PPCG partial solution to https://codegolf.stackexchange.com/q/120870/61563, C (gcc)

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/* THIS SYSTEM DOES NOT IMPLEMENT STRRSTR */
char *
strrstr(char *string, char *find)
{
	int stringlen, findlen;
	char *cp;

findlen = strlen(find);
	stringlen = strlen(string);
	if (findlen > stringlen)
		return (char*)0;

for (cp = string + stringlen - findlen; cp >= string; cp--)
		if (strncmp(cp, find, findlen) == 0)
			return cp;

return (char*)0;
}

char*s,*r,*p;
#define f(h,n) \
    s=strrstr(h,n);s&&((s-h)==0||(s-h)+strlen(n)==strlen(h))?printf("%.*s",(s-h),h),puts(s+strlen(n)):puts(h)

int main(void)
{
    /* VALID */
    f("abcde","ab");   // -> 'cde'
    f("abcde","cde");  // -> 'ab'
    f("abab","ab");    // -> 'ab'
    f("abcab","ab");   // -> 'abc' or 'cab'
    f("ababcde","ab"); // -> 'abcde'
    f("acdbcd","cd");  // -> 'acdb'
    f("abcde","abcde");// -> ''
    f("abcde","");     // -> 'abcde'
    f("","");          // -> ''

/* INVALID */
    f("abcde","ae");  // -> 'abcde'
    f("abcde","aa");  // -> 'abcde'
    f("abcde","bcd"); // -> 'abcde'
    f("abcde","xab"); // -> 'abcde'
    f("abcde","yde"); // -> 'abcde'
}

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Compilation time: 0.12 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0,35 sec

PPCG partial solution to https://codegolf.stackexchange.com/q/120870/6...