package main

import (
	"errors"
	"flag"
	"fmt"
	"image"
	"image/color"
	"image/png"
	"log"
	"os"
)

func run() error {
	flag.Parse()
	args := flag.Args()
	if len(args) < 1 {
		return errors.New("no command specified")
	}
	switch args[0] {
	case "setalpha":
		flags := flag.NewFlagSet("setalpha", flag.ContinueOnError)
		var alpha int
		flags.IntVar(&alpha, "alpha", 127, "sets the target alpha")
		flags.Parse(args[1:])
		for _, path := range flags.Args() {
			err := setAlpha(path, alpha)
			if err != nil {
				return fmt.Errorf("couldn't set alpha of '%s'; error: %v", path, err)
			}
		}
	case "gray":
		flags := flag.NewFlagSet("gray", flag.ContinueOnError)
		flags.Parse(args[1:])
		for _, path := range flags.Args() {
			err := gray(path)
			if err != nil {
				return fmt.Errorf("couldn't convert to grayscale of '%s'; error: %v", path, err)
			}
		}
	}
	return nil
}

func setAlpha(path string, alpha int) error {
	src, err := os.Open(path)
	if err != nil {
		return err
	}
	defer src.Close()
	im, _, err := image.Decode(src)
	if err != nil {
		return err
	}
	bounds := im.Bounds()
	dst := image.NewNRGBA(bounds)
	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
		for x := bounds.Min.X; x < bounds.Max.X; x++ {
			c := color.NRGBAModel.Convert(im.At(x, y)).(color.NRGBA)
			if c.A > 0 {
				c.A = uint8(alpha)
			}
			dst.Set(x, y, c)
		}
	}
	return encodePNG(path, dst)
}

func gray(path string) error {
	src, err := os.Open(path)
	if err != nil {
		return err
	}
	defer src.Close()
	im, _, err := image.Decode(src)
	if err != nil {
		return err
	}
	bounds := im.Bounds()
	dst := image.NewRGBA(bounds)
	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
		for x := bounds.Min.X; x < bounds.Max.X; x++ {
			c := im.At(x, y)
			rgba := color.NRGBAModel.Convert(c).(color.NRGBA)
			if rgba.A > 0 {
				gray := color.GrayModel.Convert(c).(color.Gray)
				rgba.R, rgba.G, rgba.B = gray.Y, gray.Y, gray.Y
				dst.Set(x, y, rgba)
			} else {
				dst.Set(x, y, c)
			}
		}
	}
	return encodePNG(path, dst)
}

func encodePNG(path string, im image.Image) error {
	dst, err := os.Create(path)
	if err != nil {
		return err
	}
	defer dst.Close()
	return png.Encode(dst, im)
}

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}