#!/usr/bin/luajit
--[[Iterated function system ASCII art animation.

GPT-4 output when asked to translate wing.pl, edited.  I’ve fixed it,
but I think that was more work than just writing the code from
scratch, because like GPT-4’s English, its Lua is a lot better at
looking right than at being right.

The original GPT-4 code had the following problems, which I fixed:

- It didn’t use the process ID; instead it used os.time() or
  os.clock(), which aren’t the same thing and aren’t useful here.
  GPT-4 claimed you couldn’t getpid() in Lua, and it's true that it's
  not in the default `os` module; but I’m using LuaJIT, not PUC Lua,
  so I can call it through the FFI, and anyway `(require
  'posix').getpid().pid` works, though it’s clumsier.

- It required treating 0 as false, like Perl and Python, but Lua
  treats it as true.  So (t-1 and 40 or 10) didn’t have the desired
  effect; it was always 40, so the IFS only had two transforms instead
  of 3.

- It had `x` and `y` reversed in the iteration assignments.

- It wasn’t using the previous values in the iteration assignments,
  instead mistakenly indexing into `a`:

        x,y = a[1][2]/2, a[2][1]/2+(t-1 and 40 or 10)

- Relatedly, it had declared `x` and `y` inside the loop, which meant
  there was no possible way to use their previous values; they would
  expire at the end of the loop body.

- That also meant there was no way to initialize them, but it didn’t
  try.

- It wasn’t rounding `x` and `y` to integers to index into the table,
  which is implicit in Perl.

- It was multiplying column numbers by 2 for some reason:

        local val = (a[c*2] and (a[c*2][r*2] or 0) or 0) + ...

- It had forgotten to multiply one of the numbers fetched from the
  array in that formula by 2, so it could get 0, 1, or 2, but not 3.

- It was using those 0-based numbers to index into the string, but
  string indices in Lua are 1-based, like table indices.

- It was randomizing the PRNG with only os.time(), which only changes
  once a second, with the result that if you ran it multiple time in a
  second, it would generate the same random numbers, which was visible
  as a noise pattern changing once a second with `watch -n .1 ./wing.lua`.

So, 11 bugs in all, which is a lot of bugs for the 21 lines of code it
came up with.

Example output (still frame):

                    ..:::'':::::':::::::'::::::''::::::'':::::''::::::'::::::':::'''
                 ..::::::   ' ':: ':::::   '''::..:::::.  ''':: ':::::   '''::  '   
               ::::'':::::      '  ':::::    :::''::::::      '  ':::.:      '      
      .   ......:::::..::..           '::.    ::::'' ':..           ':.             
      .:::::'':::::.::.' ''             ''     ':....  ''             ''            
       ':::':    '':::.::::                      ':::::::'                          
    '    :::::::::'   ':::                          ':::.                           
         ':::::'        ''::'                         ''::'                         
          '::'             '::                           '::                        
            '::::....                                                               
              ''::::'  ........                                                     
                 ':::::::'::''                                                      
                    ':::::''                                                        
                      '':....                                                       
                         ':::::::'                                                  
                            ':::                                                    
                              '':::                                                 
                                 '::                                                



]]

local posix = require 'posix'
function getpid() return posix.getpid().pid end

math.randomseed(os.time()-getpid())
local a, x, y = {}, 0, 0

for i = 1, 9999 do
    local t = math.floor(math.random() * 3)
    if t == 0 then
       y, x = .3*x - .4*y + 12, .4*x + (.2 + math.abs(getpid()%99/99-.5))*y
    else
       y, x = y/2, x/2 + (t==1 and 10 or 40)
    end
    local xi, yi = math.floor(x), math.floor(y)
    if a[xi] == nil then a[xi] = {} end
    a[xi][yi] = 1
end

local str = " .':"
for r = 0, 18 do
    for c = 0, 79 do
       local i = 0
       if a[c] then i = 2*(a[c][r*2] or 0) + (a[c][r*2+1] or 0) end
       io.write(string.sub(str, i+1, i+1))
    end 
    print()
end