minetest.register_on_generated(function(minp, maxp, seed)
    local t1 = os.clock()

    local sidelen = maxp.x - minp.x + 1
    local noise = minetest.get_perlin_map(
        {offset=0, scale=1, spread={x=200, y=125, z=200}, seed=5, octaves=5, persist=0.6},
        {x=sidelen, y=sidelen, z=sidelen}
    )
    local nvals = noise:get3dMap_flat({x=minp.x, y=minp.y, z=minp.z})

    local ni = 1
    for z = minp.z, maxp.z do
    for y = minp.y, maxp.y do
    for x = minp.x, maxp.x do
        local p = {x=x,y=y,z=z}
        if nvals[ni] - (y - 25) / 55 > 0.5 then
            if y < 1 then
                minetest.env:set_node(p, {name="default:dirt"})
            else
                minetest.env:set_node(p, {name="default:dirt_with_grass"})
            end
        elseif y < 1 then
            minetest.env:set_node(p, {name="default:water_source"})
        end
        ni = ni + 1
    end
    end
    end

    print(string.format("elapsed time: %.2fms", (os.clock() - t1) * 1000))
end)