Method DecodeNulls

cpp/src/arrow/compute/row/row_encoder_internal.cc:33–63

extract the null bitmap from the leading nullity bytes of encoded keys

// extract the null bitmap from the leading nullity bytes of encoded keys
Status KeyEncoder::DecodeNulls(MemoryPool* pool, int32_t length, uint8_t** encoded_bytes,
                               std::shared_ptr<Buffer>* null_bitmap,
                               int32_t* null_count) {
  // first count nulls to determine if a null bitmap is necessary
  *null_count = 0;
  for (int32_t i = 0; i < length; ++i) {
    *null_count += (encoded_bytes[i][0] == kNullByte);
  }

  if (*null_count > 0) {
    ARROW_ASSIGN_OR_RAISE(*null_bitmap, AllocateBitmap(length, pool));
    uint8_t* validity = (*null_bitmap)->mutable_data();

    FirstTimeBitmapWriter writer(validity, 0, length);
    for (int32_t i = 0; i < length; ++i) {
      if (encoded_bytes[i][0] == kValidByte) {
        writer.Set();
      } else {
        writer.Clear();
      }
      writer.Next();
      encoded_bytes[i] += 1;
    }
    writer.Finish();
  } else {
    for (int32_t i = 0; i < length; ++i) {
      encoded_bytes[i] += 1;
    }
  }
  return Status ::OK();
}

void BooleanKeyEncoder::AddLength(const ExecValue&, int64_t batch_length,
                                  int32_t* lengths) {