 decodes base64 string to bytes
func DecodeFromBase64(s string) ([]byte, error) {
	return hex.DecodeString(s)
}

// MarshalPublicKey marshals an ECDSA public key to bytes
func MarshalPublicKey(pubKey *ecdsa.PublicKey) []byte {
	return elliptic.Marshal(pubKey.Curve, pubKey.X, pubKey.Y)
}

// CompareBytes performs constant-time comparison of two byte slices
func CompareBytes(a, b []byte) bool {
	return SecureCompare(a, b)
}