Merge two strings by overlapped region

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# 29  
Don's is better. Wish I'd thought of it Smilie

You're not supposed to modify string.h

You can put it in your own .h file easily enough.

#ifndef __MYSTR_H__
#define __MYSTR_H__

#ifdef __cplusplus
extern "C" {

char *strmerg(const char *str1, const char *str2);

#ifdef __cplusplus


/* mystr.c */
#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
	// 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.)
	// 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);

Last edited by Corona688; 04-07-2014 at 06:51 PM..
This User Gave Thanks to Corona688 For This Post:
# 30  
Thanks Corona!

Before I start my code, I searched for a while trying to find this string merge function like strcat(), strcpy() etc in the <stdlib.h>, and turned out there is no function for this purpose, surprisingly. I feel this strmerg() function is quite daily needed for bioinformatics, although this is not exactly the longest common string problem (LCS) in algorithm, that's another story. Very good answers, thank you again!
# 31  
C's library of string functions isn't especially powerful. Regular expressions aren't included either (though there is an extension, -lregex). There's better languages for that.
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