ViggoC commented on code in PR #44241:
URL: https://github.com/apache/arrow/pull/44241#discussion_r1800487432


##########
java/vector/src/main/java/org/apache/arrow/vector/complex/RunEndEncodedVector.java:
##########
@@ -312,8 +302,142 @@ public TransferPair getTransferPair(Field field, 
BufferAllocator allocator, Call
    */
   @Override
   public TransferPair makeTransferPair(ValueVector target) {
-    throw new UnsupportedOperationException(
-        "RunEndEncodedVector does not support makeTransferPair(ValueVector)");
+    return new TransferImpl((RunEndEncodedVector) target);
+  }
+
+  private class TransferImpl implements TransferPair {
+
+    RunEndEncodedVector to;
+    TransferPair dataTransferPair;
+    TransferPair reeTransferPair;
+
+    public TransferImpl(String name, BufferAllocator allocator, CallBack 
callBack) {
+      this(new RunEndEncodedVector(name, allocator, field.getFieldType(), 
callBack));
+    }
+
+    public TransferImpl(Field field, BufferAllocator allocator, CallBack 
callBack) {
+      this(new RunEndEncodedVector(field, allocator, callBack));
+    }
+
+    public TransferImpl(RunEndEncodedVector to) {
+      this.to = to;
+      if (to.getRunEndsVector() instanceof ZeroVector) {
+        to.initializeChildrenFromFields(field.getChildren());
+      }
+      reeTransferPair = 
getRunEndsVector().makeTransferPair(to.getRunEndsVector());
+      dataTransferPair = 
getValuesVector().makeTransferPair(to.getValuesVector());
+    }
+
+    /**
+     * Transfer this vector'data to another vector. The memory associated with 
this vector is
+     * transferred to the allocator of target vector for accounting and 
management purposes.
+     */
+    @Override
+    public void transfer() {
+      to.clear();
+      dataTransferPair.transfer();
+      reeTransferPair.transfer();
+      if (valueCount > 0) {
+        to.setValueCount(valueCount);
+      }
+      clear();
+    }
+
+    /**
+     * Slice this vector at desired index and length and transfer the 
corresponding data to the
+     * target vector.
+     *
+     * @param startIndex start position of the split in source vector.
+     * @param length length of the split.
+     */
+    @Override
+    public void splitAndTransfer(int startIndex, int length) {
+      ValueVector toDataVector = dataTransferPair.getTo();
+      ValueVector toRunEndVector = reeTransferPair.getTo();
+
+      toDataVector.clear();
+      toRunEndVector.clear();
+
+      int endIndex = startIndex + length;
+      int physicalStartIndex = getPhysicalIndex(startIndex);
+      int physicalEndIndex = getPhysicalIndex(endIndex);
+      int physicalLength = physicalEndIndex - physicalStartIndex + 1;
+      dataTransferPair.splitAndTransfer(physicalStartIndex, physicalLength);
+      if (startIndex == 0) {
+        reeTransferPair.splitAndTransfer(physicalStartIndex, physicalLength);
+      } else {
+        shiftRunEndVector(
+            toRunEndVector,
+            startIndex,
+            length,
+            physicalStartIndex,
+            physicalEndIndex,
+            physicalLength);
+      }
+      getTo().setValueCount(length);
+    }
+
+    private void shiftRunEndVector(
+        ValueVector toRunEndVector,
+        int startIndex,
+        int length,
+        int physicalStartIndex,
+        int physicalEndIndex,
+        int physicalLength) {
+      toRunEndVector.setValueCount(physicalLength);
+      toRunEndVector.getValidityBuffer().setOne(0, 
toRunEndVector.getValidityBuffer().capacity());

Review Comment:
   In ListVector, it slice the validityBuffer if startIndex % 8 == 0, and copy 
data one by one when the first bit starts from the middle of a byte. 
   But for run end encoded vector, the element of run end vector can never be 
null so I just set validity buffer of RunEndVector to 1. 
   What's your concern about this code? The memory of validity buffer should be 
reused when startIndex % 8 == 0, Or we should not set the bit beyond the 
physical length?
   



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