Help to fix this program (find trigrams)

Code C - [expand]
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% FINDING TRIGRAMS AND THEIR OCCURENCES#START
#include <stdio.h>
#include <string.h>
void main(void)
{
 char mainstring[]= “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
    
char substring[20], *ret;
    int i=0;
    int x,y,z;
    x=y=z=0;
    clrscr();
    puts("enter the sub stirng to find");
    fgets(substring, sizeof(substring), stdin);
    substring[strlen(substring)-1]='\0';
    ret=strstr(mainstring,substring);
 
    if(strcmp((ret=strstr(mainstring,substring)),substring))
    {
    printf("Substring is present.\n");
 
    }
    while (mainstring[x]!='\0')
    {
    if(mainstring[x] == substring[y] && mainstring[x+1] == substring[y+1])
    {
           z=z++;
 
    }
    x++;
    }
    printf("Numer of occurance is: %d \n", z);
 
    getch();
}
 
% FINDING TRIGRAMS AND THEIR OCCURENCES#END
 
%DECRYPTING THE CIPHERTEXT#START
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
 
void upper_case(char *src)
{
    while (*src != '\0')
    {      if(islower(*src)) *src &= ~0x20;
        src++;
    }
}
 
char* decipher(const char *src, char *key, int is_encode)
{
    int i, klen, slen;
    char *dest;
 
    dest = strdup(src);
    upper_case(dest);
    upper_case(key);
 
    /* strip out non-letters */
    for (i = 0, slen = 0; dest[slen] != '\0'; slen++)
        if (isupper(dest[slen]))
            dest[i++] = dest[slen];
 
    dest[slen = i] = '\0'; /* null pad it, make it safe to use */
 
    klen = strlen(key);
    for (i = 0; i < slen; i++) {
        if (!isupper(dest[i])) continue;
        dest[i] = 'A' + (is_encode
                ? dest[i] - 'A' + key[i % klen] - 'A'
                : dest[i] - key[i % klen] + 26) % 26;
    }
 
    return dest;
}
 
int main()
{ 
const char *str = 
"XPOZTRACLKWWIVGJCYLWTWUYTPQILMLIZZAITKAMLMJVKQWAZXKWUUYSZYVFIBOTXAQZOHYBOUMIZXBIXQTNBXBNLRAMXLEIIXUBROZVWVICAAILUYFSCYLPIVZVGCICUBROGUWVCHIBROOALIGKZYSZOAPEAMLMXQTNOIZEZEIMVFTRLZOXCLXLPETRTTRVKYHJBNPGKAKCXVGZOBROZOTXJKNBRACPMLITTPLGCPMLITTLEQJHEMKKDACVUALEQJAAIUGYVLPGYXETOJXAIYZBPMTAMLMJVKQWAZXLIJJESAKKBXAKFXWJEAAMAZPFIITKPEAMVBROUMYMVZVTHWFLUYBUUUIQTNIMVIOXHJEAAIPGAMIZOAPSSKBIEOGPGAQZOTPQZAEIANYBISGUWAMTAHRBNHMFMMPGWEOAAEVSZNGPGZFSCYLMVIVZTRLZOXQWUUTRLSLFSZEHGHUAJARMYZRSCQUHAGUBLEGZOBROYHKIUAJASNGTNGPTLLWLOKRSCKCXVAKLLYKNHMLQTNTWIJYTAQTNHJISBVLVKZLVMGSECVUDRSCGZDQMYHBHIRPVIDKYRQCIOVSVLBLILOKHRBZOBRSZOXRGUBLLWASWRBZHEOAGPWXPKOTXBKY";
 
    const char *dec;
    char key[] = "EIGHT";
    clrscr();
    printf("ciphertext: %s\n", str);
    printf("keyword: %s\n", key);
    dec =decipher(str,key,0);
    printf("plaintext: %s\n", dec);
    getch();
}
 
%DECRYPTING THE CIPHERTEXT #END
%PROGRAM ENDS