feat: add sum functions for Money collections

money/src/main/java/de/tobiasschuerg/money/MoneyUtils.kt

@@ -3,3 +3,23 @@ package de.tobiasschuerg.money
 fun Money.toFloat(): Float = amount.toFloat()
 
 fun Money.toDouble(): Double = amount.toDouble()
+
+/**
+ * Sum all amounts on the collection.
+ * Will throw if currencies does not match.
+ * @see [sum(currency)]
+ */
+fun Collection<Money>.sum(): Money {
+    return fold(Money.ZERO) { acc, money ->
+        if (money.isZero()) acc else acc + money
+    }
+}
+
+/**
+ * Sum all amounts on the collection and auto apply currency conversion.
+ */
+fun Collection<Money>.sum(currency: Currency): Money {
+    return fold(Money.ZERO) { acc, money ->
+        if (money.isZero()) acc else acc + money.convertInto(currency)
+    }
+}

money/src/test/java/de/tobiasschuerg/money/MoneyListTest.kt

@@ -1,5 +1,6 @@
 package de.tobiasschuerg.money
 
+import de.tobiasschuerg.money.Currencies.EURO
 import org.junit.Assert
 import org.junit.Test
 
@@ -8,8 +9,8 @@ import org.junit.Test
  */
 class MoneyListTest {
 
-    private val currency = Currency("EUR", "Euro", 1.0)
-    private val list = MoneyList(currency)
+    private val currency = EURO
+    private val list = MoneyList(EURO)
 
     init {
         list.add(Money(3, currency))
@@ -58,4 +59,17 @@ class MoneyListTest {
         Assert.assertEquals(1, sublist.min()?.amount?.intValueExact())
         Assert.assertEquals(6, sublist.max()?.amount?.intValueExact())
     }
+
+    @Test
+    fun `test that sum of same currencies is calculated correctly`() {
+        val result = list.sum()
+        Assert.assertEquals(45.00, result.amount.toDouble(), 0.01)
+    }
+
+    @Test
+    fun `test that sum of different currencies is calculated correctly`() {
+        val list2: List<Money> = list.plus(Money(5.37, Currencies.USDOLLAR))
+        val result = list2.sum(EURO)
+        Assert.assertEquals(49.93, result.amount.toDouble(), 0.01)
+    }
 }