Add a test to check the serialized size of a BloomFilter.

android/guava-tests/test/com/google/common/hash/BloomFilterTest.java

@@ -19,6 +19,7 @@
 import static com.google.common.hash.BloomFilter.toBloomFilter;
 import static com.google.common.hash.Funnels.unencodedCharsFunnel;
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth.assertWithMessage;
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.junit.Assert.assertThrows;
@@ -353,6 +354,45 @@ public void testBitSize() {
     }
   }
 
+  /**
+   * Tests that bitSize() can be used to predict the serialization size produced by writeTo().
+   *
+   * <p>The serialization format consists of a 6-byte header (1 byte strategy, 1 byte hash
+   * functions, 4 bytes array length) followed by the bit array data (bitSize / 8 bytes).
+   */
+  public void testBitSizeMatchesSerializationSize() throws Exception {
+    int[] expectedInsertionValues = {1, 10, 100, 1000, 10000};
+    double[] fppValues = {0.01, 0.03, 0.1};
+
+    for (int expectedInsertions : expectedInsertionValues) {
+      for (double fpp : fppValues) {
+        BloomFilter<String> bf =
+            BloomFilter.create(Funnels.unencodedCharsFunnel(), expectedInsertions, fpp);
+
+        // Add some elements
+        for (int i = 0; i < expectedInsertions / 2; i++) {
+          bf.put("element" + i);
+        }
+
+        // Calculate expected size based on bitSize()
+        // Header: 1 byte (strategy) + 1 byte (hash functions) + 4 bytes (array length) = 6 bytes
+        // Data: bitSize / 8 bytes
+        long predictedSize = bf.bitSize() / 8 + 6;
+
+        // Serialize and measure actual size
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        bf.writeTo(out);
+        int actualSize = out.size();
+
+        assertWithMessage(
+                "Serialization size mismatch for expectedInsertions=%s, fpp=%s",
+                expectedInsertions, fpp)
+            .that(actualSize)
+            .isEqualTo(predictedSize);
+      }
+    }
+  }
+
   public void testApproximateElementCount() {
     int numInsertions = 1000;
     BloomFilter<Integer> bf = BloomFilter.create(Funnels.integerFunnel(), numInsertions);

guava-tests/test/com/google/common/hash/BloomFilterTest.java

@@ -19,6 +19,7 @@
 import static com.google.common.hash.BloomFilter.toBloomFilter;
 import static com.google.common.hash.Funnels.unencodedCharsFunnel;
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth.assertWithMessage;
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.junit.Assert.assertThrows;
@@ -355,6 +356,45 @@ public void testBitSize() {
     }
   }
 
+  /**
+   * Tests that bitSize() can be used to predict the serialization size produced by writeTo().
+   *
+   * <p>The serialization format consists of a 6-byte header (1 byte strategy, 1 byte hash
+   * functions, 4 bytes array length) followed by the bit array data (bitSize / 8 bytes).
+   */
+  public void testBitSizeMatchesSerializationSize() throws Exception {
+    int[] expectedInsertionValues = {1, 10, 100, 1000, 10000};
+    double[] fppValues = {0.01, 0.03, 0.1};
+
+    for (int expectedInsertions : expectedInsertionValues) {
+      for (double fpp : fppValues) {
+        BloomFilter<String> bf =
+            BloomFilter.create(Funnels.unencodedCharsFunnel(), expectedInsertions, fpp);
+
+        // Add some elements
+        for (int i = 0; i < expectedInsertions / 2; i++) {
+          bf.put("element" + i);
+        }
+
+        // Calculate expected size based on bitSize()
+        // Header: 1 byte (strategy) + 1 byte (hash functions) + 4 bytes (array length) = 6 bytes
+        // Data: bitSize / 8 bytes
+        long predictedSize = bf.bitSize() / 8 + 6;
+
+        // Serialize and measure actual size
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        bf.writeTo(out);
+        int actualSize = out.size();
+
+        assertWithMessage(
+                "Serialization size mismatch for expectedInsertions=%s, fpp=%s",
+                expectedInsertions, fpp)
+            .that(actualSize)
+            .isEqualTo(predictedSize);
+      }
+    }
+  }
+
   public void testApproximateElementCount() {
     int numInsertions = 1000;
     BloomFilter<Integer> bf = BloomFilter.create(Funnels.integerFunnel(), numInsertions);