Backpack = {
    list = {}
}

setmetatable(Backpack, {
    __call = function (self, ...)
        return self:create(...)
    end
})

function Backpack:create(player, slots)
    if Backpack.list[player] then
        return Backpack.list[player]
    end

    local new = {
        items = {},
        slots = slots or 20,
        owner = player
    }
    setmetatable(new, {__index = Backpack})
    Backpack.list[player] = new

    return new
end

function Backpack:getSlots()
    return self.slots
end

function Backpack:getEmptySlot()
    for i = 1, self.slots do
        if not self.items[i] then
            return i
        end
    end
    return false
end

function Backpack:getItems(withAmount)
    local items = {}

    for index, slot in pairs(self.items) do
        if not withAmount then
            items[#items + 1] = slot.item
        else
            items[#items + 1] = slot
        end
    end

    return items
end

function Backpack:hasItem(item)
    for index, slot in pairs(self.items) do
        if slot.item == item then
            return true
        end
    end
    return false
end

function Backpack:getOwner()
    return self.owner
end

function Backpack:addItem(item, amount)
    if not self:hasItem(item) then
        local slot = self:getEmptySlot()

        if slot then
            self.items[slot] = {item = item, amount = amount}
            return true
        else
            return false
        end
    else
        for index, slot in pairs(self.items) do
            if slot.item == item then
                slot.amount = slot.amount + amount
                return true
            end
        end
        return false
    end
end

function Backpack:removeItem(item, amount)
    if not self:hasItem(item) then
        return false
    end

    for index, slot in pairs(self.items) do
        if slot.item == item then
            if not amount or (slot.amount - amount) <= 0 then
                self.items[index] = nil
            else
                slot.amount = slot.amount - amount
            end
            return true
        end
    end

    return false
end

function Backpack:getWeight()
    local weight = 0

    for index, slot in pairs(self.items) do
        weight = weight + (slot.item:getWeight() * slot.amount)
    end

    return weight
end