package main

import (
    "image"
    "image/color"
    "image/draw"
    "image/png"
    "log"
    "os"
)

const (
    width, height = 800, 600
    maxDepth      = 11                    // how far to recurse, between 1 and 20 is reasonable
    colFactor     = uint8(255 / maxDepth) // adjusts the colour so leaves get greener further out
    fileName      = "pythagorasTree.png"
)

func main() {
    img := image.NewNRGBA(image.Rect(0, 0, width, height)) // create new image
    bg := image.NewUniform(color.RGBA{255, 255, 255, 255}) // prepare white for background
    draw.Draw(img, img.Bounds(), bg, image.ZP, draw.Src)   // fill the background

    drawSquares(340, 550, 460, 550, img, 0) // start off near the bottom of the image

    imgFile, err := os.Create(fileName)
    if err != nil {
        log.Fatal(err)
    }
    defer imgFile.Close()
    if err := png.Encode(imgFile, img); err != nil {
        imgFile.Close()
        log.Fatal(err)
    }
}

func drawSquares(ax, ay, bx, by int, img *image.NRGBA, depth int) {
    if depth > maxDepth {
        return
    }
    dx, dy := bx-ax, ay-by
    x3, y3 := bx-dy, by-dx
    x4, y4 := ax-dy, ay-dx
    x5, y5 := x4+(dx-dy)/2, y4-(dx+dy)/2
    col := color.RGBA{0, uint8(depth) * colFactor, 0, 255}
    drawLine(ax, ay, bx, by, img, col)
    drawLine(bx, by, x3, y3, img, col)
    drawLine(x3, y3, x4, y4, img, col)
    drawLine(x4, y4, ax, ay, img, col)
    drawSquares(x4, y4, x5, y5, img, depth+1)
    drawSquares(x5, y5, x3, y3, img, depth+1)
}

func drawLine(x0, y0, x1, y1 int, img *image.NRGBA, col color.RGBA) {
    dx := abs(x1 - x0)
    dy := abs(y1 - y0)
    var sx, sy int = -1, -1
    if x0 < x1 {
        sx = 1
    }
    if y0 < y1 {
        sy = 1
    }
    err := dx - dy
    for {
        img.Set(x0, y0, col)
        if x0 == x1 && y0 == y1 {
            break
        }
        e2 := 2 * err
        if e2 > -dy {
            err -= dy
            x0 += sx
        }
        if e2 < dx {
            err += dx
            y0 += sy
        }
    }
}
func abs(x int) int {
    if x < 0 {
        return -x
    }
    return x
}