package crypto

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

// Cipher 用主密钥派生的 AES-256-GCM 做敏感字段加解密。
type Cipher struct {
	aead cipher.AEAD
}

// NewCipher 以 SHA-256 从主密钥字符串派生 32 字节密钥，
// 允许 DATA_KEY 是任意长度的强随机字符串。
func NewCipher(masterKey string) (*Cipher, error) {
	if masterKey == "" {
		return nil, fmt.Errorf("new cipher: empty master key")
	}
	sum := sha256.Sum256([]byte(masterKey))
	block, err := aes.NewCipher(sum[:])
	if err != nil {
		return nil, fmt.Errorf("new cipher: %w", err)
	}
	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("new cipher: %w", err)
	}
	return &Cipher{aead: aead}, nil
}

// EncryptString 加密明文，返回 base64(nonce | ciphertext)。
func (c *Cipher) EncryptString(plaintext string) (string, error) {
	nonce := make([]byte, c.aead.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		return "", fmt.Errorf("encrypt: %w", err)
	}
	out := c.aead.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(out), nil
}

// DecryptString 解密 EncryptString 的输出。
func (c *Cipher) DecryptString(encoded string) (string, error) {
	raw, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", fmt.Errorf("decrypt: %w", err)
	}
	ns := c.aead.NonceSize()
	if len(raw) < ns {
		return "", fmt.Errorf("decrypt: ciphertext too short")
	}
	plain, err := c.aead.Open(nil, raw[:ns], raw[ns:], nil)
	if err != nil {
		return "", fmt.Errorf("decrypt: %w", err)
	}
	return string(plain), nil
}