#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);
  int isUniqueID(struct products *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*);
  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)
	{
        printf("Memory allocation error");
		return 1;
    }
    products->next = NULL;
    if (load_products(products) == 1)
	{
	  printf("Memory allocation error");
      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;
    }
    return 0;
 }

   int isUniqueID(struct products *products, unsigned long long id)
  {
	while(products)
    {
		if(products->val.id == id)
		{
			printf("ID already exist!\n");
 			return 0;
		}
		products = products->next;
        }
  return 1;
 }

   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", "ab")) == NULL)
	  {
        printf("The file cannot be opened");
        exit(2);
      }
       while (1)
	  {
         products = products->next;
         if (products == NULL)
		 {
            break;
         }

        if (fwrite(&products->val, sizeof(struct product), 1, fp) != 1)
		{
            printf("writening error");
            exit(3);
        }
     }
	 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)
		{
		    printf("The warehouse is empty.");
            exit(212);
        }
        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;
   do{
	    printf("Insert id: ");
		fflush(stdin);
		scanf("%llu", &id);
	}while(!(isUniqueID(products, id)) || !id || id > 999999999999|| id<100000000000);

    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 == NULL)
	{
        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)
		{
           printf("The warehouse is empty or the date is wrong entered.");
			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);
    if (products_with_id == NULL)
    printf("Missing product!\n");
    else
	{
       int new_quantity = products_with_id->val.quantity + quantity;
       if (new_quantity < 0)
		     printf("Quantity to change is bigger than product quantity!\n");
       else if (new_quantity > 0)
		       products_with_id->val.quantity = new_quantity;
       else if (new_quantity == 0)
		       delete_by_id(products, id);
    }
 return 0;
}