How to use the electrumx.lib.util function in electrumX

max_row_size = self.max_hist_row_entries * 4
        full_hist = b''.join(hist_list)
        nrows = (len(full_hist) + max_row_size - 1) // max_row_size
        if nrows > 4:
            self.logger.info('hashX {} is large: {:,d} entries across '
                             '{:,d} rows'
                             .format(hash_to_hex_str(hashX),
                                     len(full_hist) // 4, nrows))

        # Find what history needs to be written, and what keys need to
        # be deleted.  Start by assuming all keys are to be deleted,
        # and then remove those that are the same on-disk as when
        # compacted.
        write_size = 0
        keys_to_delete.update(hist_map)
        for n, chunk in enumerate(util.chunks(full_hist, max_row_size)):
            key = hashX + pack_be_uint16(n)
            if hist_map.get(key) == chunk:
                keys_to_delete.remove(key)
            else:
                write_items.append((key, chunk))
                write_size += len(chunk)

        assert n + 1 == nrows
        self.comp_flush_count = max(self.comp_flush_count, n)

        return write_size

def db_class(name):
    '''Returns a DB engine class.'''
    for db_class in util.subclasses(Storage):
        if db_class.__name__.lower() == name.lower():
            db_class.import_module()
            return db_class
    raise RuntimeError('unrecognised DB engine "{}"'.format(name))

def header_hash(cls, header):
        timestamp = util.unpack_le_uint32_from(header, 68)[0]
        if timestamp > 1550246400:
            import x16rt_hash
            return x16rt_hash.getPoWHash(header)
        elif timestamp > 1525651200:
            import lyra2z_hash
            return lyra2z_hash.getPoWHash(header)
        import neoscrypt
        return neoscrypt.getPoWHash(header)

def lookup_coin_class(cls, name, net):
        '''Return a coin class given name and network.

        Raise an exception if unrecognised.'''
        req_attrs = ['TX_COUNT', 'TX_COUNT_HEIGHT', 'TX_PER_BLOCK']
        for coin in util.subclasses(Coin):
            if (coin.NAME.lower() == name.lower() and
                    coin.NET.lower() == net.lower()):
                coin_req_attrs = req_attrs.copy()
                missing = [attr for attr in coin_req_attrs
                           if not hasattr(coin, attr)]
                if missing:
                    raise CoinError('coin {} missing {} attributes'
                                    .format(name, missing))
                return coin
        raise CoinError('unknown coin {} and network {} combination'
                        .format(name, net))

def __init__(self, db, prefix, reverse):
        self.prefix = prefix
        if reverse:
            self.iterator = reversed(db.iteritems())
            nxt_prefix = util.increment_byte_string(prefix)
            if nxt_prefix:
                self.iterator.seek(nxt_prefix)
                try:
                    next(self.iterator)
                except StopIteration:
                    self.iterator.seek(nxt_prefix)
            else:
                self.iterator.seek_to_last()
        else:
            self.iterator = db.iteritems()
            self.iterator.seek(prefix)