Function SetBitmapImpl

cpp/src/arrow/util/bit_util.cc:73–118

template <bool value>
void SetBitmapImpl(uint8_t* data, int64_t offset, int64_t length) {
  //                 offset  length
  // data              |<------------->|
  //   |--------|...|--------|...|--------|
  //                   |<--->|   |<--->|
  //                     pro       epi
  if (ARROW_PREDICT_FALSE(length == 0)) {
    return;
  }

  constexpr uint8_t set_byte = value ? UINT8_MAX : 0;

  auto prologue = static_cast<int32_t>(bit_util::RoundUp(offset, 8) - offset);
  DCHECK_LT(prologue, 8);

  if (length < prologue) {  // special case where a mask is required
    //             offset length
    // data             |<->|
    //   |--------|...|--------|...
    //         mask --> |111|
    //                  |<---->|
    //                     pro
    uint8_t mask = bit_util::kPrecedingBitmask[8 - prologue] ^
                   bit_util::kPrecedingBitmask[8 - prologue + length];
    data[offset / 8] = value ? data[offset / 8] | mask : data[offset / 8] & ~mask;
    return;
  }

  // align to a byte boundary
  data[offset / 8] = bit_util::SpliceWord(8 - prologue, data[offset / 8], set_byte);
  offset += prologue;
  length -= prologue;

  // set values per byte
  DCHECK_EQ(offset % 8, 0);
  std::memset(data + offset / 8, set_byte, length / 8);
  offset += bit_util::RoundDown(length, 8);
  length -= bit_util::RoundDown(length, 8);

  // clean up
  DCHECK_LT(length, 8);
  if (length > 0) {
    data[offset / 8] =
        bit_util::SpliceWord(static_cast<int32_t>(length), set_byte, data[offset / 8]);
  }
}

void SetBitmap(uint8_t* data, int64_t offset, int64_t length) {
  SetBitmapImpl<true>(data, offset, length);