Allow FST builder to use different writer (#12543)

Author: dungba88

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

@@ -26,7 +26,7 @@
 // TODO: merge with PagedBytes, except PagedBytes doesn't
 // let you read while writing which FST needs
 
-class BytesStore extends DataOutput implements FSTReader {
+class BytesStore extends DataOutput implements FSTWriter {
 
   private static final long BASE_RAM_BYTES_USED =
       RamUsageEstimator.shallowSizeOfInstance(BytesStore.class)
@@ -359,6 +359,7 @@ public void truncate(long newLen) {
     assert newLen == getPosition();
   }
 
+  @Override
   public void finish() {
     if (current != null) {
       byte[] lastBuffer = new byte[nextWrite];
@@ -368,6 +369,18 @@ public void finish() {
     }
   }
 
+  /** Writes all of our bytes to the target {@link FSTWriter}. */
+  public void writeTo(FSTWriter out) throws IOException {
+    // TODO: if the FSTWriter is also BytesStore we are doing double write
+    // once to reverse the bytes and once to write to the BytesStore
+    // maybe we should combine it into reverseAndWriteTo()?
+    reverse(0, getPosition() - 1);
+    finish();
+    for (byte[] block : blocks) {
+      out.writeBytes(block, 0, block.length);
+    }
+  }
+
   /** Writes all of our bytes to the target {@link DataOutput}. */
   @Override
   public void writeTo(DataOutput out) throws IOException {

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

@@ -0,0 +1,93 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.util.fst;
+
+import java.io.Closeable;
+import java.io.IOException;
+import org.apache.lucene.store.DataOutput;
+import org.apache.lucene.util.Accountable;
+import org.apache.lucene.util.RamUsageEstimator;
+
+/**
+ * An {@link FSTWriter} which write to a {@link DataOutput}. It only supports writing to the {@link
+ * DataOutput} and not reading from it To read, you must construct a corresponding {@link
+ * org.apache.lucene.store.DataInput} and use the {@link FSTStore} to read
+ *
+ * @lucene.experimental
+ */
+public class DataOutputFSTWriter implements FSTWriter {
+
+  private static final long BASE_RAM_BYTES_USED =
+      RamUsageEstimator.shallowSizeOfInstance(DataOutputFSTWriter.class);
+
+  private final DataOutput dataOutput;
+
+  private long size = 0L;
+
+  /**
+   * ctor
+   *
+   * @param dataOutput the data output to write to
+   */
+  public DataOutputFSTWriter(DataOutput dataOutput) {
+    this.dataOutput = dataOutput;
+  }
+
+  @Override
+  public long ramBytesUsed() {
+    long size = BASE_RAM_BYTES_USED;
+    if (dataOutput instanceof Accountable) {
+      size += ((Accountable) dataOutput).ramBytesUsed();
+    }
+    return size;
+  }
+
+  @Override
+  public long size() {
+    return size;
+  }
+
+  @Override
+  public void writeByte(byte b) throws IOException {
+    size++;
+    dataOutput.writeByte(b);
+  }
+
+  @Override
+  public void writeBytes(byte[] b, int offset, int length) throws IOException {
+    size += length;
+    dataOutput.writeBytes(b, offset, length);
+  }
+
+  @Override
+  public void finish() throws IOException {
+    if (dataOutput instanceof Closeable) {
+      ((Closeable) dataOutput).close();
+    }
+  }
+
+  @Override
+  public void writeTo(DataOutput out) throws IOException {
+    throw new UnsupportedOperationException(
+        "writeTo(DataOutput) is not supported by DataOutputFSTWriter");
+  }
+
+  @Override
+  public FST.BytesReader getReverseBytesReader() {
+    return null;
+  }
+}