Add nth-prime exercise

Tests and example implementation based on the ruby exercise.

Completes a task for exercism#3.

Author: counterleft

config.json

@@ -37,7 +37,8 @@
     "octal",
     "luhn",
     "pig-latin",
-    "pascals-triangle"
+    "pascals-triangle",
+    "nth-prime"
   ],
   "deprecated": [
   ],

nth-prime/build.gradle

@@ -0,0 +1,12 @@
+apply plugin: "java"
+apply plugin: "eclipse"
+apply plugin: "idea"
+
+repositories {
+  mavenCentral()
+}
+
+dependencies {
+  testCompile "junit:junit:4.10"
+  testCompile "org.assertj:assertj-core:3.2.0"
+}

nth-prime/src/example/java/.keep

@@ -0,0 +1,42 @@
+import java.util.stream.IntStream;
+
+public final class Prime {
+    public static int nth(int nth) {
+        if (nth < 1) {
+            throw new IllegalArgumentException();
+        }
+
+        int primesFound = 0;
+        int possiblePrime = 1;
+
+        while (primesFound < nth) {
+            possiblePrime++;
+
+            if (isPrime(possiblePrime)) {
+                primesFound++;
+            }
+        }
+
+        return possiblePrime;
+    }
+
+    private static boolean isPrime(int n) {
+        if (n == 1) {
+            return false;
+        }
+
+        if (n == 2) {
+            return true;
+        }
+
+        boolean divisible = IntStream
+                .rangeClosed(2, (int) Math.ceil(Math.sqrt(n)))
+                .anyMatch((int i) -> n % i == 0);
+
+        if (divisible) {
+            return false;
+        }
+
+        return true;
+    }
+}

nth-prime/src/example/java/Prime.java

@@ -0,0 +1,30 @@
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class PrimeTest {
+    @Test
+    public void testFirstPrime() throws Exception {
+        assertThat(Prime.nth(1)).isEqualTo(2);
+    }
+
+    @Test
+    public void testSecondPrime() throws Exception {
+        assertThat(Prime.nth(2)).isEqualTo(3);
+    }
+
+    @Test
+    public void testSixthPrime() throws Exception {
+        assertThat(Prime.nth(6)).isEqualTo(13);
+    }
+
+    @Test
+    public void testBigPrime() throws Exception {
+        assertThat(Prime.nth(10001)).isEqualTo(104743);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testUndefinedPrime() throws Exception {
+        Prime.nth(0);
+    }
+}