emkornfield commented on a change in pull request #6985:
URL: https://github.com/apache/arrow/pull/6985#discussion_r419855848
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File path: cpp/src/arrow/util/bit_util.h
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@@ -610,6 +618,71 @@ class FirstTimeBitmapWriter {
}
}
+ /// Appends number_of_bits from word to valid_bits and valid_bits_offset.
+ ///
+ /// \param[in] word The LSB bitmap to append. Any bits past number_of_bits
are assumed
+ /// to be unset (i.e. 0).
+ /// \param[in] number_of_bits The number of bits to append from word.
+ void AppendWord(uint64_t word, int64_t number_of_bits) {
+#if defined(ARROW_LITTLE_ENDIAN)
+ if (ARROW_PREDICT_FALSE(number_of_bits == 0)) {
+ return;
+ }
+
+ // Location that the first byte needs to be written to.
+ uint8_t* append_position = bitmap_ + byte_offset_;
+
+ // Update state variables except for current_byte_ here.
+ position_ += number_of_bits;
+ int64_t bit_offset =
BitUtil::CountTrailingZeros(static_cast<uint32_t>(bit_mask_));
+ bit_mask_ = BitUtil::kBitmask[(bit_offset + number_of_bits) % 8];
+ byte_offset_ += (bit_offset + number_of_bits) / 8;
+
+ if (bit_offset != 0) {
+ // We are in the middle of the byte. This code updates the byte and
shifts
+ // bits appropriately within word so it can be memcpy'd below.
+ int64_t bits_to_carry = 8 - bit_offset;
+ // Carry over bits from word to current_byte_. We assume any extra bits
in word
+ // unset so no additional accounting is needed for when number_of_bits <
+ // bits_to_carry.
+ current_byte_ |= (word & BitUtil::kPrecedingBitmask[bits_to_carry]) <<
bit_offset;
+ // Check if everything is transfered into current_byte_.
+ if (ARROW_PREDICT_FALSE(number_of_bits < bits_to_carry)) {
+ return;
+ }
+ *append_position = current_byte_;
+ append_position++;
+ // Move the carry bits off of word.
+ word = word >> bits_to_carry;
+ number_of_bits -= bits_to_carry;
+ }
+
+ int64_t bytes_for_word = ::arrow::BitUtil::BytesForBits(number_of_bits);
+ std::memcpy(append_position, &word, bytes_for_word);
+ // At this point, the previous current_byte_ has been written to bitmap_.
+ // The new current_byte_ is either the last relevant byte in 'word'
+ // or cleared if the new position is byte aligned (i.e. a fresh byte).
+ if (bit_mask_ == 0x1) {
+ current_byte_ = 0;
+ } else {
+ // current_byte_ = *(reinterpret_cast<uint8_t*>(&word) + bytes_for_word
- 1) : 0;
+ current_byte_ = *(append_position + bytes_for_word - 1);
+ }
+
+#else // big-endian
+ for (int x = 0; x < number_of_bits; x++) {
Review comment:
maybe I should just remove this since I can't verify?
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