(function () {
  'use strict'

  var COUNT = 20
  var MAX_RADIUS = 3

  var PALETTE = [
    [255, 255, 230],
    [255, 245, 200],
    [255, 230, 180],
    [220, 255, 220],
    [200, 230, 255],
  ]

  var canvas, ctx, w, h, flies, id, running

  function rand(a, b) { return a + Math.random() * (b - a) }

  function init() {
    canvas = document.createElement('canvas')
    canvas.id = 'fireflies-canvas'
    canvas.setAttribute('aria-hidden', 'true')
    document.body.prepend(canvas)
    ctx = canvas.getContext('2d')
    resize()
    spawn()
    running = true
    tick()
    addEventListener('resize', onResize)
    document.addEventListener('visibilitychange', onVisibility)
  }

  function resize() {
    w = canvas.width = innerWidth
    h = canvas.height = innerHeight
  }

  function onResize() {
    resize()
    if (!flies) return
    for (var i = 0; i < flies.length; i++) {
      var f = flies[i]
      if (f.x > w) f.x = w * 0.9
      if (f.y > h) f.y = h * 0.9
    }
  }

  function spawn() {
    flies = []
    for (var i = 0; i < COUNT; i++) flies.push(makeFly())
  }

  function makeFly() {
    var c = PALETTE[Math.floor(Math.random() * PALETTE.length)]
    return {
      x: rand(0, w),
      y: rand(0, h),
      vx: rand(-0.08, 0.08),
      vy: rand(-0.08, 0.08),
      r: rand(1.5, MAX_RADIUS),
      cr: c[0], cg: c[1], cb: c[2],
      a: rand(0.15, 0.35),
      phase: rand(0, 6.2832),
      flashSpeed: rand(0.002, 0.006),
      wanderPhase: rand(0, 6.2832),
      wanderSpeed: rand(0.002, 0.008),
      driftX: rand(-0.002, 0.002),
      driftY: rand(-0.002, 0.002),
    }
  }

  function tick() {
    if (!running) return
    update()
    draw()
    id = requestAnimationFrame(tick)
  }

  function update() {
    for (var i = 0; i < flies.length; i++) {
      var f = flies[i]
      f.wanderPhase += f.wanderSpeed
      var angle = Math.sin(f.wanderPhase) * 1.5
      f.vx += Math.cos(angle) * 0.004 + f.driftX
      f.vy += Math.sin(angle) * 0.004 + f.driftY
      f.vx *= 0.98
      f.vy *= 0.98
      var sp = Math.sqrt(f.vx * f.vx + f.vy * f.vy)
      if (sp > 0.3) {
        f.vx = (f.vx / sp) * 0.3
        f.vy = (f.vy / sp) * 0.3
      }
      f.x += f.vx
      f.y += f.vy
      if (f.x < -40) f.x = w + 40
      if (f.x > w + 40) f.x = -40
      if (f.y < -40) f.y = h + 40
      if (f.y > h + 40) f.y = -40
    }
  }

  function draw() {
    ctx.clearRect(0, 0, w, h)
    if (matchMedia('(prefers-reduced-motion: reduce)').matches) return

    for (var i = 0; i < flies.length; i++) {
      var f = flies[i]
      f.phase += f.flashSpeed
      var pulse = 0.5 + Math.sin(f.phase) * 0.5
      var alpha = f.a * pulse
      if (alpha < 0.01) continue

      ctx.save()

      var glowR = f.r * 4
      var g = ctx.createRadialGradient(f.x, f.y, 0, f.x, f.y, glowR)
      g.addColorStop(0, 'rgba(' + f.cr + ',' + f.cg + ',' + f.cb + ',' + alpha + ')')
      g.addColorStop(0.15, 'rgba(' + f.cr + ',' + f.cg + ',' + f.cb + ',' + (alpha * 0.25) + ')')
      g.addColorStop(1, 'rgba(' + f.cr + ',' + f.cg + ',' + f.cb + ',0)')
      ctx.beginPath()
      ctx.arc(f.x, f.y, glowR, 0, 6.2832)
      ctx.fillStyle = g
      ctx.fill()

      var coreR = f.r * 1.2
      var g2 = ctx.createRadialGradient(f.x, f.y, 0, f.x, f.y, coreR)
      g2.addColorStop(0, 'rgba(' + f.cr + ',' + f.cg + ',' + f.cb + ',' + (alpha * 0.85) + ')')
      g2.addColorStop(1, 'rgba(' + f.cr + ',' + f.cg + ',' + f.cb + ',0)')
      ctx.beginPath()
      ctx.arc(f.x, f.y, coreR, 0, 6.2832)
      ctx.fillStyle = g2
      ctx.fill()

      ctx.restore()
    }
  }

  function onVisibility() {
    if (document.hidden) {
      running = false
      if (id) { cancelAnimationFrame(id); id = null }
    } else {
      running = true
      if (!id) tick()
    }
  }

  if (document.readyState === 'loading') {
    document.addEventListener('DOMContentLoaded', init)
  } else {
    init()
  }
})()