define _CRT_SECURE_NO_WARNINGS
  #include <stdio.h>
  #include <string.h>
  #include <stdlib.h>
  #include <time.h>

  struct product {
     unsigned long long id;
     char name[51];
     double price;
     int quantity;
     char date[11];
  };

  struct products {
      struct product val;
      struct products *next;
  };

  struct products* find_by_id(struct products*, unsigned long long id);
  void delete_by_id(struct products*, unsigned long long id);
  int change_quantity_of_product(struct products*);
  int print_all_expired_products(struct products*);
  int print_product_with_id(struct products*);
  int add_new_product(struct products*);
  int load_products(struct products*);
  int save_products(struct products*);
  int showproducts(struct products*);
  time_t string_to_time(char*);

  int main()
   {
    int option = 0,exit_flag = 1;
    struct products *products = NULL;
    products = malloc(sizeof(struct products));
    if (products == NULL)
    {
        return 1;
    }
    products->next = NULL;
    if (load_products(products) == 1)
    {
      return 1;
    }

       while(exit_flag)
    {
        system("@cls||clear");
    printf(" *****************/MENU\\*****************\n");
    printf("*                                        *\n");
    printf("*   1.Add new item in the warehouse      *\n");
    printf("*   2.Change the quantity                *\n");
    printf("*   3.Display the expired items          *\n");
    printf("*   4.Display information for item       *\n");
    printf("*   5.Exit                               *\n");
    printf("*                                        *\n");
    printf("******************************************\n");
        printf("Insert option: ");
        if (scanf("%d", &option) == EOF)
        {
            break;
        }

        switch (option) {
            case 1:
                if (add_new_product(products)) {
                    return -1;
                }
                break;
            case 2:
                if (change_quantity_of_product(products)) {
                    return -1;
                }
                break;
            case 3:
                if (print_all_expired_products(products)) {
                    return -1;
                }
                break;
            case 4:
                if (print_product_with_id(products))
              {
                    return -1;
                }
                break;
            case 5:
                exit_flag = 0;
                break;
            default:
                break;
        }

      system("pause");
    }

   if (save_products(products))
    {
        return -1;
    }
    free_list(products);
    return 0;
 }


   int load_products(struct products *products)
   {
      FILE *fp;
      if ((fp = fopen("data", "rb")) == NULL)
        {
          printf("The file cannot be opened");
          exit(1);
        }
      struct products *prev_products = products;
      struct products *curr_products = NULL;
      while (1)
      {
        struct product product;
          if (fread(&product, sizeof(struct product), 1, fp) != 1)
           {
              if (feof(fp))
              {
                  break;
              }
             printf("reading error");
             break;
           }
        curr_products = malloc(sizeof(struct products));
        if (curr_products == NULL)
        {
            printf("Memory allocation error");
            return 1;
        }
        curr_products->val = product;
        curr_products->next = NULL;
        prev_products->next = curr_products;
        prev_products = curr_products;
      }
      fclose(fp);
      return 0;
    }

    int save_products(struct products *products)
    {
      FILE *fp  = NULL;
      if ((fp = fopen("data", "wb")) == NULL)
      {
       printf("The file cannot be opened");
       return 2;
      }
        while (1)
        {
         products = products->next;
         if (products == NULL)
         {
            break;
         }
         if (fwrite(&products->val, sizeof(struct product), 1, fp) != 1)
        {
            printf("writening error");
            break;
        }
      }
     fclose(fp);
     return 0;
    }

    struct products* find_by_id(struct products *products,unsigned long long id)
   {
    struct products *iterate_products = products->next;
    while(1)
    {
        if (iterate_products == NULL)
        {
            break;
        }
        if (iterate_products->val.id == id)
        {
            return iterate_products;
        }
       iterate_products = iterate_products->next;
    }
    return NULL;
   }

   void delete_by_id(struct products *products, unsigned long long id)
  {
      struct products *prev_products = products;
      struct products *iterate_products = products->next;
      while(1)
      {
        if (iterate_products == NULL)
        {
            printf("The warehouse is empty.");
            break;
        }
        if (iterate_products->val.id == id)
        {
            prev_products->next = iterate_products->next;
            break;
        }
        iterate_products = iterate_products->next;
        prev_products = prev_products->next;
      }
  }

  time_t string_to_time(char *string_time)
  {
      time_t result = 0;
      int day = 0, month = 0, year = 0;

      if (sscanf(string_time, "%2d.%2d.%4d", &day, &month, &year) == 3)
    {
       struct tm time = {0};
       time.tm_year = year - 1900;
       time.tm_mon = month - 1;
       time.tm_mday = day;
    if ((result = mktime(&time)) == (time_t) - 1)
        {
            perror("mktime");
            return 1;
        }
    }
    return result;
  }

   int add_new_product(struct products *products)
    {
      struct product product;
      unsigned long long id=0;
      char name[51] = "",date[11] = "";
      double price=0;
      int quantity=0;

      printf("Insert id: ");

      scanf("%llu", &id);

       printf("Insert name: ");
       scanf("%50s",name);
       printf("Insert price: ");
       scanf("%lf", &price);
       printf("Insert quantity: ");
       scanf("%d", &quantity);
       printf("Insert date /in dd.mm.yyyy format/: ");
       scanf("%s", date);
       product.id = id;
       strcpy(product.name, name);
       product.price = price;
       product.quantity = quantity;
       strcpy(product.date, date);
       struct products *last_products = products;
       while(last_products->next)
       {
        last_products = last_products->next;
       }
       struct products *next_products = NULL;
       next_products = malloc(sizeof(struct products));
       if (!next_products)
       {
        printf("Memory allocation error.");
        return -1;
       }
        next_products->val = product;
        next_products->next = NULL;
        last_products->next = next_products;
        return 0;
    }

    int print_product_with_id(struct products *products)
    {
        unsigned long long id=0;
        printf("Insert id: ");
        scanf("%llu", &id);
        struct products *products_with_id = find_by_id(products, id);
        if (products_with_id == NULL)
        printf("Missing product!\n");
        else
         {
        printf("id: %llu\n", products_with_id->val.id);
        printf("name: %s\n", products_with_id->val.name);
        printf("price: %.2f\n", products_with_id->val.price);
        printf("quantity: %d\n", products_with_id->val.quantity);
        printf("date: %s\n", products_with_id->val.date);
         }
    return 0;
    }

  int print_all_expired_products(struct products *products)
  {
        char date[11];
        printf("Insert date to compare /in dd.mm.yyyy format/: ");
        scanf("%s",date);
        time_t date_t = string_to_time(date);
        if (date_t == -1)
       {
        return -1;
       }
        struct products *iterate_products = products->next;
        while(1)
        {
          if(iterate_products == NULL)
          {
            break;
          }
          time_t product_date_t = string_to_time(iterate_products->val.date);
          if (product_date_t == -1)
          {
            return -1;
          }
         if (difftime(product_date_t, date_t) < 0)
         {
            printf("id: %llu\n", iterate_products->val.id);
            printf("name: %s\n", iterate_products->val.name);
            printf("price: %.2f\n", iterate_products->val.price);
            printf("quantity: %d\n", iterate_products->val.quantity);
            printf("date: %s\n", iterate_products->val.date);
         }
         iterate_products = iterate_products->next;
        }
     return 0;
   }
   int change_quantity_of_product(struct products *products)
   {
        unsigned long long id=0;int quantity=0;
        printf("Insert id: ");
        scanf("%llu", &id);
        printf("Insert quantity to change[(+)for adding and (-)for subtraction]:");
        scanf("%d", &quantity);
        struct products *products_with_id = find_by_id(products, id);


        int new_quantity = products_with_id->val.quantity + quantity;
if (new_quantity > 0)
products_with_id->val.quantity = new_quantity;
    return 0;
   }