strstr.c

char *strstr4(char *payload, char *token) {
  int found = 0;
  char *current = token;

  while (*payload != '\0') {
    if (*payload == *current) {
      current++;
      if (*current == '\0') {
        found = 1;
        break;
      }
    } else {
      current = token;
    }
    payload++;
  }

  if (!found)
    return NULL;
  else
    return payload;
}

int indexOf(char *str, char *token) {
  int i = 0;
  int start = 0;
  char *ot = token;
  while (str[i] != '\0') {
    if (str[i] == *token) {
      if (strcmp(token, ot) == 0)
      	start = i;
      token++;
      if (*token == '\0') {
        return start;
      }
    } else {
      token = ot;
    }
    i++;
  }
  return -1;
}

int main() {
  char *s = "Hello World!";
  char *token = "Wor";
  char *res;

  res = strstr4(s, token);

  printf("%s\n", res);

  res = strstr4(s, "za");

  if (res == NULL) {
    printf("Not found\n");
  } else {
    printf("%s\n", res);
  }

  res = strstr4(s, token);
  printf("s2: %s\n", res);
  res = strstr4(s, "H");
  printf("s2: %s\n", res);

  printf("%d\n", indexOf("Hi", "i"));
  printf("%d\n", indexOf("Hi", "z"));
  printf("%d\n", indexOf("big powers | yolo string.zaajsd", "g.z"));

  return 0;
}