Метод сиськи

#include <stdio.h>
#include <string.h>
#include <malloc.h>

int l[500];
int nods[500];
int nl=0;


int nod(int a,int b)
{
    int i;
    for (i=a;i>1;--i)
        if ((a%i==0)&&(b%i==0)) return i;
    return 1;
}



int main()
{
    char *s=(char *)calloc(1,sizeof(char));
    char fname[100];
    char c;
    unsigned int n=0,i,j,keylen;
    FILE *f;
    printf("file name: ");
    scanf("%s",fname);
    f=fopen(fname,"r");
    if (f==NULL){
            printf("file not found\n");
            return 1;
    }

    for (i=0;i<500;++i)
        nods[i]=0;


    while (!feof(f))
        if (fread(&c,1,1,f)){
            s[n++]=c;
            s=(char *)realloc(s,n+1);
        }

    s[n]='\0';

    char str1[4],str2[4];

    for (i=0;i<strlen(s);++i)
    {
        str1[0]=s[i];
        str1[1]=s[i+1];
        str1[2]=s[i+2];
        str1[3]='\0';

        for (j=i+1;j<strlen(s);++j)
        {
            str2[0]=s[j];
            str2[1]=s[j+1];
            str2[2]=s[j+2];
            str2[3]='\0';

            if (!strcmp(str1,str2)) l[nl++]=j-i;

        }
    }

    for (i=0;i<nl;++i)
        for (j=i+1;j<nl;++j)
            nods[nod(l[i],l[j])]++;


    keylen=0;
    for (i=2;i<500;++i)
        if (nods[keylen]<nods[i]) keylen=i;

    printf("%d\n",keylen);

    free(s);
    return 0;
}