Tail Call Optimization

src/main/kotlin/math/Factorial.kt

@@ -1,4 +1,4 @@
-package mathematics
+package math
 
 import java.security.InvalidParameterException
 
@@ -10,9 +10,19 @@ import java.security.InvalidParameterException
 fun getFactorial(number: Long): Long {
     if (number < 0L) {
         throw InvalidParameterException("The number of which to calculate the factorial must be greater or equal to zero.")
-    } else return when (number) {
-        0L -> 1
-        1L -> number
-        else -> number * getFactorial(number - 1)
+    } else return getFactorialOptimized(number, 1)
+}
+
+/**
+ * Calculates the factorial using tail recursion to optimize code.
+ * If the number is too high, the tail recursion returns 0.
+ * @param number The number of which to calculate the factorial.
+ * @param accumulator The accumulator of the factorial value
+ * @return The factorial of the number passed as parameter.
+ */
+private tailrec fun getFactorialOptimized(number: Long, accumulator: Long): Long {
+    return when (number) {
+        0L -> accumulator
+        else -> getFactorialOptimized(number - 1, number * accumulator)
     }
 }

src/test/kotlin/math/FactorialTest.kt

@@ -1,5 +1,6 @@
-package mathematics
+package math
 
+import math.getFactorial
 import org.junit.Test
 import java.security.InvalidParameterException
 
@@ -25,4 +26,11 @@ class FactorialTest {
         assert(getFactorial(input) == expectedFactorial)
     }
 
+    @Test
+    fun testFactorialOfHighValue() {
+        val input = 20000L
+        val expectedFactorial = 0L
+        assert(getFactorial(input) == expectedFactorial)
+    }
+
 }