-- Title: Spiral approach to object
-- Author: Chris Laurel


-- constants
km = 1.0/9460730.4725808
sec = 1.0/86400
deg = math.pi / 180

target = celestia:find("Sol/Jupiter")
sun = celestia:find("Sol")

startdist = 1000000*km
enddist = 250000*km

-- moveduration = 10*sec
-- startangle = 0
-- angle = math.pi


function smoothstep(x)
   return (3 - 2 * x) * x * x
end


function spiral_approach(params)
   local obs = celestia:getobserver()

   -- Get the parameters
   local target = params.target or celestia:find("Sol/Earth")
   local sun = params.sun or celestia:find("Sol")
   local duration = params.duration or 10*sec
   local startangle = params.startangle or 0
   local endangle = params.endangle or math.pi
   local startdist = params.startdist or target:radius()*km * 20
   local enddist = params.enddist or target:radius()*km * 4

   obs:follow(target)
   celestia:select(target)

   -- Get the sun direction
   local dir = sun:getposition():vectorto(target:getposition())
   dir = dir:normalize()

   -- Get the orbital velocity vector for the target
   local vel = target:getposition(celestia:gettime() + 0.1) - target:getposition()
   vel = vel:normalize()

   local start = celestia:gettime()

   local v = celestia:newvector(0, 0, 0)

   -- set the observer position over the duration
   while celestia:gettime() <= start + duration do

      -- smoothly interpolate distance and angle
      local t
      if duration == 0 then
	 t = 1
      else
	 t = (celestia:gettime() - start) / duration
      end
      t = smoothstep(t)
      local dist = startdist + t * (enddist - startdist)
      local theta = startangle + t * (endangle - startangle)

      v = -math.cos(theta) * dir + math.sin(theta) * vel

      obs:setposition(target:getposition() + v * dist)
      obs:lookat(target:getposition(), celestia:newvector(0.0, 1.0, 0.0))

      wait(0)
   end
end

t0 = celestia:gettime()
celestia:find("Sol/Earth"):preloadtexture()
celestia:find("Sol/Earth/Moon"):preloadtexture()
spiral_approach{
   target = celestia:find("Sol/Earth"),
   sun = celestia:find("Sol"),
   enddist = 300000*km,
   endangle = 180*deg,
   duration = 0
}

wait(0)
wait(0)
celestia:flash((celestia:gettime() - t0)*86400)

spiral_approach{
	    target = celestia:find("Sol/Earth"),
	    sun = celestia:find("Sol"),
	    startdist = 300000*km,
	    enddist = 30000*km,
	    startangle = 180*deg,
	    endangle = 0*deg,
	    duration = 10*sec
	 }