package xy_sh

import (
	"bytes"
	"crypto/cipher"
	"crypto/hmac"
	"crypto/rand"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"io"
	"time"

	"github.com/tjfoc/gmsm/sm3"
	"github.com/tjfoc/gmsm/sm4"
)

// GenerateTimestampMillis 生成毫秒级时间戳字符串
func GenerateTimestampMillis() string {
	return fmt.Sprintf("%d", time.Now().UnixMilli())
}

// SM4Encrypt SM4-CBC模式加密
// 使用随机IV，IV拼在密文前面，最终输出base64编码字符串
func SM4Encrypt(plaintext []byte, key []byte) (string, error) {
	if len(key) != 16 {
		return "", fmt.Errorf("SM4密钥长度必须为16字节")
	}

	block, err := sm4.NewCipher(key)
	if err != nil {
		return "", fmt.Errorf("创建SM4 cipher失败: %v", err)
	}

	padded := pkcs7Padding(plaintext, block.BlockSize())

	iv := make([]byte, block.BlockSize())
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		return "", fmt.Errorf("生成IV失败: %v", err)
	}

	mode := cipher.NewCBCEncrypter(block, iv)
	ciphertext := make([]byte, len(padded))
	mode.CryptBlocks(ciphertext, padded)

	result := append(iv, ciphertext...)
	return base64.StdEncoding.EncodeToString(result), nil
}

// SM4Decrypt SM4-CBC模式解密
// 输入为base64编码字符串，前16字节为IV，后续为密文
func SM4Decrypt(encryptedData string, key []byte) ([]byte, error) {
	if len(key) != 16 {
		return nil, fmt.Errorf("SM4密钥长度必须为16字节")
	}

	data, err := base64.StdEncoding.DecodeString(encryptedData)
	if err != nil {
		return nil, fmt.Errorf("base64解码失败: %v", err)
	}

	block, err := sm4.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("创建SM4 cipher失败: %v", err)
	}

	blockSize := block.BlockSize()
	if len(data) < blockSize {
		return nil, fmt.Errorf("数据长度不足")
	}
	iv := data[:blockSize]
	ciphertext := data[blockSize:]

	mode := cipher.NewCBCDecrypter(block, iv)
	plaintext := make([]byte, len(ciphertext))
	mode.CryptBlocks(plaintext, ciphertext)

	plaintext, err = pkcs7UnPadding(plaintext)
	if err != nil {
		return nil, fmt.Errorf("去除填充失败: %v", err)
	}

	return plaintext, nil
}

// GenerateSign 生成签名
// 签名规则：将毫秒级时间戳timestamp字符串与encryptedData字符串直接拼接，
// 使用HMAC-SM3算法结合预先约定的SM3 salt进行加密，输出十六进制字符串
func GenerateSign(timestamp string, encryptedData string, sm3Salt []byte) string {
	data := timestamp + encryptedData
	mac := hmac.New(sm3.New, sm3Salt)
	mac.Write([]byte(data))
	return hex.EncodeToString(mac.Sum(nil))
}

// pkcs7Padding PKCS7填充
func pkcs7Padding(data []byte, blockSize int) []byte {
	padding := blockSize - len(data)%blockSize
	padText := bytes.Repeat([]byte{byte(padding)}, padding)
	return append(data, padText...)
}

// pkcs7UnPadding 去除PKCS7填充
func pkcs7UnPadding(data []byte) ([]byte, error) {
	length := len(data)
	if length == 0 {
		return nil, fmt.Errorf("数据为空")
	}
	padding := int(data[length-1])
	if padding > length || padding == 0 {
		return nil, fmt.Errorf("无效的填充")
	}
	for i := length - padding; i < length; i++ {
		if data[i] != byte(padding) {
			return nil, fmt.Errorf("无效的填充")
		}
	}
	return data[:length-padding], nil
}