package main

import (
  "fmt"
  "github.com/lucasb-eyer/go-colorful"
  "math"
  "math/rand"
  "syscall/js"
)

var (
  width  float64
  height float64
  ctx    js.Value
)

type Ball struct {
  posX  float64
  posY  float64
  dirX  float64
  dirY  float64
  color colorful.Color
  size  float64
}

func main() {

  // Init Canvas stuff
  doc := js.Global().Get("document")
  canvasEl := doc.Call("getElementById", "mycanvas")
  width = doc.Get("body").Get("clientWidth").Float()
  height = doc.Get("body").Get("clientHeight").Float()
  canvasEl.Set("width", width)
  canvasEl.Set("height", height)
  ctx = canvasEl.Call("getContext", "2d")

  done := make(chan struct{}, 0)

  bm := BallManager{
    speed: 160,
    size:  10,
  }

  bm.SetNBalls(160)
  var renderFrame js.Func
  var tmark float64
  var markCount = 0
  var tdiffSum float64

  renderFrame = js.FuncOf(func(this js.Value, args []js.Value) interface{} {
    now := args[0].Float()
    tdiff := now - tmark
    tdiffSum += now - tmark
    markCount++
    if markCount > 10 {
      doc.Call("getElementById", "fps").Set("innerHTML", fmt.Sprintf("FPS: %.01f", 1000/(tdiffSum/float64(markCount))))
      tdiffSum, markCount = 0, 0
    }
    tmark = now

    // Pull window size to handle resize
    curBodyW := doc.Get("body").Get("clientWidth").Float()
    curBodyH := doc.Get("body").Get("clientHeight").Float()
    if curBodyW != width || curBodyH != height {
      width, height = curBodyW, curBodyH
      canvasEl.Set("width", width)
      canvasEl.Set("height", height)
    }
    bm.Update(tdiff / 1000)

    js.Global().Call("requestAnimationFrame", renderFrame)
    return nil
  })
  defer renderFrame.Release()

  // Start running
  js.Global().Call("requestAnimationFrame", renderFrame)

  <-done

}

// BallManager manager
type BallManager struct {
  balls []*Ball
  speed float64
  size  int
}

// Update updates the dot positions and draws
func (bm *BallManager) Update(dtTime float64) {
  if bm.balls == nil {
    return
  }
  ctx.Call("clearRect", 0, 0, width, height)

  // Update
  for _, ball := range bm.balls {

    if ball.posX < 0 {
      ball.posX = 0
      ball.dirX *= -1
    }
    if ball.posX > width {
      ball.posX = width
      ball.dirX *= -1
    }

    if ball.posY < 0 {
      ball.posY = 0
      ball.dirY *= -1
    }
    if ball.posY > height {
      ball.posY = height
      ball.dirY *= -1
    }

    dirX := ball.dirX
    dirY := ball.dirY

    ctx.Set("globalAlpha", 0.6)
    ctx.Call("beginPath")
    ctx.Set("fillStyle", ball.color.Hex())
    ctx.Call("arc", ball.posX, ball.posY, bm.size, 0, 2*math.Pi)
    ctx.Call("fill")

    ball.posX += dirX * bm.speed * dtTime
    ball.posY += dirY * bm.speed * dtTime
  }
}

// SetNBalls initializes balls with n size
func (bm *BallManager) SetNBalls(n int) {
  bm.balls = make([]*Ball, n)
  for i := 0; i < n; i++ {
    bm.balls[i] = &Ball{
      posX:  rand.Float64() * width,
      posY:  rand.Float64() * height,
      dirX:  rand.NormFloat64(),
      dirY:  rand.NormFloat64(),
      color: colorful.FastHappyColor(),
      size:  10,
    }
  }
}