package main

import (
	"bufio"
	flag "github.com/spf13/pflag"
	"fmt"
	"io"
	"os"
	"os/exec"
)

type selpgargs struct {
	start_page  int
	end_page    int
	input_file  string
	destination string
	page_len    int
	form_deli   string
}

var progname string

func FlagInit(args *selpgargs) {
	flag.IntVarP(&args.start_page, "startPage", "s", -1, "Start page number")
	flag.IntVarP(&args.end_page, "endPage", "e", -1, "End page number")
	flag.IntVarP(&args.page_len, "pageLength", "l", 72, "Line numbers for a page")
	flag.StringVarP(&args.form_deli, "type", "f", "l", "Determine form-feed-delimited")
	flag.StringVarP(&args.destination, "dest", "d", "", "set printer")
	flag.Usage = func () {
		fmt.Printf("%s is a tool to select pages from your chosen file\n\n", progname)
		fmt.Printf("Usage:\n\n")
		fmt.Printf("\tselpg -s start_page -e end_page [-f (speciy how the page is sperated)| -l lines_per_page_default_72] [-d dest] [filename]\n\n")
		fmt.Printf("If no file specified, %s will read input from stdin, and use control-D to end.\n\n", progname)
	}
	flag.Parse()
}

func ProcessArgs(args *selpgargs) {
	if len(os.Args)<3  {
		fmt.Fprintf(os.Stderr, "%s: not enough arguments\n\n", progname)
		flag.Usage()
		os.Exit(1)
	}

	if os.Args[1]!= "-s" {
		fmt.Fprintf(os.Stderr, "%s: 1st arg should be -s\n\n", progname)
		flag.Usage()
		os.Exit(1)
	}

	end_index := 2
	//若default则为2
	if len(os.Args[1]) == 2 {
		end_index = 3
	}

	if os.Args[end_index] != "-e" {
		fmt.Fprintf(os.Stderr, "%s: 2st arg should be -e\n\n", progname)
		flag.Usage()
		os.Exit(1)
	}

	if args.start_page > args.end_page || args.start_page < 0 ||
		args.end_page < 0 {
		fmt.Fprintln(os.Stderr, "Invalid arguments")
		flag.Usage()
		os.Exit(1)
	}
}

func ProcessInput(args *selpgargs) {
	var stdin io.WriteCloser
	var err error
	var cmd *exec.Cmd

	if args.destination != "" {
		cmd = exec.Command("cat", "-n")
		stdin, err = cmd.StdinPipe()
		if err != nil {
			fmt.Println(err)
		}
	} else {
		stdin = nil
	}

	if flag.NArg() > 0 {
		args.input_file = flag.Arg(0)
		output, err := os.Open(args.input_file)
		if err != nil {
			fmt.Println(err)
			os.Exit(1)
		}
		reader := bufio.NewReader(output)
		if args.form_deli != "l" {
			for pageNum := 0; pageNum <= args.end_page; pageNum++ {
				line, err := reader.ReadString('\f')
				if err != io.EOF && err != nil {
					fmt.Println(err)
					os.Exit(1)
				}
				if err == io.EOF {
					break
				}
				printOrWrite(args, string(line), stdin)
			}
		} else {
			count := 0
			for {
				line, _, err := reader.ReadLine()
				if err != io.EOF && err != nil {
					fmt.Println(err)
					os.Exit(1)
				}
				if err == io.EOF {
					break
				}
				if count/args.page_len >= args.start_page {
					if count/args.page_len > args.end_page {
						break
					} else {
						printOrWrite(args, string(line), stdin)
					}
				}
				count++
			}

		}
	} else {
		scanner := bufio.NewScanner(os.Stdin)
		count := 0
		target := ""
		for scanner.Scan() {
			line := scanner.Text()
			line += "\n"
			if count/args.page_len >= args.start_page {
				if count/args.page_len <= args.end_page {
					target += line
				}
			}
			count++
		}
		printOrWrite(args, string(target), stdin)
	}

	if args.destination != "" {
		stdin.Close()
		cmd.Stdout = os.Stdout
		cmd.Run()
	}
}

func printOrWrite(args *selpgargs, line string, stdin io.WriteCloser) {
	if args.destination != "" {
		stdin.Write([]byte(line + "\n"))
	} else {
		fmt.Println(line)
	}
}

func main() {
	progname = os.Args[0]
	var args selpgargs
	FlagInit(&args)
	ProcessArgs(&args)
	ProcessInput(&args)
}