From procedural to functional with nim

Being a multiparadigm programming language, nim lets you code in many different styles, in this post I show you how I went from a procedural code to a functional(ish) one. Making the code gain readability and expressiveness, without any performance penalty.

Thee following it's a subset of the nim implementation of my raybench project.

Original code:

proc main() =  
  var data = newSeq[seq[V3]]()
  let world = world_new()
  let vdu = (world.camera.rt - world.camera.lt) / float32(WIDTH)
  let vdv = (world.camera.lb - world.camera.lt) / float32(HEIGHT)

  randomize()

  for y in 0..(HEIGHT-1):
    var row = newSeq[V3]()
    for x in 0..(WIDTH-1):
      var color = zero
      var ray:Ray

      ray.origin = world.camera.eye

      for i in 1..SAMPLES:
        ray.direction = ((world.camera.lt + (vdu * (float32(x) + float32(random(1'f32))) +
                        vdv * (float32(y) + float32(random(1'f32))))) -
                        world.camera.eye).unit
        color = color + trace(world, ray, 0)

      color = color / float32(SAMPLES)
      row.add(color)
    data.add(row)
  writeppm(data)

Improved Code:

proc main() =  
  let world = world_new()
  let vdu = (world.camera.rt - world.camera.lt) / float32(WIDTH)
  let vdv = (world.camera.lb - world.camera.lt) / float32(HEIGHT)
  let ss = toSeq(1..SAMPLES)
  let hs = toSeq(0..<HEIGHT)
  let ws = toSeq(0..<WIDTH)

  randomize()

  hs.map(proc (y:int): auto =
    ws.map(proc (x:int): auto =
      foldl(ss.mapIt(
        trace(world, (
          world.camera.eye,
          ((world.camera.lt + (vdu * (float32(x) + random(1.0)) +
                        vdv * (float32(y) + random(1.0)))) -
                        world.camera.eye).unit
          ), 0)), a + b) / float32(SAMPLES)
      )
  ).writeppm

Stats

Lines Characters
Original 26 731
Improved 21 628

The comparison shows a decrease in the number of characters used to describe the same computation.

Performance wise, the results are as shown (best out of three, running on a Core-i7 6700):

Original:

$ time ./nimrb

real    0m25.924s  
user    0m25.781s  
sys     0m0.125s  

Improved:

$ time ./nimrb_map

real    0m25.016s  
user    0m24.828s  
sys     0m0.156s  

There seems to be a very small improvement in speed, but I wouldn't make much of it.

While changes in the code improved readability, it also allowed me to make another small improvement. Using the Parallel Map function described here: http://blog.ubergarm.com/10-nim-one-liners-to-impress-your-friends/ and changing a single line from this:

  hs.map(proc (y:int): auto =

to this:

  hs.pMap(proc (y:int): auto =

We are taking advantage of nim's threadpool library and, with it, of all the resources available from the current CPU, improving the running speed.

$ time ./nimrb_pmap

real    0m8.884s  
user    1m4.156s  
sys     0m0.250s  

Making the code run almost 3 times as fast as the single threaded version, with no significant changes to the code.

I find this very interesting, and will be looking forward to keep playing around with nim some more.