ALP: Wire up Encoding.ALP in column Encoding enum and AlpValuesWriter

Add ALP entry to org.apache.parquet.column.Encoding with getValuesReader
returning AlpValuesReaderForFloat/Double. Update AlpValuesWriter.getEncoding()
to return Encoding.ALP instead of the PLAIN placeholder.

Author: sfc-gh-pgaur

parquet-column/src/main/java/org/apache/parquet/column/Encoding.java

@@ -29,6 +29,8 @@
 import org.apache.parquet.bytes.BytesUtils;
 import org.apache.parquet.column.page.DictionaryPage;
 import org.apache.parquet.column.values.ValuesReader;
+import org.apache.parquet.column.values.alp.AlpValuesReaderForDouble;
+import org.apache.parquet.column.values.alp.AlpValuesReaderForFloat;
 import org.apache.parquet.column.values.bitpacking.ByteBitPackingValuesReader;
 import org.apache.parquet.column.values.bytestreamsplit.ByteStreamSplitValuesReaderForDouble;
 import org.apache.parquet.column.values.bytestreamsplit.ByteStreamSplitValuesReaderForFLBA;
@@ -147,6 +149,24 @@ public ValuesReader getValuesReader(ColumnDescriptor descriptor, ValuesType valu
     }
   },
 
+  /**
+   * Adaptive Lossless floating-Point (ALP) encoding for FLOAT and DOUBLE columns.
+   */
+  ALP {
+    @Override
+    public ValuesReader getValuesReader(ColumnDescriptor descriptor, ValuesType valuesType) {
+      switch (descriptor.getType()) {
+        case FLOAT:
+          return new AlpValuesReaderForFloat();
+        case DOUBLE:
+          return new AlpValuesReaderForDouble();
+        default:
+          throw new ParquetDecodingException(
+              "Encoding ALP is only supported for type FLOAT and DOUBLE, got " + descriptor.getType());
+      }
+    }
+  },
+
   /**
    * @deprecated This is no longer used, and has been replaced by {@link #RLE}
    * which is combination of bit packing and rle

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

@@ -54,10 +54,9 @@ protected AlpValuesWriter() {
     this(DEFAULT_VECTOR_SIZE);
   }
 
-  // TODO: Replace with Encoding.ALP once ALP is added to parquet-format Thrift spec.
   @Override
   public Encoding getEncoding() {
-    return Encoding.PLAIN;
+    return Encoding.ALP;
   }
 
   @Override