Add method in DirectWriter to compute how many bytes are written for encoding a number of values using a number of bits per value.

lucene/CHANGES.txt

@@ -153,6 +153,9 @@ API Changes
   (with optional timeout) or asynchronously via CompletableFuture. Backward compatible -
   existing code that ignores the return value works unchanged. (Salvatore Campagna)
 
+* GITHUB#15422: Add a new method in DirectWriter to compute how many bytes are written for encoding a number of values using a number of
+  bits per value. (Ignacio Vera)
+
 New Features
 ---------------------
 * GITHUB#15328: VectorSimilarityFunction.getValues() now implements doubleVal allowing its

lucene/core/src/java/org/apache/lucene/util/packed/DirectWriter.java

@@ -149,11 +149,20 @@ public void finish() throws IOException {
     }
     assert !finished;
     flush();
+
     // add padding bytes for fast io
+    final int paddingBytesNeeded = paddingBytesNeeded(bitsPerValue);
+    for (int i = 0; i < paddingBytesNeeded; i++) {
+      output.writeByte((byte) 0);
+    }
+    finished = true;
+  }
+
+  private static int paddingBytesNeeded(int bitsPerValue) {
     // for every number of bits per value, we want to be able to read the entire value in a single
     // read e.g. for 20 bits per value, we want to be able to read values using ints so we need
     // 32 - 20 = 12 bits of padding
-    int paddingBitsNeeded;
+    final int paddingBitsNeeded;
     if (bitsPerValue > Integer.SIZE) {
       paddingBitsNeeded = Long.SIZE - bitsPerValue;
     } else if (bitsPerValue > Short.SIZE) {
@@ -166,20 +175,20 @@ public void finish() throws IOException {
     assert paddingBitsNeeded >= 0;
     final int paddingBytesNeeded = (paddingBitsNeeded + Byte.SIZE - 1) / Byte.SIZE;
     assert paddingBytesNeeded <= 3;
-
-    for (int i = 0; i < paddingBytesNeeded; i++) {
-      output.writeByte((byte) 0);
-    }
-    finished = true;
+    return paddingBytesNeeded;
   }
 
   /** Returns an instance suitable for encoding {@code numValues} using {@code bitsPerValue} */
   public static DirectWriter getInstance(DataOutput output, long numValues, int bitsPerValue) {
+    checkBitsPerValue(bitsPerValue);
+    return new DirectWriter(output, numValues, bitsPerValue);
+  }
+
+  private static void checkBitsPerValue(int bitsPerValue) {
     if (Arrays.binarySearch(SUPPORTED_BITS_PER_VALUE, bitsPerValue) < 0) {
       throw new IllegalArgumentException(
           "Unsupported bitsPerValue " + bitsPerValue + ". Did you use bitsRequired?");
     }
-    return new DirectWriter(output, numValues, bitsPerValue);
   }
 
   /**
@@ -224,4 +233,17 @@ public static int unsignedBitsRequired(long maxValue) {
 
   static final int[] SUPPORTED_BITS_PER_VALUE =
       new int[] {1, 2, 4, 8, 12, 16, 20, 24, 28, 32, 40, 48, 56, 64};
+
+  /**
+   * Returns how many bytes are written for encoding {@code numValues} using {@code bitsPerValue}.
+   *
+   * @param numValues total number of values
+   * @param bitsPerValue the number of bits required per value
+   * @return The amount of bytes written
+   */
+  public static long bytesRequired(long numValues, int bitsPerValue) {
+    checkBitsPerValue(bitsPerValue);
+    final long bytes = (numValues * bitsPerValue + Byte.SIZE - 1) / 8;
+    return bytes + paddingBytesNeeded(bitsPerValue);
+  }
 }

lucene/core/src/test/org/apache/lucene/util/packed/TestDirectPacked.java

@@ -128,6 +128,8 @@ private void doTestBpv(Directory directory, int bpv, long offset, boolean merge)
       writer.finish();
       output.close();
       IndexInput input = directory.openInput(name, IOContext.DEFAULT);
+      assertEquals(
+          input.length() - offset, DirectWriter.bytesRequired(original.length, bitsRequired));
       LongValues reader;
       if (merge) {
         reader =