feat: add money value object

internal/howmuch/model/money.go

@@ -0,0 +1,66 @@
+// MIT License
+//
+// Copyright (c) 2024 vinchent <vinchent@vinchent.xyz>
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package model
+
+type Currency string
+
+// TODO: may handle a more complexe logic with the exchange rate.
+
+// XXX: Here we suppose that the currency is the same for every piece
+// of money involved in the calculate.
+
+const (
+	EUR Currency = "EUR"
+	USD Currency = "USD"
+	CNY Currency = "CNY"
+)
+
+type Money struct {
+	ammount  int
+	currency Currency
+}
+
+func MakeMoney(amount int, currency Currency) Money {
+	return Money{amount, currency}
+}
+
+func Add(cur Currency, money ...Money) Money {
+	var sum Money
+	sum.currency = cur
+
+	for _, m := range money {
+		sum.ammount += m.ammount
+	}
+
+	return sum
+}
+
+func Diff(cur Currency, money1 Money, money2 Money) Money {
+	var diff Money
+
+	diff.currency = cur
+
+	diff.ammount = money1.ammount - money2.ammount
+
+	return diff
+}