#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <err.h>

static void
usage(const char *argv0)
{
   fprintf(stderr, "usage: %s needle [window-size] < haystack\n", argv0);
   exit(EXIT_FAILURE);
}

static const char*
search(const char *haystack, const char *needle, const size_t window_size)
{
   for (const char *s = haystack; s < haystack + window_size; ++s) {
      if (memcmp(s, needle, window_size))
         continue;
      return s;
   }
   return NULL;
}

static void
search_and_exit_if_match(const char *haystack, const char *needle, const size_t window_size, const size_t offset)
{
   const char *match;
   if ((match = search(haystack, needle, window_size))) {
      printf("%zu\n", offset + match - haystack);
      free((void*)needle);
      free((void*)haystack);
      exit(EXIT_SUCCESS);
   }
}

int
main(int argc, const char *argv[])
{
   // default incase failure, or cant get size of file
   size_t window_size = 4096 * 1024;
   bool has_window_size = false;

   if (argc < 2)
      usage(argv[0]);
   else if (argc > 2) {
      window_size = strtoull(argv[2], NULL, 10);
      has_window_size = true;
   }

   FILE *f;
   if (!(f = fopen(argv[1], "rb")))
      err(EXIT_FAILURE, "fopen(%s)", argv[1]);

   if (!has_window_size) {
      fseek(f, 0, SEEK_END);
      const long tell = ftell(f);
      window_size = (tell > 0 ? (size_t)tell : window_size);
      fseek(f, 0, SEEK_SET);
   }

   char *needle;
   if (!(needle = malloc(window_size)))
      err(EXIT_FAILURE, "malloc");

   window_size = fread(needle, 1, window_size, f);
   fclose(f);

   char *haystack;
   if (!(haystack = calloc(2, window_size)))
         err(EXIT_FAILURE, "calloc");

   size_t rd = 0, offset = 0;
   while ((rd = fread(haystack + window_size * !!offset, 1 + !offset, window_size, stdin))) {
      search_and_exit_if_match(haystack, needle, (rd >= window_size ? window_size : 0), offset);
      offset += window_size;
      memmove(haystack, haystack + window_size, window_size);
   }

   search_and_exit_if_match(haystack, needle, (rd >= window_size ? window_size : 0), offset);
   free(needle);
   free(haystack);
   return EXIT_FAILURE;
}