HHH-19935 Make sure that increment to current sequence can not be zero

Author: cigaly

hibernate-core/src/main/java/org/hibernate/id/uuid/UuidVersion7Strategy.java

@@ -55,6 +55,13 @@ public long millis() {
 return lastTimestamp.toEpochMilli();
 }
 
+/**
+ * Sub-miliseconds part of timestamp (micro- and nanoseconds) mapped to 12 bit integral value.
+ * Calculated as nanos / 1000000 * 4096
+ *
+ * @param timestamp
+ * @return
+ */
 private static long nanos(Instant timestamp) {
 return (long) ((timestamp.getNano() % 1_000_000L) * 0.004096);
 }
@@ -65,17 +72,24 @@ public State getNextState() {
 lastTimestamp.toEpochMilli() == now.toEpochMilli() && nanos < nanos( now ) ) {
 return new State( now, randomSequence() );
 }
-final long nextSequence = lastSequence + Holder.numberGenerator.nextLong( 0xFFFF_FFFFL );
-if ( nextSequence > MAX_RANDOM_SEQUENCE ) {
-return new State( lastTimestamp.plusNanos( 250 ), randomSequence() );
-}
 else {
-return new State( lastTimestamp, nextSequence );
+final long nextSequence = randomSequence();
+/*
+ *	If next random sequence is less or equal to last one sub-millisecond part
+ *	should be incremented to preserve monotonicity of generated UUIDs.
+ *	To do this smallest number of nanoseconds that will always increase
+ *	sub-millisecond part mapped to 12 bits is
+ *	1_000_000 (nanons per milli) / 4096 (12 bits) = 244.14...
+ *	So 245 is used as smallest integer larger than this value.
+ */
+return lastSequence >= nextSequence
+? new State( lastTimestamp.plusNanos( 245 ), nextSequence )
+: new State( lastTimestamp, nextSequence );
 }
 }
 
 private static long randomSequence() {
-return Holder.numberGenerator.nextLong( MAX_RANDOM_SEQUENCE );
+return Holder.numberGenerator.nextLong( MAX_RANDOM_SEQUENCE + 1 );
 }
 }
 
@@ -117,7 +131,7 @@ public UUID generateUuid(final SharedSessionContractImplementor session) {
 | state.nanos() & 0xFFFL,
 // LSB bits 0-1 - variant = 4
 0x8000_0000_0000_0000L
-// LSB bits 2-15 - pseudorandom counter
+// LSB bits 2-63 - pseudorandom counter
 | state.lastSequence
 );
 }