[GitHub] [arrow] nirandaperera commented on a change in pull request #10410: ARROW-10640: [C++] A "where" kernel to combine two arrays based on a mask

[GitBox <gi...@apache.org>]

nirandaperera commented on a change in pull request #10410:
URL: https://github.com/apache/arrow/pull/10410#discussion_r640660867



##########
File path: cpp/src/arrow/compute/kernels/scalar_if_else.cc
##########
@@ -0,0 +1,838 @@
+// 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.
+
+#include <arrow/compute/api.h>
+#include <arrow/util/bit_block_counter.h>
+#include <arrow/util/bitmap.h>
+#include <arrow/util/bitmap_ops.h>
+
+#include "codegen_internal.h"
+
+namespace arrow {
+using internal::BitBlockCount;
+using internal::BitBlockCounter;
+using internal::Bitmap;
+
+namespace compute {
+
+namespace {
+
+enum { COND_ALL_VALID = 1, LEFT_ALL_VALID = 2, RIGHT_ALL_VALID = 4 };
+
+// if the condition is null then output is null otherwise we take validity from the
+// selected argument
+// ie. cond.valid & (cond.data & left.valid | ~cond.data & right.valid)
+Status PromoteNullsVisitor(KernelContext* ctx, const ArrayData& cond, const Scalar& left,
+                           const Scalar& right, ArrayData* output) {
+  uint8_t flag = right.is_valid * 4 + left.is_valid * 2 + !cond.MayHaveNulls();
+
+  if (flag < 6 && flag != 3) {
+    // there will be a validity buffer in the output
+    ARROW_ASSIGN_OR_RAISE(output->buffers[0], ctx->AllocateBitmap(cond.length));
+  }
+
+  // if the condition is null then output is null otherwise we take validity from the
+  // selected argument
+  // ie. cond.valid & (cond.data & left.valid | ~cond.data & right.valid)
+  switch (flag) {
+    case COND_ALL_VALID | LEFT_ALL_VALID | RIGHT_ALL_VALID:  // = 7
+      break;
+    case LEFT_ALL_VALID | RIGHT_ALL_VALID:  // = 6
+      // out_valid = c_valid
+      output->buffers[0] = SliceBuffer(cond.buffers[0], cond.offset, cond.length);
+      break;
+    case COND_ALL_VALID | RIGHT_ALL_VALID:  // = 5
+      // out_valid = ~cond.data
+      arrow::internal::InvertBitmap(cond.buffers[1]->data(), cond.offset, cond.length,
+                                    output->buffers[0]->mutable_data(), 0);
+      break;
+    case RIGHT_ALL_VALID:  // = 4
+      // out_valid = c_valid & ~cond.data
+      arrow::internal::BitmapAndNot(cond.buffers[0]->data(), cond.offset,
+                                    cond.buffers[1]->data(), cond.offset, cond.length, 0,
+                                    output->buffers[0]->mutable_data());
+      break;
+    case COND_ALL_VALID | LEFT_ALL_VALID:  // = 3
+      // out_valid = cond.data
+      output->buffers[0] = SliceBuffer(cond.buffers[1], cond.offset, cond.length);
+      break;
+    case LEFT_ALL_VALID:  // = 2
+      // out_valid = cond.valid & cond.data
+      arrow::internal::BitmapAnd(cond.buffers[0]->data(), cond.offset,
+                                 cond.buffers[1]->data(), cond.offset, cond.length, 0,
+                                 output->buffers[0]->mutable_data());
+      break;
+    case COND_ALL_VALID:  // = 1
+      // out_valid = 0 --> nothing to do; but requires out_valid to be a all-zero buffer

Review comment:
       I think there is a mismatch in the kernel comment and the impl. 
   https://github.com/apache/arrow/blob/29130ca54cd773f6f52e17bc78d37fb72d53eb49/cpp/src/arrow/compute/kernel.cc#L56
   It looks like it is zeroed out for bitmaps




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