Soundex (C)

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/*
 * This implementation of the Soundex algorithm is released 
 * to the public domain: anyone may use it for any purpose. 
 *
 * N. Dean Pentcheff
 * 1/13/89
 * Dept. of Zoology
 * University of California
 * Berkeley, CA  94720
 * E-mail: dean@violet.berkeley.edu
 *
 * char * soundex( char * )
 *
 * Given as argument: Pointer to a character string.
 * Returns: Pointer to a static string, 4 characters long, plus a terminal
 *    '\0'.  This string is the Soundex key for the argument string.
 * Side effects and limitations:
 *    Does not clobber the string passed in as the argument.
 *    No limit on argument string length.
 *    Assumes a character set with continuously ascending and contiguous
 *       letters within each case and within the digits (e.g. this works 
 *       for ASCII and bombs in EBCDIC.  But then, most things do.).
 * Reference: Adapted from Knuth, D.E. (1973) The art of computer 
 *    programming; Volume 3: Sorting and searching.  
 *    Addison-Wesley Publishing Company:
 *    Reading, Mass. Page 392.
 * Special cases:
 *    Leading or embedded spaces, numerals, or punctuation are squeezed 
 *    out before encoding begins.
 *    Null strings or those with no encodable letters return the 
 *    code 'Z000'.
 * Test data from Knuth (1973):
 *    Euler   Gauss   Hilbert Knuth   Lloyd   Lukasiewicz
 *    E460    G200    H416    K530    L300    L222
 */
#include        <string.h> 
#include        <ctype.h> 
char *soundex(const char *in)
{
   static  int code[] =
      {  0,1,2,3,0,1,2,0,0,2,2,4,5,5,0,1,2,6,2,3,0,1,0,2,0,2 };
      /* a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z */
   static  char key[5];
   register        char ch;
   register        int last;
   register        int count;
   /* Set up default key, complete with trailing '0's */
   strcpy(key, "Z000");
   /* Advance to the first letter.  If none present, 
      return default key */
   while (*in != '\0'  &&  !isalpha(*in))
      ++in;
   if (*in == '\0')
      return key;
   /* Pull out the first letter, uppercase it, and 
      set up for main loop */
   key[0] = toupper(*in);
   last = code[key[0] - 'A'];
   ++in;
   /* Scan rest of string, stop at end of string or 
      when the key is full */
   for (count = 1;  count < 4  &&  *in != '\0';  ++in) {
      /* If non-alpha, ignore the character altogether */
      if (isalpha(*in)) {
         ch = tolower(*in);
         /* Fold together adjacent letters sharing the same code */
         if (last != code[ch - 'a']) {
            last = code[ch - 'a'];
            /* Ignore code==0 letters except as separators */
            if (last != 0)
               key[count++] = '0' + last;
         }
      }
   }
   return key;
}
#ifdef  TESTPROG
/*
 * If compiled with -DTESTPROG, main() will print back the key for each
 * line from stdin.
 */
#include    <stdio.h>   
int main()
{
   char    instring[80];
   while (fgets(instring, sizeof instring, stdin) != NULL)
      printf("%s\n", soundex(instring));
   return 0;
}
#endif  TESTPROG

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