What is a good hash function for strings? Easiest way to convert int to string in C++. E.g., my struct is { char* data; char link{'A', 'B', .., 'a', 'b', ' ', ..}; } and it will test root for whether (node->link['x'] != NULL) to get to the possible words starting with "x". This is an example of the folding approach to designing a hash function. As a cryptographic function, it was broken about 15 years ago, but for non cryptographic purposes, it is still very good, and surprisingly fast. %PDF-1.3 In general, the hash is much smaller than the input data, hence hash functions are sometimes called compression functions.
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�VF��jS�`]�!-�_U��k� �`���ܶ5&cO�OkL� In hashing there is a hash function that maps keys to some values. I've considered CRC32 (but where to find good implementation?) If bucket i contains xi elements, then a good measure of clustering is (∑ i(xi2)/n) - α. The hash function is a perfect hash function when it uses all the input data. I'm implementing a hash table with this hash function and the binary tree that you've outlined in other answer. This video walks through how to develop a good hash function. x��X�r�F��W���Ƴ/�ٮ���$UX��/0��A��V��yX�Mc�+"KEh��_��7��[���W�q�P�xe��3�v��}����;�g�h��$H}�Mw�z�Y��'��B��E���={ލ��z焆t� e� �^y��r��!��,�+X�?.��PnT2�
>�xE�+���\������5��-����a��ĺ��@�.��'��đȰ�tHBj���H�E This can be faster than hashing. %��������� I've not tried it, so I can't vouch for its performance. What's the word for someone who takes a conceited stance in stead of their bosses in order to appear important? You might get away with CRC16 (~65,000 possibilities) but you would probably have a lot of collisions to deal with. On the other hand, a collision may be quicker to deal with than than a CRC32 hash. You could just take the last two 16-bit chars of the string and form a 32-bit int 4 Choosing a Good Hash Function Goal: scramble the keys.! endobj So the contents of the string are interpreted as a raw number, no worries about characters anymore, and you then bit-shift this the precision needed (you tweak this number to the best performance, I've found 2 works well for hashing strings in set of a few thousands). In simple terms, a hash function maps a big number or string to a small integer that can be used as the index in the hash table. With a good hash function, even a 1-bit change in a message will produce a different hash (on average, half of the bits change). Why did the design of the Boeing 247's cockpit windows change for some models? On collision, increment index until you hit an empty bucket.. quick and simple. 0��j$`��L[yHjG-w�@�q\s��h`�D I�.p �5ՠx���$0���> /Font << /F1.0 Now assumming you want a hash, and want something blazing fast that would work in your case, because your strings are just 6 chars long you could use this magic: Explanation: ��X{G���,��SC�O���O�ɐnU.��k�ץx;g����G���r�W�-$���*�%:��]����^0��3_Se��u'We�ɀ�TH�i�i�m�\ګ�ɈP��7K׆-��$�N����\Q. x��YMo�H�����ͬ6=�M�J{�D����%Ҟ Ɔ 6 �����;�c� `,ٖ!��U��������N1�-HC��Y hŠ��X����CTo�e���� R?s�yh�wd�|q�`TH�|Hsu���xW5��Vh��p� R6�A8�@0s��S�����������F%�����3R�iė�4t'm�4ڈ�a�����͎t'�ŀ5��'8����H?k6H�R���o��)�i��l�8S�r���l�D:�ę�ۜ�H��ܝ��
�j�$�!�ýG�H�QǍ�ڴ8�D���$�R�C$R#�FP�k$q!��6���FPc�E Since you store english words, most of your characters will be letters and there won't be much variation in the most significant two bits of your data. Prerequisite: Hashing data structure The hash function is the component of hashing that maps the keys to some location in the hash table. It is a one-way function, that is, a function which is practically infeasible to invert. The size of the table is important too, to minimize collisions. /Resources 10 0 R /Filter /FlateDecode >> Sybol Table: Implementations Cost Summary fix: use repeated doubling, and rehash all keys S orted ay Implementation Unsorted list lgN Get N Put N Get N / 2 /2 Put N Remove N / 2 Worst Case Average Case Remove N Separate chaining N N N 1* 1* 1* * assumes hash function is random The mapped integer value is used as an index in the hash table. Lookup about heaps and priority queues. 3) The hash function "uniformly" distributes the data across the entire set of possible hash values. There's no avalanche effect at all... And if you can guarentee that your strings are always 6 chars long without exception then you could try unrolling the loop. The mid square method is a very good hash function. He is B.Tech from IIT and MS from USA. Using these would probably be save much work opposed to implementing your own classes. The good and widely used way to define the hash of a string s of length n ishash(s)=s[0]+s[1]⋅p+s[2]⋅p2+...+s[n−1]⋅pn−1modm=n−1∑i=0s[i]⋅pimodm,where p and m are some chosen, positive numbers.It is called a polynomial rolling hash function. If the hash table size M is small compared to the resulting summations, then this hash function should do a good job of distributing strings evenly among the hash table slots, because it gives equal weight to all characters in the string.
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