Function PackedMaxSpreadBytes

cpp/src/arrow/util/bpacking_dispatch_internal.h:63–74

Compute the maximum spread in bytes that a packed integer can cover. This is assuming contiguous packed integer starting with the given bit offset away from a byte boundary. This function is non-monotonic, for instance with zero offset, three bit integers will be split on the first byte boundary (hence having a spread of two bytes) while four bit integer will be well behaved and never spread over

/// four bit integer will be well behaved and never spread over byte boundary (hence
/// having a spread of one).
ARROW_FORCE_INLINE constexpr int PackedMaxSpreadBytes(int width, int bit_offset) {
  int max = static_cast<int>(bit_util::BytesForBits(width));
  int start = bit_offset;
  do {
    const int byte_start = start / 8;
    const int byte_end = (start + width - 1) / 8;  // inclusive end bit
    const int spread = byte_end - byte_start + 1;
    max = spread > max ? spread : max;
    start += width;
  } while (start % 8 != bit_offset);
  return max;
}

/// Compute the maximum spread in bytes that a packed integer can cover across all bit
/// offsets.