add geometric progression

realDuYuanChao · realDuYuanChao · commit de039287fc7d · 2020-10-18T17:08:16.000+08:00
diff --git a/src/main/java/com/examplehub/maths/GeometricProgression.java b/src/main/java/com/examplehub/maths/GeometricProgression.java
@@ -0,0 +1,30 @@
+package com.examplehub.maths;
+
+public class GeometricProgression {
+
+    /**
+     * Get nth item of geometric progression.
+     *
+     * @param firstItem   the first item of geometric progression.
+     * @param commonRatio the common ratio of geometric progression.
+     * @param nth         the nth.
+     * @return the nth item of geometric progression.
+     */
+    public static double getNth(double firstItem, double commonRatio, int nth) {
+        return firstItem * Math.pow(commonRatio, nth - 1);
+    }
+
+    /**
+     * Sum of geometric progression.
+     *
+     * @param firstItem   the first item of geometric progression.
+     * @param commonRatio the common ratio of geometric progression.
+     * @param nth         the nth.
+     * @return the sum of geometric progression.
+     */
+    public static double sum(double firstItem, double commonRatio, int nth) {
+        return commonRatio == 1 ?
+                firstItem * nth :
+                firstItem * (1 - Math.pow(commonRatio, nth)) / (1 - commonRatio);
+    }
+}
diff --git a/src/test/java/com/examplehub/maths/GeometricProgressionTest.java b/src/test/java/com/examplehub/maths/GeometricProgressionTest.java
@@ -0,0 +1,21 @@
+package com.examplehub.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class GeometricProgressionTest {
+    @Test
+    void testGetNth() {
+        assertEquals(1, GeometricProgression.getNth(1, 1, 100));
+        assertEquals(8, GeometricProgression.getNth(1, 2, 4));
+        assertEquals(1024, GeometricProgression.getNth(1, 2, 11));
+    }
+
+    @Test
+    void testSum() {
+        assertEquals(15, GeometricProgression.sum(1, 2, 4));
+        assertEquals(15, GeometricProgression.sum(1, 2, 4));
+        assertEquals(100, GeometricProgression.sum(1, 1, 100));
+    }
+}
