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

char *
strmerg(const char *str1, const char *str2) {
	size_t	str1_l = strlen(str1),	// length of string1
		str2_l = strlen(str2),	// length of string2
		// match_l is longest possible match (length of shortest input)
		match_l = str1_l < str2_l ? str1_l : str2_l;
		// base is ptr to start of longest possible match in string1
	char	*base = (char *)str1 + str1_l - match_l,
		c1,		// 1st char in string2 possible match
		c2 = *str2,	// 1st char in string2
		*out;
	// At this point, match_l is the longest possible match (based on the
	// lengths of string1 and string2) and base points to the spot in
	// string1 where the longest possible match could start.

	// While there is a mismatch...
	while((c1 = *base) && ((c1 != c2) || strncmp(base, str2, match_l))) {
		// Decrement the longest possible match, and increment the
		// spot in string1 where the longest possible match could
		// still start.  Note that even though we aren't checking
		// the value of match_l in this loop, we are guaranteed that
		// the loop will end with match_l greater than or equal to
		// zero.  (If it is zero, the only match is the terminating
		// null byte in string1.)
		match_l--;
		base++;
	}
	// When we get to this point, match_l is the length of the longest
	// substring of the tail of string1 that matches the head of string2.

	// Allocate space to hold the resulting string:	length of string1 +
	// length of string2 - length of matched substring + 1 for the
	// terminating null byte.  NOTE:  The caller is responsible for
	// freeing the allocated space when it is no longer needed.
	out = malloc(str1_l + str2_l - match_l + 1);
	if(out) {
		// Copy string1 and unmatched portion of string2 to out.
		strcpy(out, str1);
		strcpy(out + str1_l, str2 + match_l);
	}
	return(out);
}

int
main(int argc, char *argv[]) {
	char *result;	// Space for the pointer that strmerg() will return

	if (argc != 3) {
		fprintf(stderr, "Error! Bad arg count\nUsage: %s %s\n",
			argv[0], "string1 string2");
		exit(EXIT_FAILURE);
	}

	printf("string1: \"%s\" (%"PRIuMAX" bytes)\n",
		argv[1], (intmax_t)strlen(argv[1]));
	printf("string2: \"%s\" (%"PRIuMAX" bytes)\n",
		argv[2], (intmax_t)strlen(argv[2]));

	result = strmerg(argv[1], argv[2]);
	if(result == NULL) {
		perror("strmerg() failed:");
		exit(EXIT_FAILURE);
	}
	printf("strmerg(string1, string2) output: \"%s\" (%"PRIuMAX" bytes)\n",
		result, (intmax_t)strlen(result));
	free(result);
	result = strmerg(argv[2], argv[1]);
	if(result == NULL) {
		perror("strmerg() failed:");
		exit(EXIT_FAILURE);
	}
	printf("strmerg(string2, string1) output: \"%s\" (%"PRIuMAX" bytes)\n",
		result, (intmax_t)strlen(result));
	free(result);
	return(0);
}

./a.out ACGTGCCC CCCCCGTGTGTGT

string1: "ACGTGCCC" (8 bytes)
string2: "CCCCCGTGTGTGT" (13 bytes)
strmerg(string1, string2) output: "ACGTGCCCCCGTGTGTGT" (18 bytes)
strmerg(string2, string1) output: "CCCCCGTGTGTGTACGTGCCC" (21 bytes)

./a.out aaaaaaaaaa1 1aaaaaaaaaaaa

string1: "aaaaaaaaaa1" (11 bytes)
string2: "1aaaaaaaaaaaa" (13 bytes)
strmerg(string1, string2) output: "aaaaaaaaaa1aaaaaaaaaaaa" (23 bytes)
strmerg(string2, string1) output: "1aaaaaaaaaaaa1" (14 bytes)