// Copyright 2015 by Sascha L. Teichmann
// Use of this source code is governed by the MIT license
// that can be found in the LICENSE file.

package main

import (
	"bufio"
	"fmt"
	"image"
	"image/color"
	"image/draw"
	"os"

	_ "image/jpeg"
	_ "image/png"
)

func loadRGBA(filename string) (*image.RGBA, error) {

	file, err := os.Open(filename)
	if err != nil {
		return nil, err
	}
	defer file.Close()

	var img image.Image
	if img, _, err = image.Decode(bufio.NewReader(file)); err != nil {
		return nil, fmt.Errorf("Decoding '%s' failed: %s", filename, err)
	}

	if rgba, ok := img.(*image.RGBA); ok {
		return rgba, nil
	}
	bounds := img.Bounds()
	rgba := image.NewRGBA(bounds)
	draw.Draw(rgba, bounds, img, image.ZP, draw.Src)

	return rgba, nil
}

func averageColor(filename string) (color.Color, error) {

	img, err := loadRGBA(filename)
	if err != nil {
		return nil, err
	}

	bounds := img.Bounds()
	if bounds.Empty() {
		return color.Black, nil
	}
	y := img.PixOffset(bounds.Min.X, bounds.Min.Y)
	yEnd := img.PixOffset(bounds.Min.X, bounds.Max.Y)
	w := bounds.Dx() * 4
	pix := img.Pix

	var r, g, b, a uint64

	for ; y < yEnd; y += img.Stride {
		for pos, end := y, y+w; pos < end; pos += 4 {
			pa := uint64(pix[pos+3])
			r += pa * uint64(pix[pos])
			g += pa * uint64(pix[pos+1])
			b += pa * uint64(pix[pos+2])
			a += pa
		}
	}

	r /= 255
	g /= 255
	b /= 255

	if s := a / 255; s > 0 {
		r /= s
		g /= s
		b /= s
	}

	col := color.RGBA{
		R: uint8(r),
		G: uint8(g),
		B: uint8(b),
		A: uint8(a / uint64(bounds.Dx()*bounds.Dy()))}

	return &col, nil
}