gravity

if arg[#arg] == "-debug" then require("mobdebug").start() end

function love.load()
  love.window.setMode(1900,1000)

  G = 1

  planetList = {}

  worldSize = {w=1900,h=1000}

  tool = {pause=false, active=false,x=0,y=0,dx=0,dy=0,mass=10,lastID = nil}
  tool.radius=math.sqrt(tool.mass/math.pi)

  love.graphics.clear = function() end

end

function newPlanet(x,y,m,vx,vy)
  local p = #planetList+1
  planetList[p] = {}
  planetList[p].ID = p
  planetList[p].x = x
  planetList[p].y = y
  planetList[p].m = m
  planetList[p].radius = math.sqrt(m/math.pi)
  planetList[p].velocityX = vx or 0
  planetList[p].velocityY = vy or 0
  tool.lastID = p
end

function collision(me,target)
  me.m = me.m + target.m
  me.velocityX = me.velocityX + target.velocityX*target.m/me.m
  me.velocityY = me.velocityY + target.velocityY*target.m/me.m
  me.radius = math.sqrt(me.m/math.pi)
  planetList[target.ID] = nil
end

function updatePlanet(me)
  dt = 0.001
  for _,target in pairs(planetList) do
    if target.ID ~= me.ID then
      local dx = target.x - me.x
      local dy = target.y - me.y
      local distance = math.sqrt((dx)^2+(dy)^2)
      local force = (G*(me.m * target.m)/distance^2)*dt
      me.velocityX = me.velocityX + dx*force/me.m
      me.velocityY = me.velocityY + dy*force/me.m
      if distance < me.radius then collision(me,target) end
    end
  end
  me.x = me.x + me.velocityX
  me.y = me.y + me.velocityY
end

function addRandomPlanets(n)
  for i=1,n do
    newPlanet(math.random(0,worldSize.w),math.random(0,worldSize.h),math.random(5,10+n),math.random(-2,2),math.random(-2,2))
  end
end

function updateAllPlanets()
  for _,planet in pairs(planetList) do
    updatePlanet(planet)
  end
end

function drawPlanet(me)
  love.graphics.setColor(0,1,0)
  love.graphics.circle("fill",me.x,me.y,me.radius)
end

function drawAllPlanets()
  for _,planet in pairs(planetList) do
    drawPlanet(planet)
  end
end


function love.mousepressed(x,y,b)
  if b==1 then
    tool.active = true
    tool.x=x
    tool.y=y
    tool.dx=x
    tool.dy=y
  end

  if b==2 then
    planetList = nil
    planetList = {}
    love.graphics.setCanvas(canvas)
      love.graphics.clear()
    love.graphics.setCanvas()
  end

end

function love.keypressed(key)

  if key=="1" then
    tool.mass = 10
  end

  if key=="r" then
    addRandomPlanets(100)
  end

  if key=="space" then
    tool.pause = not tool.pause
  end

  if key=="z" and love.keyboard.isScancodeDown("lctrl") then
    planetList[tool.lastID] = nil
  end
end

function love.mousemoved(x,y,dx,dy)
  if tool.active then
    tool.dx=x
    tool.dy=y
  end
end

function love.wheelmoved(x,y)
  if love.keyboard.isScancodeDown("lctrl") then y = y*20 end
  tool.mass=tool.mass+y*3
  if tool.mass<5 then
    tool.mass = 5
  end
  tool.radius=math.sqrt(tool.mass/math.pi)
end

function love.mousereleased(x,y,b)
  if b==1 then
    tool.dx=x
    tool.dy=y
    tool.active=false
    newPlanet(tool.x,tool.y,tool.mass,(tool.x-tool.dx)/200,(tool.y-tool.dy)/200)
  end
end

function love.update()
  if not tool.pause then
    updateAllPlanets()
  end
end

function love.draw()
  love.graphics.setColor(0,0,0,0.1)
  love.graphics.rectangle("fill",0,0,worldSize.w,worldSize.h)

  drawAllPlanets()

  love.graphics.setColor(1,1,1,0.2)
  love.graphics.circle("line",love.mouse.getX(),love.mouse.getY(),tool.radius)

  if tool.active then
    love.graphics.setColor(0,1,0,0.7)
    love.graphics.circle("line",tool.x,tool.y,tool.radius)
    love.graphics.line(tool.x,tool.y,tool.dx,tool.dy)
  end
end