package main

import (
  "fmt"
  "time"
  "net"
  "strings"
)

const host0 string = "192.168.1.4";
const host1 string = "192.168.1.8";

func main() {

  go monitor(host0)
  go monitor(host1)

  for {
    time.Sleep(time.Second)
  }
}

func monitor(host string) {

  fmt.Printf("Pinging %s...\n", host)

  for {
    ping(host)
    time.Sleep(time.Millisecond * 100)
  }
}

func ping(host string) {

  var message_data [512]byte
  // var bytes_read   int

  c, e := net.Dial("ip4:1", host)
  if e != nil { fmt.Printf("%s %s\n", host, e); return }
  defer c.Close()

  message_data[0] = 8   // ECHO
  message_data[1] = 0
  message_data[2] = 0   // Checksum (incorrect)
  message_data[3] = 0   // Checksum (also incorrect)
  message_data[4] = 0
  message_data[5] = 15
  message_data[6] = 0 // Seq (L-byte)
  message_data[7] = 0 // Seq (H-byte)

  checksum := compute_checksum(message_data[0:8])

  message_data[2] = byte(checksum >> 8)
  message_data[3] = byte(checksum & 0xFF)

  _, e = c.Write(message_data[0:8])
  if e != nil { return }

  c.SetReadDeadline(time.Now().Add(time.Second * 5))
  _, e = c.Read(message_data[0:])

  if e != nil { return }

  source_IP := fmt.Sprintf("%d.%d.%d.%d",
                           message_data[12],
                           message_data[13],
                           message_data[14],
                           message_data[15]);


  if (!strings.Contains(host, source_IP)) {
    // BUG! We received a response from a different IP address then we sent  //
    // the ICMP Echo request to.                                             //

    fmt.Printf("X")
  } else {
    fmt.Printf(".")
  }
}

func compute_checksum(message []byte) uint16 {

  var word_index uint16 = 0
  var sum uint16        = 0
  var answer uint16     = 0

  bytes_remaining := len(message)

  for bytes_remaining > 0 {
    sum += uint16(message[word_index]) * 0x100 + uint16(message[word_index + 1])
    word_index      += 2
    bytes_remaining -= 2

    if bytes_remaining == 1 {
      sum += uint16(message[len(message) - 1])
    }
  }

  answer = ^((sum & 0xFFFF) + (sum >> 16))

  return answer
}