// Copyright 2019 Yang ZhongXi
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//     http://www.apache.org/licenses/LICENSE-2.0
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package core

import (
	"net"
	"time"
)

const echoRequestHeadLen = 8

// Ping PING指令，PING通了立即返回, host:IP地址, times: PING次数
func Ping(host string, times int) bool {
	conn, err := net.DialTimeout("ip4:icmp", host, time.Duration(time.Second))
	if err != nil {
		return false
	}
	defer conn.Close()
	id0, id1 := host[0], host[1]
	seq := 1
	for times > 0 {
		times--
		msg := []byte{
			0x08,       // 报文类型, 8-回显消息
			0x00,       // 请求和应答，都为0
			0x00, 0x00, // 校验码
			id0, id1, // ID标识符
			byte((seq >> 8) & 0xFF), byte(seq & 0xFF), // 序列号
		}
		msg = append(msg, make([]byte, 32)...) // 数据内容
		msg = append(msg, checkSum(msg)...)
		conn.SetDeadline(time.Now().Add(time.Duration(time.Second)))
		_, err = conn.Write(msg)
		if err != nil {
			return false
		}
		buffer := make([]byte, 1024)
		n, err := conn.Read(buffer)

		if err != nil {
			continue
		}
		if n > 32 {
			return true
		}
	}
	return false
}

func checkSum(data []byte) []byte {
	var (
		sum    uint32
		length int = len(data)
		index  int
	)
	//以每16位为单位进行求和，直到所有的字节全部求完或者只剩下一个8位字节（如果剩余一个8位字节说明字节数为奇数个）
	for length > 1 {
		sum += uint32(data[index])<<8 + uint32(data[index+1])
		index += 2
		length -= 2
	}
	//如果字节数为奇数个，要加上最后剩下的那个8位字节
	if length > 0 {
		sum += uint32(data[index])
	}
	//加上高16位进位的部分
	sum += (sum >> 16)
	//别忘了返回的时候先求反
	sum = ^sum
	return []byte{byte((sum >> 8) & 0xFF), byte(sum & 0xFF)}
}