added isEmpty method to HyperLogLog and UltraLogLog

src/main/java/com/dynatrace/hash4j/distinctcount/DistinctCounter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2023 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -142,6 +142,13 @@ interface DistinctCounter<T extends DistinctCounter<T, R>, R extends DistinctCou
    */
   double getStateChangeProbability();
 
+  /**
+   * Returns {@code true} if the sketch is empty, corresponding to the initial state.
+   *
+   * @return {@code true} if the sketch is empty
+   */
+  boolean isEmpty();
+
   /**
    * An estimator for a distinct counter.
    *

src/main/java/com/dynatrace/hash4j/distinctcount/HyperLogLog.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -477,6 +477,19 @@ public double getStateChangeProbability() {
     return DistinctCountUtil.unsignedLongToDouble(sum) * 0x1p-64;
   }
 
+  /**
+   * Returns {@code true} if the sketch is empty, corresponding to the initial state.
+   *
+   * @return {@code true} if the sketch is empty
+   */
+  @Override
+  public boolean isEmpty() {
+    for (byte b : state) {
+      if (b != 0) return false;
+    }
+    return true;
+  }
+
   /**
    * Resets this sketch to its initial state representing an empty set.
    *

src/main/java/com/dynatrace/hash4j/distinctcount/UltraLogLog.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -386,6 +386,19 @@ public double getStateChangeProbability() {
     return DistinctCountUtil.unsignedLongToDouble(sum) * 0x1p-64;
   }
 
+  /**
+   * Returns {@code true} if the sketch is empty, corresponding to the initial state.
+   *
+   * @return {@code true} if the sketch is empty
+   */
+  @Override
+  public boolean isEmpty() {
+    for (byte b : state) {
+      if (b != 0) return false;
+    }
+    return true;
+  }
+
   /**
    * Resets this sketch to its initial state representing an empty set.
    *

src/main/java/com/dynatrace/hash4j/random/PseudoRandomGenerator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

src/main/java/com/dynatrace/hash4j/random/PseudoRandomGeneratorProvider.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

src/main/java/com/dynatrace/hash4j/random/SplitMix64V1.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

src/test/java/com/dynatrace/hash4j/distinctcount/DistinctCounterTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -1043,6 +1043,21 @@ void testChangeProbabilityAndFinitenessOfEstimators() {
     }
   }
 
+  @Test
+  void testIsEmpty() {
+    SplittableRandom random = new SplittableRandom(0x309a6777250910bcL);
+
+    for (int p = getMinP(); p <= getMaxP(); ++p) {
+      T sketch = create(p);
+      for (int i = 0; i < 5; ++i) {
+        assertThat(sketch.isEmpty()).isTrue();
+        sketch.add(random.nextLong());
+        assertThat(sketch.isEmpty()).isFalse();
+        sketch.reset();
+      }
+    }
+  }
+
   @Test
   void testToken() {
     for (int p = getMinP(); p <= Math.min(getMaxP(), 10); ++p) {

src/test/java/com/dynatrace/hash4j/random/AbstractPseudoRandomGeneratorProviderTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2024 Dynatrace LLC
+ * Copyright 2024-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

src/test/java/com/dynatrace/hash4j/random/PseudoRandomGeneratorProviderForTesting.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023-2024 Dynatrace LLC
+ * Copyright 2023-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

src/test/java/com/dynatrace/hash4j/random/SplitMix64_v1ProviderTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2024 Dynatrace LLC
+ * Copyright 2024-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

src/test/java/com/dynatrace/hash4j/similarity/DistinctElementHashProviderTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 Dynatrace LLC
+ * Copyright 2022-2025 Dynatrace LLC
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.