add arithmetic progression

realDuYuanChao · realDuYuanChao · commit aa60af559367 · 2020-10-18T16:41:53.000+08:00
diff --git a/src/main/java/com/examplehub/maths/ArithmeticProgression.java b/src/main/java/com/examplehub/maths/ArithmeticProgression.java
@@ -0,0 +1,31 @@
+package com.examplehub.maths;
+
+/**
+ * https://en.wikipedia.org/wiki/Arithmetic_progression
+ */
+public class ArithmeticProgression {
+
+    /**
+     * Get nth item.
+     *
+     * @param firstItem        the initial term of an arithmetic progression.
+     * @param commonDifference the common difference of successive members.
+     * @param nth              the nth term of the sequence.
+     * @return the nth tern.
+     */
+    public static int getNth(int firstItem, int commonDifference, int nth) {
+        return firstItem + (nth - 1) * commonDifference;
+    }
+
+    /**
+     * Sum of n items of arithmetic progression.
+     *
+     * @param firstItem        the initial term of an arithmetic progression.
+     * @param commonDifference the common difference of successive members.
+     * @param n                the numbers of arithmetic progression.
+     * @return sum of arithmetic progression.
+     */
+    public static int sum(int firstItem, int commonDifference, int n) {
+        return (2 * firstItem + (n - 1) * commonDifference) * n / 2;
+    }
+}
diff --git a/src/test/java/com/examplehub/maths/ArithmeticProgressionTest.java b/src/test/java/com/examplehub/maths/ArithmeticProgressionTest.java
@@ -0,0 +1,21 @@
+package com.examplehub.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ArithmeticProgressionTest {
+    @Test
+    void testGetNth() {
+        assertEquals(10, ArithmeticProgression.getNth(1, 1, 10));
+        assertEquals(19, ArithmeticProgression.getNth(1, 2, 10));
+        assertEquals(90, ArithmeticProgression.getNth(100, -2, 6));
+    }
+
+    @Test
+    void testSum() {
+        assertEquals(55, ArithmeticProgression.sum(1, 1, 10));
+        assertEquals(5050, ArithmeticProgression.sum(1, 1, 100));
+        assertEquals(5050, ArithmeticProgression.sum(100, -1, 100));
+    }
+}
