Virtual World game programming involves moving many characters - both player controlled and automatic - in parallel. The movements of each character depend - to a greater or lesser extent - on the prior movements of the character, and of the other charcaters and the resultant position and condition that they're left in. Can you imagine what a complex coding task that could be?

Lua uses coroutines which allows the programmer to write the movement and state logic for each character in its own separate function - with a loop in the code of the function which moves the character and then yields when the movement has been completed. You may consider that all of these functions are running in parallel (and they are certainly all active every cycle), but of course they're really time-sliced with the loop being run in each routine once in every cycle.

I've written coroutine demos before (see [here]), but I wrote a more specific virtual world example during the Lua Programming course I was presenting today.

Scenario. Jane has a 10 metre start over 10 Johns in a chase. Jane moves forward a randon amount each second - somewhere between 0 and 0.9 metres. Each of the Johns moves forward between 0 and 1 metres. Simulate the chase.

Let's start by defining the movement routine for Jane:

  function movejane()
    local covered = 10.0
    while true do
      local stepsize = math.random()*0.9
      covered = covered + stepsize
      at = covered
      target = at
      coroutine.yield()
    end
  end

and the movement routine for each John:

  function movejohn()
    local covered = 0
    while true do
      local stepsize = math.random()
      covered = covered + stepsize
      at = covered
      if at > target then
        winner = 1
      else
        winner = 0
      end
      coroutine.yield()
    end
  end


Note that there are local variables for each of the different characters, and global variables which are used to maintain the global statuses and generally visible data.

We'll then set up a table of characters:

  characters = {coroutine.create(movejane)}
  for k = 1, 10 do
    crh = coroutine.create(movejohn)
    characters[#characters+1] = crh
  end

and ... finally ... code a loop to run our simulation for 200 seconds:

  for seconds = 1, 200 do
    for person = 1,#characters do
      coroutine.resume(characters[person])
      io.write (string.format("%6.2f ",at))
      if winner == 1 then break end
    end
    if winner == 1 then
      io.write(" WONHER\n")
      break
    end
    io.write("\n")
  end

Let's see how that runs ...

munchkin:laug11 grahamellis$ lua charmoves

 10.63   0.58   0.94   0.74   0.37   0.94   0.15   0.26   0.85   0.96   0.07

 11.46   0.79   1.77   1.31   1.05   1.69   0.95   0.38   1.49   1.94   0.09

 11.91   1.40   1.95   1.97   1.27   2.05   1.64   0.46   2.18   2.64   0.22

 12.34   1.88   2.05   2.65   1.37   2.35   2.48   0.49   2.33   3.43   1.20

 12.41   2.04   2.09   3.41   1.57   3.00   3.08   1.33   2.71   4.40   1.67

 12.43   2.45   2.47   4.18   1.85   3.55   3.19   1.89   3.11   5.03   2.12

 12.80   2.86   2.47   5.13   2.36   3.72   3.34   1.98   4.07   5.12   3.12

[etc]

 31.31  24.97  23.33  28.77  24.51  22.43  26.29  23.60  27.76  25.90  27.72

 31.57  25.86  23.66  29.50  25.46  22.58  26.37  23.81  28.26  26.46  28.51

 31.95  26.28  24.09  30.01  25.76  22.85  26.88  24.62  29.23  26.68  29.41

 32.14  26.70  24.44  30.78  26.39  23.07  27.70  25.29  30.00  27.19  30.24

 32.51  27.22  25.09  31.59  27.21  23.48  28.12  26.24  30.50  27.71  30.58

 32.94  27.45  25.18  32.53  27.98  23.81  28.59  26.55  31.31  28.32  31.11

 33.17  28.45  25.33  33.52  WONHER

munchkin:laug11 grahamellis$

Full source code [here].

(written 2011-08-17, updated 2011-08-18)

 U114 - Lua - Threading and Coroutines.
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [2314] Passing parameters to a coroutine in Lua - (2009-08-01)
  [1870] What to do with a huge crop of apples - (2008-11-04)
  [1699] If you are learning Lua, here are some more examples - (2008-07-06)
  [1691] Co-routines in Lua - co-operative processing - (2008-06-29)

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