ALP: Pre-allocate long[] encoded buffer for double vector decodes

Reuse encoded long[] buffer across double vector decompression calls
instead of allocating new long[1024] per vector. Buffer is allocated
once in initFromPage and passed through to decompressDoubleVector.

Float decode is intentionally left unchanged — JVM escape analysis
produces better code when int[] is allocated fresh per vector call.

Author: sfc-gh-pgaur

parquet-column/src/main/java/org/apache/parquet/column/values/alp/AlpCompression.java

@@ -399,12 +399,20 @@ static DoubleCompressedVector compressDoubleVector(double[] input, int count, Al
 
   static void decompressDoubleVector(DoubleCompressedVector v, double[] output) {
     long[] encoded = new long[v.numElements];
+    decompressDoubleVector(v, output, encoded);
+  }
+
+  static void decompressDoubleVector(DoubleCompressedVector v, double[] output, long[] encodedBuffer) {
     if (v.bitWidth > 0) {
-      unpackLongs(v.packedValues, v.numElements, v.bitWidth, encoded);
+      unpackLongs(v.packedValues, v.numElements, v.bitWidth, encodedBuffer);
+    } else {
+      for (int i = 0; i < v.numElements; i++) {
+        encodedBuffer[i] = 0;
+      }
     }
 
     for (int i = 0; i < v.numElements; i++) {
-      long unfored = encoded[i] + v.frameOfReference;
+      long unfored = encodedBuffer[i] + v.frameOfReference;
       output[i] = AlpEncoderDecoder.decodeDouble(unfored, v.exponent, v.factor);
     }
 

parquet-column/src/main/java/org/apache/parquet/column/values/alp/AlpValuesReaderForDouble.java

@@ -28,10 +28,12 @@
 public class AlpValuesReaderForDouble extends AlpValuesReader {
 
   private double[] decodedBuffer;
+  private long[] encodedLongBuffer;
 
   @Override
   protected void allocateDecodedBuffer(int capacity) {
     this.decodedBuffer = new double[capacity];
+    this.encodedLongBuffer = new long[capacity];
   }
 
   @Override
@@ -54,7 +56,7 @@ private void ensureVectorDecoded(int vectorIdx) {
     int dataOffset = vectorOffsets[vectorIdx];
     AlpCompression.DoubleCompressedVector cv =
         AlpCompression.DoubleCompressedVector.load(rawData, dataOffset, numElements);
-    AlpCompression.decompressDoubleVector(cv, decodedBuffer);
+    AlpCompression.decompressDoubleVector(cv, decodedBuffer, encodedLongBuffer);
     decodedVectorIndex = vectorIdx;
   }
 }