Refactor FST.saveMetadata() to FSTMetadata.save() (#13549)

* lazily write the FST padding byte

* Also write the pad byte when there is emptyOutput

* add comment

* Make Lucene90BlockTreeTermsWriter to write FST off-heap

* Add change log

* Tidy code & Add comments

* use temp IndexOutput for FST writing

* Use IOUtils to delete files

* Update CHANGES.txt

* Update CHANGES.txt

Author: dungba88

lucene/core/src/java/org/apache/lucene/util/fst/FST.java

@@ -528,56 +528,10 @@ public FSTMetadata<T> getMetadata() {
    * @param out the DataOutput to write the FST bytes to
    */
   public void save(DataOutput metaOut, DataOutput out) throws IOException {
-    saveMetadata(metaOut);
+    metadata.save(metaOut);
     fstReader.writeTo(out);
   }
 
-  /**
-   * Save the metadata to a DataOutput
-   *
-   * @param metaOut the DataOutput to write the metadata to
-   */
-  public void saveMetadata(DataOutput metaOut) throws IOException {
-    CodecUtil.writeHeader(metaOut, FILE_FORMAT_NAME, VERSION_CURRENT);
-    // TODO: really we should encode this as an arc, arriving
-    // to the root node, instead of special casing here:
-    if (metadata.emptyOutput != null) {
-      // Accepts empty string
-      metaOut.writeByte((byte) 1);
-
-      // Serialize empty-string output:
-      ByteBuffersDataOutput ros = new ByteBuffersDataOutput();
-      outputs.writeFinalOutput(metadata.emptyOutput, ros);
-      byte[] emptyOutputBytes = ros.toArrayCopy();
-      int emptyLen = emptyOutputBytes.length;
-
-      // reverse
-      final int stopAt = emptyLen / 2;
-      int upto = 0;
-      while (upto < stopAt) {
-        final byte b = emptyOutputBytes[upto];
-        emptyOutputBytes[upto] = emptyOutputBytes[emptyLen - upto - 1];
-        emptyOutputBytes[emptyLen - upto - 1] = b;
-        upto++;
-      }
-      metaOut.writeVInt(emptyLen);
-      metaOut.writeBytes(emptyOutputBytes, 0, emptyLen);
-    } else {
-      metaOut.writeByte((byte) 0);
-    }
-    final byte t;
-    if (metadata.inputType == INPUT_TYPE.BYTE1) {
-      t = 0;
-    } else if (metadata.inputType == INPUT_TYPE.BYTE2) {
-      t = 1;
-    } else {
-      t = 2;
-    }
-    metaOut.writeByte(t);
-    metaOut.writeVLong(metadata.startNode);
-    metaOut.writeVLong(numBytes());
-  }
-
   /** Writes an automaton to a file. */
   public void save(final Path path) throws IOException {
     try (OutputStream os = new BufferedOutputStream(Files.newOutputStream(path))) {
@@ -1249,5 +1203,59 @@ public FSTMetadata(
     public int getVersion() {
       return version;
     }
+
+    public T getEmptyOutput() {
+      return emptyOutput;
+    }
+
+    public long getNumBytes() {
+      return numBytes;
+    }
+
+    /**
+     * Save the metadata to a DataOutput
+     *
+     * @param metaOut the DataOutput to write the metadata to
+     */
+    public void save(DataOutput metaOut) throws IOException {
+      CodecUtil.writeHeader(metaOut, FILE_FORMAT_NAME, VERSION_CURRENT);
+      // TODO: really we should encode this as an arc, arriving
+      // to the root node, instead of special casing here:
+      if (emptyOutput != null) {
+        // Accepts empty string
+        metaOut.writeByte((byte) 1);
+
+        // Serialize empty-string output:
+        ByteBuffersDataOutput ros = new ByteBuffersDataOutput();
+        outputs.writeFinalOutput(emptyOutput, ros);
+        byte[] emptyOutputBytes = ros.toArrayCopy();
+        int emptyLen = emptyOutputBytes.length;
+
+        // reverse
+        final int stopAt = emptyLen / 2;
+        int upto = 0;
+        while (upto < stopAt) {
+          final byte b = emptyOutputBytes[upto];
+          emptyOutputBytes[upto] = emptyOutputBytes[emptyLen - upto - 1];
+          emptyOutputBytes[emptyLen - upto - 1] = b;
+          upto++;
+        }
+        metaOut.writeVInt(emptyLen);
+        metaOut.writeBytes(emptyOutputBytes, 0, emptyLen);
+      } else {
+        metaOut.writeByte((byte) 0);
+      }
+      final byte t;
+      if (inputType == INPUT_TYPE.BYTE1) {
+        t = 0;
+      } else if (inputType == INPUT_TYPE.BYTE2) {
+        t = 1;
+      } else {
+        t = 2;
+      }
+      metaOut.writeByte(t);
+      metaOut.writeVLong(startNode);
+      metaOut.writeVLong(numBytes);
+    }
   }
 }