Hashing Strings

• Consider the problem of implementing a spelling checker. It is fairly easy to break a text file up into words, but how do you check to see if all of the words are spelled correctly? Because comparing strings can take a long time, even a O(log n) binary search of a decent sized dictionary is prohibitively slow. On the other hand, there isn't nearly enough memory in the world to give each sequence of ten characters or less its own bit.

• The solution is to use a hash function, a function which takes a string and returns an integer between 0 and m, where m is larger than the number of strings in the dictionary but small enough to fit an array of size m into memory. A good hash function sends all of the strings into the dictionary to different integers. (Such a function is called a perfect hash function. If, in addition, m is equal to the size of the dictionary, it is called a minimal perfect hash function.)

• Here's an example hash function:

  unsigned int StrHash(char *lstr)
  {
      unsigned int hval = 0;
  
      while (*lstr != '\0')
          hval = ( hval* 48 + 45 + *lstr++ ) % StrTableSize;
  
      return(hval);
  }