#include <stdio.h>

int find_word(char* p1, int len, char* s)
{
    while(*s != '\0')
    {
        int check = 1;
        for(int i = 0; i < len; i++)
        {
            if(*(s+i) == '\0') return 0;
            if(*(s+i) != *(p1+i))
            {
                check = 0;
                break;
            }
        }

        if(check == 1) return 1;
        s++;
    }

    return 0;
}

void longest_common_substring(char* s1, char* s2)
{
    char* p1 = s1;
    char* p2 = s2;
    char* p3;
    char* start;
    char* end;
    int len1 = 0, len2 = 0;

    while(*p1 != '\0')
    {
        if(*p1 >= 'A' && *p1 <= 'Z') *p1 -= 'a'-'A';
        p1++;
        len1++;
    }
    while(*p2 != '\0')
    {
        if(*p2 >= 'A' && *p2 <= 'Z') *p2 -= 'a'-'A';
        p2++;
        len2++;
    }

    if(len1 > len2)
    {
        p1 = start = end = s2;
        p3 = s1;
    }
    else
    {
        p1 = start = end = s1;
        p3 = s2;
    }

    while(*p1 != '\0')
    {
        p2 = p1;
        int counter = 1;
        while(*p2 != '\0')
        {
            if(find_word(p1, counter, p3) && p2-p1 > end-start)
            {
                start = p1;
                end = p2;
            }
            p2++;
            counter++;
        }
        p1++;
    }

    while(start <= end)
    {
        printf("%c", *start);
        start++;
    }
    printf("\n");
}

int main()
{
    char* s1 = "abababcde";
    char* s2 = "cccdeccccccccc";

    longest_common_substring(s1,s2);

    return 0;
}