package audit

import (
	"crypto/hmac"
	"encoding/hex"
	"encoding/json"

	udeModel "udeServer/internal/model"

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

// CalculateHMAC computes the HMAC-SM3 of the log entry (without the
// audit-state / HMAC / timestamp / id fields) with the given secret key.
func CalculateHMAC(entry *udeModel.TSystemLog, secretKey string, macBytes []byte) (string, error) {
	// 解析log对象为字节数组,并清空旧的Hmac
	data, err := generateHmacBytes(entry)
	if err != nil {
		return "", err
	}

	// 计算Hmac
	mac := hmac.New(sm3.New, []byte(secretKey))
	mac.Write(data)

	// 写入复用缓冲区,只清空长度,不清空容量
	entryHmac := mac.Sum(macBytes[:0])

	return hex.EncodeToString(entryHmac), nil
}

// generateHmacBytes marshals the entry and strips the fields excluded from
// the MAC computation. NOTE: TSystemLog has no json tags on purpose, so the
// marshalled keys equal the Go field names and this exclusion list stays in
// sync with the struct definition.
func generateHmacBytes(entry any) ([]byte, error) {
	data, err := json.Marshal(entry)
	if err != nil {
		return nil, err
	}

	var m map[string]interface{}
	if err = json.Unmarshal(data, &m); err != nil {
		return nil, err
	}

	// 移除Hmac无关字段
	delete(m, "AuditStatus")
	delete(m, "HMac")
	delete(m, "CreatedAt")
	delete(m, "Id")

	signContent, err := json.Marshal(m)
	if err != nil {
		return nil, err
	}
	return signContent, nil
}