Function ComputeBitsShiftForBlockAndStamp

cpp/src/arrow/compute/key_map_internal.h:264–269

When log_blocks is greater than 25, there will be overlapping bits between block id and stamp within a 32-bit hash value. So we must check if this is the case when right shifting a hash value to retrieve block id and stamp. The following two functions derive the number of bits to right shift from the given log_blocks.

  // right shifting a hash value to retrieve block id and stamp. The following two
  // functions derive the number of bits to right shift from the given log_blocks.
  static int ComputeBitsShiftForBlockAndStamp(int log_blocks) {
    if (ARROW_PREDICT_FALSE(log_blocks + bits_stamp_ > bits_hash_)) {
      return 0;
    }
    return bits_hash_ - log_blocks - bits_stamp_;
  }
  static int ComputeBitsShiftForBlock(int log_blocks) {
    if (ARROW_PREDICT_FALSE(log_blocks + bits_stamp_ > bits_hash_)) {
      return bits_hash_ - log_blocks;