Add Waypoints Before Stations [Factorio]

/c
--[[ a "station" is an existing station, of which you have many.
     a "waypoint" is a single station to add before its stacker.
     the table maps station names to the new waypoint name you're adding.
     t[k] = v, where key is the station name, and v is the waypoint name. ]]
local station_waypoints = {
  ["[img=item/iron-ore] [U] @ [img=item/lab]"] = "[img=item/iron-ore] [W] @ [img=item/lab]",
  ["[img=item/copper-ore] [U] @ [img=item/lab]"] = "[img=item/copper-ore] [W] @ [img=item/lab]",
  ["[img=item/stone] [U] @ [img=item/lab]"] = "[img=item/stone] [W] @ [img=item/lab]",
  ["[img=item/coal] [U] @ [img=item/lab]"] = "[img=item/coal] [W] @ [img=item/lab]",
}
--[[ internal table to lookup waypoint names quickly ]]
local waypoints = {}
for _,v in pairs(station_waypoints) do
  waypoints[v] = true
end
local n_changed = 0
local n_already_changed = 0
local trains = game.player.force.get_trains()
--[[ loop over trains, then over the "records" in a train's schedule. ]]
for i = 1,#trains do
  local schedule = trains[i].schedule
  if schedule then
    local already_waypointed = false
    local adding_waypoint = false
    local n_records = #schedule.records
    for j = 1,n_records do
      local station = schedule.records[j].station
      --[[ if any station is a waypoint, this train is already configured.
           we don't try to redo a done train safely; we skip it. ]]
      if waypoints[station] then
        already_waypointed = true
        break
      end
      --[[ if the station has a waypoint defined in our config table, add a new record to the
           schedule for the waypoint, inserted before the current point (our loop index).
           this change doesn't "commit" until assigning the train's schedule, so it's ok to do
           it wrongly on a train that already has it, and check later before committing. ]]
      if station_waypoints[station] then
        adding_waypoint = true
        local new_record = { station=station_waypoints[station] }
        table.insert(schedule.records, j, new_record)
        --[[ inserting a record means bumping our loop index, the total # we're looping to,
             as well as the "current" station the train is using, if it's >= our index. ]]
        if schedule.current >= j then
          schedule.current = schedule.current + 1
        end
        j = j + 1
        n_records = n_records + 1
      end
    end
    --[[ commit changes, iff we should ]]
    if already_waypointed then
      n_already_changed = n_already_changed + 1
    elseif adding_waypoint then
      trains[i].schedule = schedule
      n_changed = n_changed + 1
    end
  end
end
game.player.print("Changed "..n_changed.." trains. Skipped "..n_already_changed.." already changed.")