centerpt = tpt.create(100,100,"sand")

mvid={}
mvrx={}
mvry={}

rindex=0

function updatevars()
   x = tpt.get_property("x", centerpt)
   y = tpt.get_property("y", centerpt)
end

updatevars()

function particles()
   updatevars()
   lua_CD(x,y)
   if rindex > 0 then
      for tmpid = 1, rindex, 1 do
         lua_CD(x - mvrx[tmpid], y - mvry[tmpid])
         tpt.set_property("x", x - mvrx[tmpid], mvid[tmpid])
         tpt.set_property("y", y - mvry[tmpid], mvid[tmpid])
      end
   end
end

function lua_CD(CDx, CDy)
   if tpt.get_property("type", CDx, CDy - 1) == 1 then add_particle(CDx, CDy - 1) end
   if tpt.get_property("type", CDx + 1, CDy - 1) == 1 then add_particle(CDx + 1, CDy - 1) end
   if tpt.get_property("type", CDx + 1, CDy) == 1 then add_particle(CDx + 1, CDy) end
   if tpt.get_property("type", CDx + 1, CDy + 1) == 1 then add_particle(CDx + 1, CDy + 1) end
   if tpt.get_property("type", CDx, CDy + 1) == 1 then add_particle(CDx, CDy + 1) end
   if tpt.get_property("type", CDx - 1, CDy + 1) == 1 then add_particle(CDx - 1, CDy + 1) end
   if tpt.get_property("type", CDx - 1, CDy) == 1 then add_particle(CDx - 1, CDy) end
   if tpt.get_property("type", CDx - 1, CDy - 1) == 1 then add_particle(CDx - 1, CDy - 1) end
end

function add_particle(apx, apy)
   rindex = rindex + 1
   tpt.delete(apx, apy)
   mvid[rindex] = tpt.create(apx, apy, "sand")
   mvrx[rindex] = x - apx
   mvry[rindex] = y - apy
end

tpt.register_step(particles)