--!strict
-- ReplicatedStorage/Shared/Inventory.luau — pure logic, no Roblox services.
export type Inventory = { [string]: number }

local Inventory = {}

function Inventory.add(inventory: Inventory, itemId: string, amount: number, maxStack: number): (boolean, string?)
	if amount <= 0 or amount ~= math.floor(amount) then
		return false, "invalid amount"
	end
	local current = inventory[itemId] or 0
	if current + amount > maxStack then
		return false, "stack full"
	end
	inventory[itemId] = current + amount
	return true, nil
end

return Inventory

--!strict
-- A service that receives its data store, so tests can inject an in-memory fake.
type Store = {
	GetAsync: (self: Store, key: string) -> any,
	SetAsync: (self: Store, key: string, value: any) -> (),
}

local function newCurrencyService(store: Store)
	local service = {}
	function service.award(userId: number, amount: number): number
		local key = `User_{userId}`
		local balance = (store:GetAsync(key) or 0) + amount
		store:SetAsync(key, balance)
		return balance
	end
	return service
end

-- In tests: a fake store backed by a table.
local fakeData: { [string]: any } = {}
local fakeStore: Store = {
	GetAsync = function(_self, key)
		return fakeData[key]
	end,
	SetAsync = function(_self, key, value)
		fakeData[key] = value
	end,
}
print(newCurrencyService(fakeStore).award(1, 50))

return newCurrencyService

undefined

```luau
-- src/shared/Inventory.spec.luau
local ReplicatedStorage = game:GetService("ReplicatedStorage")

local JestGlobals = require(ReplicatedStorage.DevPackages.JestGlobals)
local Inventory = require(ReplicatedStorage.Shared.Inventory)

local describe, it, expect = JestGlobals.describe, JestGlobals.it, JestGlobals.expect

describe("Inventory.add", function()
	it("stacks items up to the limit", function()
		local inventory = {}
		expect(Inventory.add(inventory, "Apple", 3, 5)).toBe(true)
		expect(inventory.Apple).toBe(3)
		local ok, reason = Inventory.add(inventory, "Apple", 3, 5)
		expect(ok).toBe(false)
		expect(reason).toBe("stack full")
	end)

	it("rejects fractional amounts", function()
		expect(Inventory.add({}, "Apple", 0.5, 5)).toBe(false)
	end)
end)