Add a method to just reset the BytesStore

Author: dungba88

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

@@ -337,6 +337,19 @@ public long size() {
     return getPosition();
   }
 
+  /**
+   * Similar to {@link #truncate(long)} with newLen=0 but keep the first block
+   * to reduce GC.
+   */
+  public void reset() {
+    if (blocks.isEmpty()) {
+      return;
+    }
+    nextWrite = 0;
+    current = blocks.get(0);
+    blocks.subList(1, blocks.size()).clear();
+  }
+
   /**
    * Pos must be less than the max position written so far! Ie, you cannot "grow" the file with
    * this!
@@ -371,13 +384,13 @@ 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);
+      if (block == current) { // last block
+        out.writeBytes(block, 0, nextWrite);
+      } else {
+        out.writeBytes(block, 0, block.length);
+      }
     }
   }
 

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

@@ -327,9 +327,6 @@ private CompiledNode compileNode(UnCompiledNode<T> nodeIn) throws IOException {
     } else {
       node = addNode(nodeIn);
     }
-    // truncate the scratch writer
-    // TODO: Rather than truncating, we can keep the first block and set nextWrite to 0 to reduce GC
-    bytes.truncate(0);
 
     assert node != -2;
 
@@ -358,7 +355,8 @@ long addNode(FSTCompiler.UnCompiledNode<T> nodeIn) throws IOException {
         return NON_FINAL_END_NODE;
       }
     }
-    // System.out.println("  startAddr=" + startAddress);
+    // reset the scratch writer to prepare for new write
+    bytes.reset();
 
     // the scratch writer must be cleaned at this point
     assert bytes.getPosition() == 0;