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

// Returns newly-allocated memory which you must 'free' later.
char *strmerg(const char *str1, const char *str2) {
        // Note:  size_t and ssize_t are technically more correct for numbers holding string length,
        // particularly for LARGE strings.  I had to use ssize_t for 'a' since I need a negative number
        // for my while loop.

        // Position in 'str1'.  Counts backwards from end.
        // Each loop, str1+a will be 1 longer, i.e.  "t", "at", "tat", "atat", "Gatat"...
        ssize_t a=strlen(str1)-1;
        // How many bytes of overlap has been found.  0 if none.
        size_t found=0;
        // How many characters of overlap to check for.  Counts up from 1.
        size_t b=1;

        while(a >= 0) // Loop until a is negative
        {
                // Compare the last 'b' bytes of str1, to the first 'b' bytes of str2.
                // Using strncmp instead of strcmp prevents it from checking ALL of str2,
                // because strncmp takes maximum length as an argument.
                // It will return 0 if they are equal.
                if(str1[a] == str2[0]) // Optimization
                if(strncmp(str1+a, str2, b) == 0) found=b;
                a--;
                b++;
        }

        b=strlen(str1); // don't count 5 megabases more times than necessary

        // Code block only because some compilers require you to declare
        // all variables at the top of a code block
        {
                char * const out=malloc(sizeof(char)*(b+strlen(str2+found)+1));
                strcpy(out, str1);
                strcpy(out+b, str2+found); // Faster than strcat
                return(out);
        }
}

int main(int argc, char *argv[])
{
        if (argc != 3) {
                printf("Error! \nUsage: ./%s string1 string2\n", argv[0]);
                exit(EXIT_FAILURE);
        }
        else
        {
                char * const out=strmerg(argv[1], argv[2]);
                printf("string 1 is %s\n", argv[1]);
                printf("string 2 is %s\n", argv[2]);
                printf("Output is %s\n", out);
                free(out);
        }
        return(0);
}