#define _GNU_SOURCE
#include <string.h>

char *strcasestr(const char *h, const char *n)
{
	const size_t nl= strlen(n);

	h = strchrnul(h, *n);

	if (!*n) return (char *)h;

	for (; *h && strncasecmp(h, n, nl); h = strchrnul(h + 1, *n));

	return *h ? (char *)h : 0;
}