Good Hash Function for Strings

Question

I m trying to think up a good hash function for strings   And I was thinking it might be a good idea to sum up the unicode values for the first five characters in the string  assuming it has five  otherwise stop where it ends    Would that be a good idea  or is it a bad one   I am doing this in Java  but I wouldn t imagine that would make much of a difference

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This will avoid any collision and it will be fast until we use the shifting in calculations    int k   key length        int sum   0      for int i   0   i  lt  k-1   i             sum    key charAt i  lt  lt  5 i

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here s a link that explains many different hash functions  for now I prefer the ELF hash function for your particular problem  It takes as input a string of arbitrary length

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If it s a security thing  you could use Java crypto  import java security MessageDigest   MessageDigest messageDigest   MessageDigest getInstance  quot SHA-256 quot    messageDigest update stringToHash getBytes     String stringHash   new String messageDigest digest

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FNV-1 is rumoured to be a good hash function for strings   For long strings  longer than  say  about 200 characters   you can get good performance out of the MD4 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  In the context of Java  you would have to convert the 16-bit char values into 32-bit words  e g  by grouping such values into pairs  A fast implementation of MD4 in Java can be found in sphlib  Probably overkill in the context of a classroom assignment  but otherwise worth a try

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sdbm this algorithm was created for sdbm  a public-domain reimplementation of ndbm  database library  static unsigned long sdbm unsigned char  str           unsigned long hash   0      int c      while  c    str                hash   c    hash  lt  lt  6     hash  lt  lt  16  - hash       return hash

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public String hashString String s  throws NoSuchAlgorithmException       byte   hash   null      try           MessageDigest md   MessageDigest getInstance  SHA-256            hash   md digest s getBytes            catch  NoSuchAlgorithmException e    e printStackTrace          StringBuilder sb   new StringBuilder        for  int i   0  i  lt  hash length    i            String hex   Integer toHexString hash i            if  hex length      1                sb append 0               sb append hex charAt hex length   - 1              else               sb append hex substring hex length   - 2                        return sb toString

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djb2 hash function unsigned long hash unsigned char  str        unsigned long hash   5381      int c       while  c    str            hash     hash  lt  lt  5    hash    c     hash   33   c         return hash      source Logic behind djb2 hash function - SO

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Guava s HashFunction  javadoc  provides decent non-crypto-strong hashing

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This function provided by Nick is good but if you use new String byte   bytes  to make the transformation to String  it failed  You can use this function to do that   private static final char   hex      0    1    2    3    4    5    6    7    8    9    a    b    c    d    e    f      public static String byteArray2Hex byte   bytes        StringBuffer sb   new StringBuffer bytes length   2       for final byte b   bytes            sb append hex  b  amp  0xF0   gt  gt  4            sb append hex b  amp  0x0F              return sb toString       public static String getStringFromSHA256 String stringToEncrypt  throws NoSuchAlgorithmException       MessageDigest messageDigest   MessageDigest getInstance  SHA-256        messageDigest update stringToEncrypt getBytes         return byteArray2Hex messageDigest digest         May be this can help somebody

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If you are doing this in Java then why are you doing it   Just call  hashCode   on the string

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Here s a simple hash function that I use for a hash table I built  Its basically for taking a text file and stores every word in an index which represents the alphabetical order    int generatehashkey const char  name            int x   tolower name 0  - 97          if  x  lt  0    x  gt  25             x   26          return x      What this basically does is words are hashed according to their first letter  So  word starting with  a  would get a hash key of 0   b  would get 1 and so on and  z  would be 25  Numbers and symbols would have a hash key of 26  THere is an advantage this provides  You can calculate easily and quickly where a given word would be indexed in the hash table since its all in an alphabetical order  something like this  Code can be found here  https   github com abhijitcpatil general     Giving the following text as input  Atticus said to Jem one day     I   d rather you shot at tin cans in the backyard  but I know you   ll go   after birds  Shoot all the blue jays you want  if you can hit    em  but   remember it   s a sin to kill a mockingbird     That was the only time I   ever heard Atticus say it was a sin to do something  and I asked Miss   Maudie about it     Your father   s right     she said     Mockingbirds don   t   do one thing except make music for us to enjoy  They don   t eat up   people   s gardens  don   t nest in corn cribs  they don   t do one thing   but sing their hearts out for us  That   s why it   s a sin to kill a   mockingbird    This would be the output   0 -- gt  a a about asked and a Atticus a a all after at Atticus 1 -- gt  but but blue birds  but backyard 2 -- gt  cribs corn can cans 3 -- gt  do don   t don   t don   t do don   t do day 4 -- gt  eat enjoy  except ever 5 -- gt  for for father   s 6 -- gt  gardens go 7 -- gt  hearts heard hit 8 -- gt  it   s in it  I it I it   s if I in 9 -- gt  jays Jem 10 -- gt  kill kill know 11 -- gt   12 -- gt  mockingbird  music make Maudie Miss mockingbird     13 -- gt  nest 14 -- gt  out one one only one 15 -- gt  people   s 16 -- gt  17 -- gt  right remember rather 18 -- gt  sin sing said  she something sin say sin Shoot shot said 19 -- gt  to That   s their thing they They to thing to time the That to the the tin to 20 -- gt  us  up us 21 -- gt   22 -- gt  why was was want 23 -- gt   24 -- gt  you you you   ll you 25 -- gt   26 -- gt     Mockingbirds        Your    em    I   d

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You should probably use String hashCode     If you really want to implement hashCode yourself      Do not be tempted to exclude   significant parts of an object from   the hash code computation to improve   performance -- Joshua Bloch  Effective Java   Using only the first five characters is a bad idea  Think about hierarchical names  such as URLs  they will all have the same hash code  because they all start with  http      which means that they are stored under the same bucket in a hash map  exhibiting terrible performance   Here s a war story paraphrased on the String hashCode from  Effective Java       The String hash function implemented   in all releases prior to 1 2 examined   at most sixteen characters  evenly   spaced throughout the string  starting   with the first character  For large   collections of hierarchical names    such as URLs  this hash function   displayed terrible behavior

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If you want to see the industry standard implementations  I d look at java security MessageDigest    Message digests are secure one-way hash functions that take arbitrary-sized data and output a fixed-length hash value

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Usually hashes wouldn t do sums  otherwise stop and pots will have the same hash   and you wouldn t limit it to the first n characters because otherwise house and houses would have the same hash   Generally hashs take values and multiply it by a prime number  makes it more likely to generate unique hashes  So you could do something like   int hash   7  for  int i   0  i  lt  strlen  i          hash   hash 31   charAt i

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Its a good idea to work with odd number when trying to develop a good hast function for string  this function takes a string and return a index value  so far its work pretty good  and has less collision  the index ranges from 0 - 300 maybe even more than that  but i haven t gotten any higher so far even with long words like  electromechanical engineering   int keyHash string key        unsigned int k    int key length        unsigned int u   0 n   0       for  Uint i 0  i lt k  i                  n    int key i           u    7 n 31            return u 139      another thing you can do is multiplying each character int parse by the index as it increase like the word  bear   0 b     1 e     2 a     3 r  which will give you an int value to play with  the first hash function above collide at  here  and  hear  but still great at give some good unique values  the one below doesn t collide with  here  and  hear  because i multiply each character with the index as it increases   int keyHash string key        unsigned int k    int key length        unsigned int u   0 n   0       for  Uint i 0  i lt k  i                  n    int key i           u    i n 31            return u 139

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