#include <stdio.h>

#define MAXLEN 80
#define NALPHA 26

char *soundexGroups[] = {
                "aeiouhyw",
                             "kcgjqsxz",
                             "td",
                             "bpfv",
                             "l",
                             "mn",
                             "r"
                        };
int soundexCodes[NALPHA];

void soundexInit() {
    /*
     * build an inverted index from the global table soundexGroups[].
     * the inverted index is stored in global soundexCodes[].
     */
    int i;
    char *sptr;

    for(i=sizeof(soundexGroups)/sizeof(char *)−1; i>=0; -i)
        for(sptr=soundexGroups[i]; *sptr; ++sptr)
            soundexCodes[*sptr-'a'] = i;
}

int compareCodes(int *soundex1, int *soundex2) {
    int *ptr1, *ptr2;

    for(ptr1=soundex1, ptr2=soundex2; *ptr1!=−1 && *ptr2!=−1 && *ptr1==*ptr2; ++
ptr1, ++ptr2)
           ;
    return *ptr1 == *ptr2;
}

void findSoundex(char *s, int *soundex, char lastchar) {
    /*
     * find the soundex code for s and save in soundex.
     * the stored value is the index in the array of soundex codes.
     * function is recursive.
     * start by changing multiple occurrences of chars in consecutive positions
     * by single occurrences.
     * end soundex by −1.
     * lastchar == −1 implies this is the first call to this function.
     */
    if(!*s)
        *soundex = −1, lastchar = 0;
    else if(*s == lastchar)
        findSoundex(s+1, soundex, lastchar);
    else if(lastchar == −1) // *s is the first char.
        *soundex=soundexCodes[*s-'a'], findSoundex(s+1, soundex+1, *s);
    else if(soundexCodes[*s-'a'] == 0) // vowel group.
        findSoundex(s+1, soundex, *s);
    else
        *soundex=soundexCodes[*s-'a'], findSoundex(s+1, soundex+1, *s);
}

int compareSoundex(char *s1, char *s2) {
    /*
     * find soundex codes for s1 and s2.
     * return 1 if codes are equal else 0.
     */
   int soundex1[MAXLEN], soundex2[MAXLEN];

   findSoundex(s1, soundex1, −1);
   findSoundex(s2, soundex2, −1);

   return compareCodes(soundex1, soundex2);
}

int main() {
    char s1[MAXLEN];
    char s2[MAXLEN];
    soundexInit();

    printf("Enter string 1: ");
    gets(s1);

    while(*s1) {
        printf("Enter string 2: ");
        gets(s2);
        printf("(%s == %s) = %d.\n", s1, s2, compareSoundex(s1, s2));
        printf("Enter string 1(enter to end): ");
        gets(s1);
    }
    return 0;
}

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