package main

import (
	"errors"
	"flag"
	"fmt"
	"image"
	"image/gif"
	"image/jpeg"
	"image/png"
	"os"
	"strings"

	"golang.org/x/image/bmp"
	"golang.org/x/image/tiff"
	_ "golang.org/x/image/webp"
)

func load(filename string) (image.Image, error) {
	file := os.Stdin
	if filename != "-" {
		var err error
		file, err = os.Open(filename)
		if err != nil {
			return nil, err
		}
		defer file.Close()
	}

	img, _, err := image.Decode(file)
	if err != nil {
		return nil, fmt.Errorf("decode %q: %s", filename, err)
	}

	return img, nil
}

func run() error {
	args := flag.Args()
	if len(args) != 2 {
		return errors.New("usage: convert input-file output-file")
	}
	inputFileName, outputFileName := args[0], args[1]

	n := strings.LastIndex(outputFileName, ".")
	if n < 0 {
		return errors.New("unknown output file extension")
	}
	ext := outputFileName[n+1:]
	switch ext {
	case "gif", "jpeg", "jpg", "png", "bmp", "tiff":
	default:
		return fmt.Errorf("unknown output file extension %q", ext)
	}

	input, err := load(inputFileName)
	if err != nil {
		return err
	}

	outputFile, err := os.Create(outputFileName)
	if err != nil {
		return err
	}
	defer outputFile.Close()
	switch ext {
	case "gif":
		err = gif.Encode(outputFile, input, nil)
	case "jpeg", "jpg":
		err = jpeg.Encode(outputFile, input, nil)
	case "png":
		err = png.Encode(outputFile, input)
	case "bmp":
		err = bmp.Encode(outputFile, input)
	case "tiff":
		err = tiff.Encode(outputFile, input, nil)
	}
	if err != nil {
		return err
	}
	return outputFile.Close()
}

func main() {
	flag.Usage = func() {
		fmt.Fprint(os.Stderr, `Usage: convert input-file output-file
Supported formats:
  png
  jpeg
  bmp
  tiff
  webp (input only)
`)
	}
	flag.Parse()

	if err := run(); err != nil {
		fmt.Fprintf(os.Stderr, "convert: %s\n", err)
		os.Exit(1)
	}
}