package main

import (
	"fmt"
	"os"
	"strconv"
	"strings"
)

// Options contains the arguments understood by this small example CLI.
type Options struct {
	Name      string
	Count     int
	Verbose   bool
	Help      bool
	Arguments []string
}

// ParseArgs parses long options, short options, and positional arguments.
func ParseArgs(args []string) (Options, error) {
	opts := Options{Count: 1}

	for len(args) > 0 {
		arg := args[0]
		args = args[1:]

		switch {
		case arg == "--":
			opts.Arguments = append(opts.Arguments, args...)
			return opts, nil
		case arg == "-h" || arg == "--help":
			opts.Help = true
		case arg == "-v" || arg == "--verbose":
			opts.Verbose = true
		case arg == "-n" || arg == "--name":
			if len(args) == 0 {
				return opts, fmt.Errorf("%s requires a value", arg)
			}
			opts.Name, args = args[0], args[1:]
		case strings.HasPrefix(arg, "--name="):
			opts.Name = strings.TrimPrefix(arg, "--name=")
		case arg == "-c" || arg == "--count":
			if len(args) == 0 {
				return opts, fmt.Errorf("%s requires a value", arg)
			}
			var err error
			opts.Count, err = positiveInt(args[0])
			if err != nil {
				return opts, fmt.Errorf("%s: %w", arg, err)
			}
			args = args[1:]
		case strings.HasPrefix(arg, "--count="):
			var err error
			opts.Count, err = positiveInt(strings.TrimPrefix(arg, "--count="))
			if err != nil {
				return opts, fmt.Errorf("--count: %w", err)
			}
		case strings.HasPrefix(arg, "-"):
			return opts, fmt.Errorf("unknown option %q", arg)
		default:
			opts.Arguments = append(opts.Arguments, arg)
		}
	}

	return opts, nil
}

func positiveInt(value string) (int, error) {
	n, err := strconv.Atoi(value)
	if err != nil || n < 1 {
		return 0, fmt.Errorf("expected a positive integer, got %q", value)
	}
	return n, nil
}

func usage() {
	fmt.Fprintf(os.Stderr, `Usage: %s [options] [arguments...]

Options:
  -n, --name NAME    name used in the greeting
  -c, --count N      number of greetings (default 1)
  -v, --verbose      print additional information
  -h, --help         show this help
`, os.Args[0])
}

func main() {
	opts, err := ParseArgs(os.Args[1:])
	if err != nil {
		fmt.Fprintln(os.Stderr, "error:", err)
		usage()
		os.Exit(2)
	}
	if opts.Help {
		usage()
		return
	}

	name := opts.Name
	if name == "" {
		name = "world"
	}

	if opts.Verbose {
		fmt.Fprintf(os.Stderr, "name=%q count=%d arguments=%q\n",
			name, opts.Count, opts.Arguments)
	}

	for i := 0; i < opts.Count; i++ {
		fmt.Printf("Hello, %s!\n", name)
	}
}