Function which returns a shot-pattern thunk

spread_shot.lua

-- fires bullets in a spread, with an optional delay between shots;
-- this can produce anything from shotgun blasts to elegant spirals
--
-- shot_options:
-- n: number of shots
-- arc: {from, to}, as ratios where 0.0 is up and 0.5 is down; can be > 1.0
-- interval: frames between shots
spread_shot = function(npc, create_bullet, interval, options)
  -- TODO: this cooldown stuff begs to be refactored
  local cooldown = interval
  local elapsed = 0
  local n = 0
  local angle = 0.0
  local arc_step = (options.arc.to - options.arc.from) / options.n
                                                                            
  return function()
    if cooldown == 0 then
      n = options.n -- load new batch
      angle = options.arc.from
      elapsed = 0
      cooldown = interval
    else
      cooldown -= 1
    end
                                                                            
    if n > 0 and elapsed % options.interval == 0 then
      add(bullets, create_bullet(npc, angle))
      angle += arc_step
      n -= 1
    end
                                                                            
    elapsed += 1
  end
end