Momentum-Apps/memmem.c

/* Copyright (C) 1991-2025 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License along
   with this program; if not, write to the Free Software Foundation,
   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */

/*

@deftypefn Supplemental void* memmem (const void *@var{haystack}, @
  size_t @var{haystack_len} const void *@var{needle}, size_t @var{needle_len})

Returns a pointer to the first occurrence of @var{needle} (length
@var{needle_len}) in @var{haystack} (length @var{haystack_len}).
Returns @code{NULL} if not found.

@end deftypefn

*/

#include <stddef.h>
#include <string.h>

#ifndef _LIBC
#define __builtin_expect(expr, val) (expr)
#endif

#undef memmem

/* Return the first occurrence of NEEDLE in HAYSTACK.  */
void* memmem(const void* haystack, size_t haystack_len, const void* needle, size_t needle_len) {
    const char* begin;
    const char* const last_possible = (const char*)haystack + haystack_len - needle_len;

    if(needle_len == 0)
        /* The first occurrence of the empty string is deemed to occur at
       the beginning of the string.  */
        return (void*)haystack;

    /* Sanity check, otherwise the loop might search through the whole
     memory.  */
    if(__builtin_expect(haystack_len < needle_len, 0)) return NULL;

    for(begin = (const char*)haystack; begin <= last_possible; ++begin)
        if(begin[0] == ((const char*)needle)[0] &&
           !memcmp((const void*)&begin[1], (const void*)((const char*)needle + 1), needle_len - 1))
            return (void*)begin;

    return NULL;
}