// Ruff provides a pop-up web server to Retrieve/Upload Files Fast over
// LAN, inspired by WOOF (Web Offer One File) by Simon Budig.
//
// It's based on the idea that not every device has <insert neat file transfer
// tool here>, but just about every device that can network has an HTTP client,
// making a hyper-simple HTTP server a viable option for file transfer with
// zero notice or setup as long as *somebody* has a copy of RUFF.
//
// Why create RUFF when WOOF exists? WOOF is no longer in the debian repos and
// it's easier to `go get` a tool than it is to hunt down Simon's website for
// the latest copy.
//
// Why use RUFF over something like Transfer.sh? Transfer.sh is fantastic for
// sharing files over the net, but you have to upload, wait for that, then wait
// on it to download on the destination. If you're sharing a WiFi network with
// your target device, it's a lot simpler and potentially MUCH faster to skip
// the middle man and chuck your file straight to its new home.
package main

import (
	"context"
	"html/template"
	"net"
	"net/http"
	"net/url"
	"time"

	"io"
	"mime/multipart"
	"os"
	"path"

	"errors"
	"flag"
	"fmt"
	"github.com/mdp/qrterminal"
)

// Config stores all settings for an instance of RUFF.
type Config struct {
	Downloads int
	Port      int
	FilePath  string
	FileName  string
	HideQR    bool
	Uploading bool
	Multiple  bool
}

// getConfig fills in a Config struct based on the command line arguments.
func getConfig() (Config, error) {
	conf := Config{
		Downloads: 1,
		Port:      8008,
		HideQR:    false,
		Uploading: false,
		Multiple:  true,
	}

	flag.IntVar(&conf.Downloads, "count", conf.Downloads, "number of downloads before exiting. set to -1 for unlimited downloads.")
	flag.IntVar(&conf.Port, "port", conf.Port, "port to serve file on.")
	flag.BoolVar(&conf.HideQR, "hide-qr", conf.HideQR, "hide the QR code.")
	flag.BoolVar(&conf.Uploading, "upload", false, "upload files instead of downloading")
	flag.BoolVar(&conf.Multiple, "multiple", conf.Multiple, "allow uploading multiple files at once")

	flag.IntVar(&conf.Downloads, "c", conf.Downloads, "number of downloads before exiting. set to -1 for unlimited downloads. (shorthand)")
	flag.IntVar(&conf.Port, "p", conf.Port, "port to serve file on. (shorthand)")
	flag.BoolVar(&conf.HideQR, "q", conf.HideQR, "hide the QR code. (shorthand)")
	flag.BoolVar(&conf.Uploading, "u", false, "upload files instead of downloading (shorthand)")
	flag.BoolVar(&conf.Multiple, "m", conf.Multiple, "allow uploading multiple files at once (shorthand)")

	flag.Parse()
	conf.FilePath = flag.Arg(0)
	conf.FileName = path.Base(conf.FilePath)

	if conf.FilePath == "" && !conf.Uploading {
		return conf, errors.New("no file provided")
	}

	return conf, nil
}

// getIP uses the net package to try and determine the local address of the
// device it's running on.
//
// Note: I guess since this is a UDP connection, nothing is actually sent, no
// connection is established. Target doesn't even need to really exist for us
// to be able to grab the local address.
func getIP() (string, error) {
	conn, err := net.Dial("udp", "8.8.8.8:80")
	if err != nil {
		return "", err
	}

	localAddr := conn.LocalAddr().(*net.UDPAddr)

	return localAddr.IP.String(), nil
}

// done is used to signal that the HTTP server has finished gracefully
// shutting down.
var done = make(chan struct{})

func main() {
	conf, err := getConfig()
	if err != nil {
		fmt.Printf("config error: %v\n", err)
		os.Exit(1)
	}

	server := &http.Server{
		Addr:         fmt.Sprintf(":%v", conf.Port),
		ReadTimeout:  10 * time.Second,
		WriteTimeout: 10 * time.Second,
	}

	if conf.Uploading {
		setupUpload(server, conf)
	} else {
		setupDownload(server, conf)
	}

	ip, err := getIP()
	if err != nil {
		fmt.Printf("failed to look up local IP: %v\n", err)
		os.Exit(1)
	}

	url := fmt.Sprintf("http://%s:%v/%s", ip, conf.Port, conf.FileName)
	if conf.Uploading {
		url = fmt.Sprintf("http://%s:%v", ip, conf.Port)
	}
	if !conf.HideQR {
		qrterminal.GenerateHalfBlock(url, qrterminal.M, os.Stdout)
	}
	fmt.Println(url)

	err = server.ListenAndServe()
	if err != nil && !errors.Is(err, http.ErrServerClosed) {
		fmt.Printf("server exited with error: %v\n", err)
		os.Exit(1)
	}

	// Wait for the server to finish any transfers, up to 3 seconds
	select {
	case <-done:
	case <-time.After(3 * time.Second):
	}
}

// setupDownload sets up the HTTP server for sending a file to a remote device.
func setupDownload(server *http.Server, conf Config) {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Printf("Incoming request: %v: %v %v %v\n", r.RemoteAddr, r.Proto, r.Method, r.URL)
		// 303 redirect to real file.
		http.RedirectHandler("/"+conf.FileName, http.StatusSeeOther).ServeHTTP(w, r)
	})

	downloads := conf.Downloads
	http.HandleFunc("/"+conf.FileName, func(w http.ResponseWriter, r *http.Request) {
		fmt.Printf("Incoming request: %v: %v %v %v\n", r.RemoteAddr, r.Proto, r.Method, r.URL)

		w.Header().Set("Content-Disposition", "attachment; filename=\""+url.PathEscape(conf.FileName)+"\"")
		// http.ServeFile handles all the nitty gritty details of hauling the file
		// off, but maybe it shouldn't? ServeFile does content ranges and I really
		// don't see that working with limited download counts unless we reimplement
		// all that logic ourselves.
		http.ServeFile(w, r, conf.FilePath)

		downloads--
		if downloads == 0 {
			go shutdown(server)
		}
	})
}

var baseHeader = `<!DOCTYPE html>
<html>
	<head>
		<title>{{.}}</title>
		<style>
			body {
				padding: 18pt;
				text-align: center;
				font: 16pt monospace;
				color: #212121;
			}
			form {
				display: inline-block;
				text-align: left;
			}
			input {
				font: inherit;
			}
		</style>
	</head>
	<body>`

var baseFooter = `</body>
</html>`

var uploadTemplate = `{{template "BaseHeader" "RUFF - Upload Form"}}
		<form enctype="multipart/form-data" action="/" method="post">
			<label for="file">Select a file for upload:</label><br><br>
			<input type="file" name="file"{{if .Multiple}} multiple{{end}}>
			<input type="submit" value="Upload">
		</form>
{{template "BaseFooter"}}`

var errorTemplate = `{{template "BaseHeader" "RUFF - Upload Error"}}
		<p>{{.}}</p>
		<p><a href="/">Go back</a></p>
{{template "BaseFooter"}}`

var messageTemplate = `{{template "BaseHeader" (print "RUFF - " .)}}
		<p>{{.}}</p>
{{template "BaseFooter"}}`

// setupUpload sets up the HTTP server for receiving a file from another device
// through an upload form and a small stack of templates.
//
// When go1.16 gets more widespread maybe I'll hack the templates off into
// their own files.
func setupUpload(server *http.Server, conf Config) {
	tpl := template.Must(template.New("BaseHeader").Parse(baseHeader))
	template.Must(tpl.New("BaseFooter").Parse(baseFooter))
	template.Must(tpl.New("UploadForm").Parse(uploadTemplate))
	template.Must(tpl.New("UploadError").Parse(errorTemplate))
	template.Must(tpl.New("UploadMessage").Parse(messageTemplate))

	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Printf("Incoming request: %v: %v %v %v\n", r.RemoteAddr, r.Proto, r.Method, r.URL)

		// Display upload form
		if r.Method != http.MethodPost {
			err := tpl.ExecuteTemplate(w, "UploadForm", conf)
			if err != nil {
				panic(err)
			}
			return
		}

		// Handle POSTed upload
		// Buffer a maximum of 20MB of form data in memory.
		r.ParseMultipartForm(20 << 20)

		// Collect all files from the form.
		// They're stored in a map of slices of file headers.
		files := make([]*multipart.FileHeader, 0, 1)
		for _, field := range r.MultipartForm.File {
			for _, header := range field {
				// Make sure there's only one file if we only expect one.
				if len(files) > 0 && !conf.Multiple {
					err := errors.New("multiple files found, only expected one file. start RUFF with -m for multiple file uploads.")
					tpl.ExecuteTemplate(w, "UploadError", err)
					fmt.Println(err)
					return
				}
				files = append(files, header)
			}
		}

		// Save all files to disk.
		for i := range files {
			err := saveFile(files[i])
			if err != nil {
				err = fmt.Errorf("could not save file %v: %w", files[i].Filename, err)
				tpl.ExecuteTemplate(w, "UploadError", err)
				fmt.Println(err)
				return
			}
		}

		tpl.ExecuteTemplate(w, "UploadMessage", "Upload successful!")
		fmt.Println("upload successful")
		go shutdown(server)
	})
}

// saveFile saves a fileHeader to the current working directory.
func saveFile(header *multipart.FileHeader) error {
	inFile, err := header.Open()
	if err != nil {
		return fmt.Errorf("could not open uploaded file: %w", err)
	}
	defer inFile.Close()

	outFile, err := os.Create(header.Filename)
	// TODO: This might fail if the file already exists, we should handle this
	// case specially.
	if err != nil {
		return fmt.Errorf("could not save uploaded file: %w", err)
	}
	defer outFile.Close()

	// TODO: If the file is large enough to be dumped to disk, we could assert it
	// as an os.File and move the file itself rather than copying it bit by bit.
	_, err = io.Copy(outFile, inFile)
	if err != nil {
		return fmt.Errorf("could not copy uploaded file to disk: %w", err)
	}

	fmt.Printf("Received file: %v\n", header.Filename)
	return nil
}

// shutdown shuts down the HTTP server, sending a signal when it's complete.
func shutdown(server *http.Server) {
	server.Shutdown(context.Background())
	done <- struct{}{}
}