package main

import (
	"errors"
	"flag"
	"fmt"
	"io"
	"mime"
	"mime/multipart"
	"net/mail"
	"os"
	"strings"
)

func main() {
	flag.Usage = func() {
		fmt.Fprintf(flag.CommandLine.Output(), "Usage: mimeslicer <command> [options]\n\n")
		fmt.Fprintf(flag.CommandLine.Output(), "Commands:\n")
		os.Exit(0)
	}

	if len(os.Args) < 2 {
		fmt.Println("Error: Command missing.")
		flag.Usage()
	}

	inspectCmd := flag.NewFlagSet("inspect", flag.ExitOnError)
	inspectCmdInputFile := inspectCmd.String("i", "", "input file")

	switch os.Args[1] {
	case "inspect":
		inspectCmd.Parse(os.Args[2:])

		if err := inspect(*inspectCmdInputFile); err != nil {
			fmt.Printf("Error: %s", err.Error())
			os.Exit(1)
		}
	case "-h", "--help", "help":
		flag.Usage()
	default:
		fmt.Printf("Error: unknown command %q\n\n", os.Args[1])
		flag.Usage()
	}
}

func inspect(file string) error {
	fd, err := os.Open(file)
	if err != nil {
		return err
	}
	defer fd.Close()

	msg, err := mail.ReadMessage(fd)
	if err != nil {
		return err
	}

	for k, v := range msg.Header {
		for _, val := range v {
			fmt.Printf("%s: %s\n", k, val)
		}
	}

	contentType := msg.Header.Get("Content-Type")
	mediaType, params, err := mime.ParseMediaType(contentType)
	if err != nil {
		return errors.Join(err, errors.New("failed to parse mediate type from  contentType"))
	}

	if !strings.HasPrefix(mediaType, "multipart/") {
		return errors.New("expected multipler/ TOOD")
	}

	boundary, ok := params["boundary"]
	if !ok {
		return errors.New("Error: no boundary in Content-Type header for multipart media type")
	}

	mr := multipart.NewReader(msg.Body, boundary)

	partIndex := 0
	fmt.Printf("\nMIME body parts\n\n")

	for {
		part, err := mr.NextPart()
		if err == io.EOF {
			break
		}

		fmt.Printf("[%d]\n", partIndex)

		fmt.Printf("Headers:\n")
		for k, v := range part.Header {
			for _, val := range v {
				fmt.Printf("  %s: %s\n", k, val)
			}
		}

		partIndex++
	}

	return nil
}