[#5] Generate ULID compatible with RFC-4122 UUID v4

Now it can generate UUIDs and ULIDs compatible with RFC-4122 UUID v4.

List of Changes:

Created UlidCreator.getUlid4();
Created UlidCreator.getUlidString4();
Updated README.md

Coverage 94.1%

README.md

@@ -33,7 +33,7 @@ Add these lines to your `pom.xml`.
 <dependency>
   <groupId>com.github.f4b6a3</groupId>
   <artifactId>ulid-creator</artifactId>
-  <version>2.2.0</version>
+  <version>2.3.0</version>
 </dependency>
 ```
 See more options in [maven.org](https://search.maven.org/artifact/com.github.f4b6a3/ulid-creator).
@@ -54,6 +54,12 @@ The default random number generator is `java.security.SecureRandom`.
 UUID ulid = UlidCreator.getUlid();
 ```
 
+```java
+// GUID based on ULID spec
+// Compatible with RFC-4122 UUID v4
+UUID ulid = UlidCreator.getUlid4();
+```
+
 Sequence of GUIDs based on ULID spec:
 
 ```text
@@ -90,6 +96,12 @@ See the section on GUIDs to know how the 128 bits are generated in this library.
 String ulid = UlidCreator.getUlidString();
 ```
 
+```java
+// String based on ULID spec
+// Compatible with RFC-4122 UUID v4
+String ulid = UlidCreator.getUlidString4();
+```
+
 Sequence of Strings based on ULID spec:
 
 ```text

src/main/java/com/github/f4b6a3/ulid/UlidCreator.java

@@ -80,6 +80,20 @@ public final class UlidCreator {
 		return UlidSpecCreatorHolder.INSTANCE.create();
 	}
 
+	/**
+	 * Returns a ULID as GUID.
+	 * 
+	 * It is compatible with the RFC-4122 UUID v4.
+	 * 
+	 * The random component is generated by a secure random number generator:
+	 * {@link java.security.SecureRandom}.
+	 * 
+	 * @return a UUID
+	 */
+	public static UUID getUlid4() {
+		return UlidSpecCreatorHolder.INSTANCE.create4();
+	}
+
 	/**
 	 * Returns a ULID string.
 	 * 
@@ -94,6 +108,22 @@ public final class UlidCreator {
 		return UlidSpecCreatorHolder.INSTANCE.createString();
 	}
 
+	/**
+	 * Returns a ULID string.
+	 * 
+	 * It is compatible with the RFC-4122 UUID v4.
+	 * 
+	 * The returning string is encoded to Crockford's base32.
+	 * 
+	 * The random component is generated by a secure random number generator:
+	 * {@link java.security.SecureRandom}.
+	 * 
+	 * @return a ULID string
+	 */
+	public static String getUlidString4() {
+		return UlidSpecCreatorHolder.INSTANCE.createString4();
+	}
+
 	/**
 	 * Return a GUID creator for direct use.
 	 * 

src/main/java/com/github/f4b6a3/ulid/creator/UlidSpecCreator.java

@@ -125,6 +125,18 @@ public class UlidSpecCreator {
 		return struct.toUuid();
 	}
 
+	/**
+	 * 
+	 * Return a GUID based on the ULID specification.
+	 * 
+	 * It is compatible with the RFC-4122 UUID v4.
+	 * 
+	 * @return {@link UUID} a GUID value
+	 */
+	public synchronized UUID create4() {
+		return applyVersion4(create());
+	}
+
 	/**
 	 * Returns a ULID string.
 	 * 
@@ -140,6 +152,24 @@ public class UlidSpecCreator {
 		return struct.toString();
 	}
 
+	/**
+	 * Returns a ULID string.
+	 * 
+	 * It is compatible with the RFC-4122 UUID v4.
+	 * 
+	 * The returning string is encoded to Crockford's base32.
+	 * 
+	 * The random component is generated by a secure random number generator:
+	 * {@link java.security.SecureRandom}.
+	 * 
+	 * @return a ULID string
+	 */
+	public synchronized String createString4() {
+		UUID uuid = applyVersion4(create());
+		final UlidStruct struct = new UlidStruct(uuid);
+		return struct.toString();
+	}
+
 	/**
 	 * Return the current timestamp and resets or increments the random part.
 	 * 
@@ -245,4 +275,18 @@ public class UlidSpecCreator {
 	protected long extractRandom2(UUID uuid) {
 		return uuid.getLeastSignificantBits() & HALF_RANDOM_COMPONENT;
 	}
+
+	/**
+	 * Apply the RFC-4122 version 4 to a given UUID.
+	 * 
+	 * It makes the returning UUID compatible with RFC-4122 UUID v4.
+	 * 
+	 * @param ulid a UUID
+	 * @return a UUID
+	 */
+	private static UUID applyVersion4(UUID ulid) {
+		final long msb = (ulid.getMostSignificantBits() & 0xffffffffffff0fffL) | 0x0000000000004000L; // set version
+		final long lsb = (ulid.getLeastSignificantBits() & 0x3fffffffffffffffL) | 0x8000000000000000L; // set variant
+		return new UUID(msb, lsb);
+	}
 }

src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorTest.java

@@ -3,12 +3,14 @@ package com.github.f4b6a3.ulid.ulid;
 import org.junit.Test;
 
 import com.github.f4b6a3.ulid.UlidCreator;
+import com.github.f4b6a3.ulid.util.UlidConverter;
 import com.github.f4b6a3.ulid.util.UlidUtil;
 import com.github.f4b6a3.ulid.util.UlidValidator;
 
 import static org.junit.Assert.*;
 import java.util.Arrays;
 import java.util.HashSet;
+import java.util.UUID;
 
 public class UlidCreatorTest {
 
@@ -32,6 +34,25 @@ public class UlidCreatorTest {
 		checkOrdering(list);
 		checkCreationTime(list, startTime, endTime);
 	}
+	
+	@Test
+	public void testGetUlid4() {
+		String[] list = new String[DEFAULT_LOOP_MAX];
+
+		long startTime = System.currentTimeMillis();
+
+		for (int i = 0; i < DEFAULT_LOOP_MAX; i++) {
+			list[i] = UlidCreator.getUlidString4();
+		}
+
+		long endTime = System.currentTimeMillis();
+
+		checkNullOrInvalid(list);
+		checkUniqueness(list);
+		checkOrdering(list);
+		checkCreationTime(list, startTime, endTime);
+		checkVersion4(list);
+	}
 
 	private void checkNullOrInvalid(String[] list) {
 		for (String ulid : list) {
@@ -73,4 +94,11 @@ public class UlidCreatorTest {
 			assertEquals("The ULID list is not ordered", list[i], other[i]);
 		}
 	}
+	
+	private void checkVersion4(String[] list) {
+		for (String ulid : list) {
+			UUID uuid = UlidConverter.fromString(ulid);
+			assertEquals(String.format("ULID is is not version 4 %s", uuid), 4, uuid.version());
+		}
+	}
 }