rem https://en.wikipedia.org/wiki/L-system
const fg_col = rgb(154, 170, 53)
const bg_col = rgb(238, 241, 221)
color fg_col, bg_col

sub draw_turtle(script, x, y, stem, turn, angle)
  local a, stack, i, idx, x2, y2, col
  dim stack
  cls
  for i = 0 to len(script)
    select case mid(script, i, 1)
    case "F", "1", "0" ' draw forward from current position 
      a = rad(angle)
      x2 = x - (stem * cos(a))
      y2 = y - (stem * sin(a))
      line x, y, x2, y2, fg_col
      x = x2
      y = y2
    case "-" 'turn left
      angle -= turn 
    case "+" 'turn right
      angle += turn 
    case "[" ' push current position and angle
      stack << [x, y, angle] 
    case "]" ' pop saved position and angle
      idx = len(stack) - 1
      [x, y, angle] = stack(idx)
      delete stack, idx, 1
    end select
  next i
  showpage
end

func create_lsystem(byref rules, byref variables, maxDepth, s, depth)
  local result, i, ch
  if (depth < maxDepth) then
    result = ""
    for i = 0 to len(s)
      ch = mid(s, i, 1)
      if (ch in variables) then
        result += rules(ch)
      else 
        result += ch
      end if
    next i
    result = create_lsystem(rules, variables, maxDepth, result, depth + 1)
  else
    result = s
  endif
  return result
end

sub show_plant
  local x = xmax / 2
  local y = ymax - 10
  local maxDepth = 6
  local rules = {"X" : "F+[[X]-X]-F[-FX]+X", "F" : "FF"}
  local variables = ["X", "F"]
  local stem = 3
  local script = create_lsystem(rules, variables, maxDepth, "X", 0)
  local a = 35

  draw_turtle(script, x, y, stem, 35, 90)
  delay(1200)
  for a = 35 to 80 step 1
    draw_turtle(script, x, y, stem, 45-a, 90)
    delay(20)
  next a
end

sub show_tree
  local x = xmax / 2
  local y = ymax
  local maxDepth = 6
  local rules = {"1":"11", "0":"1[-0]+0"}
  local variables = ["1", "0"]
  local angle = 70
  local stem = 2
  for d = 2 to 8
    local script = create_lsystem(rules, variables, d, "0", 0)
    draw_turtle(script, x, y, stem, angle, 90)
    delay(120)
  next d
end

'show_tree
'delay(1000)
show_plant
pause