From b22465f0b0113e11d301c244fb05c4995358b5db Mon Sep 17 00:00:00 2001
From: Haidong Ji
Date: Mon, 27 Aug 2018 22:01:02 -0500
Subject: Last digit of sum of Fibs done!

User of spreadsheet really helped here. I should have thought of adding
rows earlier.---
 .../sources/LastDigitOfFibSum.java                 | 38 ++++++++++++++++++++++
 .../tests/LastDigitOfFibSumTest.java               | 27 +++++++++++++++
 2 files changed, 65 insertions(+)
 create mode 100644 AlgoDesignAndTechniqueEdxJava/sources/LastDigitOfFibSum.java
 create mode 100644 AlgoDesignAndTechniqueEdxJava/tests/LastDigitOfFibSumTest.java

diff --git a/AlgoDesignAndTechniqueEdxJava/sources/LastDigitOfFibSum.java b/AlgoDesignAndTechniqueEdxJava/sources/LastDigitOfFibSum.java
new file mode 100644
index 0000000..0783154
--- /dev/null
+++ b/AlgoDesignAndTechniqueEdxJava/sources/LastDigitOfFibSum.java
@@ -0,0 +1,38 @@
+import java.util.Scanner;
+
+public class LastDigitOfFibSum {
+
+	final static int FIB_PISANO_PERIOD_FOR_TEN = 60;
+
+	public static int getFibNModM(long n) {
+		long r = n % FIB_PISANO_PERIOD_FOR_TEN;
+		if (r == 0)
+			return 0;
+		int firstN = 0;
+		int secondN = 1;
+		int tempHolder = 1;
+		for (int i = 1; i < r; i++) {
+			tempHolder = (firstN + secondN) % 10;
+			firstN = secondN;
+			secondN = tempHolder;
+		}
+		return secondN;
+	}
+
+	public static int getLastDigit(long n) {
+		if (n <= 1)
+			return (int) n;
+		int result = getFibNModM(n + 2);
+		if (result == 0)
+			return 9;
+		else
+			return result - 1;
+	}
+
+	public static void main(String args[]) {
+		Scanner in = new Scanner(System.in);
+		long n = in.nextLong();
+
+		System.out.println(getLastDigit(n));
+	}
+}
diff --git a/AlgoDesignAndTechniqueEdxJava/tests/LastDigitOfFibSumTest.java b/AlgoDesignAndTechniqueEdxJava/tests/LastDigitOfFibSumTest.java
new file mode 100644
index 0000000..49df83d
--- /dev/null
+++ b/AlgoDesignAndTechniqueEdxJava/tests/LastDigitOfFibSumTest.java
@@ -0,0 +1,27 @@
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+public class LastDigitOfFibSumTest {
+	@Test
+	void testLastDigitOfFibSum0() {
+		assertEquals(0, LastDigitOfFibSum.getLastDigit(0));
+	}
+	@Test
+	void testLastDigitOfFibSum1() {
+		assertEquals(1, LastDigitOfFibSum.getLastDigit(1));
+	}
+	@Test
+	void testLastDigitOfFibSum6() {
+		assertEquals(0, LastDigitOfFibSum.getLastDigit(6));
+	}
+	@Test
+	void testLastDigitOfFibSum9() {
+		assertEquals(8, LastDigitOfFibSum.getLastDigit(9));
+	}
+	@Test
+	void testLastDigitOfFibSum100() {
+		assertEquals(5, LastDigitOfFibSum.getLastDigit(100));
+	}
+
+}
-- 
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