advanced_npc/trade/trade.lua

-- NPC trading abilities by Zorman2000

npc.trade = {}

npc.trade.CASUAL = "casual"
npc.trade.TRADER = "trader"
npc.trade.NONE = "none"

npc.trade.OFFER_BUY = "buy"
npc.trade.OFFER_SELL = "sell"

-- This table holds all responses for trades  
npc.trade.results = {
  casual = {},
  formal = {}
}

-- Casual trader NPC dialogues definition
-- Casual buyer
npc.trade.CASUAL_TRADE_BUY_DIALOGUE = {
  text = "I'm looking to buy some items, are you interested?",
  casual_trade_type = npc.trade.OFFER_BUY,
  responses = {
    [1] = {
      text = "Yes, let's see what are you looking for",
      action_type = "function",
      response_id = 1,
      action = function(self, player)
        npc.trade.show_casual_trade_formspec(self, player, npc.trade.OFFER_BUY)
      end
    }
  }
}

-- Casual seller
npc.trade.CASUAL_TRADE_SELL_DIALOGUE = {
  text = "I have some items to sell, are you interested?",
  casual_trade_type = npc.trade.OFFER_SELL,
  responses = {
    [1] = {
      text = "Yes, let's see what you have",
      action_type = "function",
      response_id = 1,
      action = function(self, player)
        npc.trade.show_casual_trade_formspec(self, player, npc.trade.OFFER_SELL)
      end
    }
  }
}

function npc.trade.show_casual_trade_formspec(self, player, offer_type)
  
  -- Strings for formspec, to include international support later
  local prompt_string = " offers to buy from you"
  local for_string = "for"
  local buy_sell_string = "Sell"

  -- Get offer. As this is casual trading, NPCs will only have
  -- one trade offer
  local trade_offer = self.trader_data.buy_offers[1]
  if offer_type == npc.trade.OFFER_SELL then
    trade_offer = self.trader_data.sell_offers[1]
    prompt_string = " wants to sell to you"
    buy_sell_string = "Buy"
  end

  local formspec = "size[8,4]"..
                default.gui_bg..
                default.gui_bg_img..
                default.gui_slots..
                "label[2,0.1;"..self.nametag..prompt_string.."]"..
                "item_image_button[2,1.3;1.2,1.2;"..trade_offer.item..";item;]"..
                "label[3.75,1.75;"..for_string.."]"..
                "item_image_button[4.8,1.3;1.2,1.2;"..trade_offer.price..";price;]"..
                "button_exit[1,3.3;2.9,0.5;yes_option;"..buy_sell_string.."]"..
                "button_exit[4.1,3.3;2.9,0.5;no_option;"..npc.dialogue.NEGATIVE_ANSWER_LABEL.."]" 

  -- Create entry into results table
  npc.trade.results.casual[player:get_player_name()] = {
    trade_offer = trade_offer
  }
  -- Show formspec to player
  minetest.show_formspec(player:get_player_name(), "advanced_npc:casual_trade", formspec)
end

function npc.trade.get_random_trade_status()
	local chance = math.random(1,10)

	if chance < 3 then
		-- Non-trader
    return npc.trade.NONE
	elseif 3 <= chance and chance <= 7 then
    -- Casual trader
    return npc.trade.CASUAL
	elseif chance > 7 then
    -- Trader by profession
    return npc.trade.TRADER
	end
end

-- This function will return an offer object, based
-- on the items the NPC has.
-- Criteria: If having a near empty inventory, (< 6) NPC
-- will offer to buy with a 70% chance.
-- If NPC has a near full inventory  (> 10 items), NPC
-- will offer to sell. The prices will be selected using:
-- item_price * (+/- price_item * 0.2) so item will be
-- more or less 20% of the item price.
function npc.trade.get_casual_trade_offer(self, offer_type)
  return {
            offer_type = offer_type, 
            item = "default:wood 10", 
            price = "default:iron_lump 20"
         }
end


-- Handler for chat formspec
minetest.register_on_player_receive_fields(function (player, formname, fields)
  -- Additional checks for other forms should be handled here
  -- Handle yes/no dialogue
  if formname == "advanced_npc:casual_trade" then
    local player_name = player:get_player_name()

    if fields then
      local player_response = npc.trade.results.casual[player_name]
      if fields.yes_option then
        -- Check if offer is a buy or sell
        if player_response.trade_offer.offer_type == npc.trade.OFFER_BUY then
          -- If NPC is buying from player, then player loses item, gets price
          local inv = minetest.get_inventory({type = "player", name = player_name})
          -- Check NPC has the item being buyed
          if inv:contains_item("main", ItemStack(player_response.trade_offer.item)) then
            -- Remove item from player
            inv:remove_item("main", ItemStack(player_response.trade_offer.item))
            -- TODO: Add to NPC
            -- Add price items to player
            inv:add_item("main", ItemStack(player_response.trade_offer.price))
          end
        else

        end
      elseif fields.no_option then
        minetest.chat_send_player(player_name, "Talk to me if you change your mind!")
      end
    end
  end

end)