-- Collison concave polygon
--
-- 多角形の内外判定（javascriptでの実装を添えて） - 加具留矢流余
-- https://mikebird28.hatenablog.jp/entry/2020/09/23/002409

function love.load()
  polys = {
    {20, 80, 80, 20, 120, 140},
    {140, 80, 200, 20, 260, 100, 200, 180},
    {340, 20, 400, 60, 380, 160, 320, 160, 280, 80},
    {460, 60, 520, 20, 600, 60, 600, 160, 520, 180, 460, 140},
    {140, 340, 260, 340, 280, 420, 100, 440, 20, 360, 40, 200, 100, 200},
    {500, 320, 460, 220, 380, 240, 440, 340, 300, 360, 340, 440, 500, 420, 520, 460, 620, 420, 600, 320}
  }

  hits = {}
  for i = 1, #polys do
    hits[i] = false
  end
end

function sign(x)
  return (x < 0 and -1) or 1
end

function getDeg(x1, y1, x2, y2)
  local abs1, abs2, theta, s
  abs1 = math.sqrt(x1 * x1 + y1 * y1)
  abs2 = math.sqrt(x2 * x2 + y2 * y2)
  theta = math.acos((x1 * x2 + y1 * y2) / (abs1 * abs2))
  s = sign(x1 * y2 - y1 * x2)
  return theta * s
end

function checkInPoly(pos, mx, my)
  local thetaSum = 0
  local x0, y0, x1, y1
  local v1x, v1y, v2x, v2y
  x0, y0 = pos[#pos-1], pos[#pos]
  if x0 == mx and y0 == my then return true end
  for i = 1, #pos-1, 2 do
    x1, y1 = pos[i], pos[i+1]
    if x1 == mx and y1 == my then return true end
    v1x, v1y = x0 - mx, y0 - my
    v2x, v2y = x1 - mx, y1 - my
    thetaSum = thetaSum + getDeg(v1x, v1y, v2x, v2y)
    x0, y0 = x1, y1
  end
  thetaSum = math.abs(thetaSum)
  if thetaSum >= 0.1 then return true end
  return false
end

function love.update(dt)
  local mx, my
  mx, my = love.mouse.getPosition()

  for i = 1, #polys do
    hits[i] = checkInPoly(polys[i], mx, my)
  end
end

function love.draw()
  -- clear canvas
  love.graphics.clear(0, 0, 0, 1)

  -- draw polygon
  for i = 1, #polys do
    if hits[i] then
      love.graphics.setColor(1, 0, 0, 1)
    else
      love.graphics.setColor(0, 1, 0, 1)
    end
    love.graphics.polygon("line", polys[i])
  end

  -- draw text
  love.graphics.setColor(1, 1, 1, 1)
  love.graphics.print("ESC to exit.", 8, 8)
end

function love.keypressed(key, scancode, isrepeat)
  if key == "escape" then
    love.event.quit()
  end
end