package main

import (
	"bufio"
	"flag"
	"os"
	"time"
)

var (
	file = flag.String("file", "", "File Path")
)

func init() {
	flag.Parse()
}

// tail prints the last n lines of the file, and
// then polls waiting for more data to be written
// and printing it.
func Tail(f *os.File, n int) {
	lines := make([]string, n) // circular buffer
	i := 0
	n = 0
	r := bufio.NewReader(f)
	w := bufio.NewWriter(os.Stdout)
	for {
		line, err := r.ReadString('\n')
		if len(line) > 0 {
			lines[i] = line
			i = (i + 1) % len(lines)
			if n < len(lines) {
				n++
			}
		}
		if err != nil {
			break
		}
	}
	for j := (i - n + len(lines)) % len(lines); n > 0; j, n =
		(j+1)%len(lines), n-1 {
		w.WriteString(lines[j])
	}
	w.Flush()
	buf := make([]byte, 8192)
	for {
		time.Sleep(1e9)
		for {
			n, err := f.Read(buf)
			if n > 0 {
				w.Write(buf[0:n])
			}
			if err != nil {
				break
			}
		}
		w.Flush()
		f.Seek(0, 2) // in case the file has been truncated.
	}
}

func main() {
	f, err := os.Open(*file)
	defer f.Close()
	if err != nil {
		flag.Usage()
		os.Exit(1)
	}
	Tail(f, 10)
}