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Time point c

Работа с датой и временем на С++

Моя программа должна обрабатывать дату и время, мне нужно:

  1. получать текущую дату/время от операционной системы;
  2. вычислять разницу в днях/часах/секундах между двумя моментами времени;
  3. создавать объект для произвольного момента времени (например в файле или базе данных находится дата регистрации пользователя, которую должна обработать моя программа).

Как решить эти задачи на С++?

В стандарте С++11 появился набор классов для работы со временем, он позволяет больше чем старый модуль time.h и расположен в пространстве имен std::chrono . В нем определены классы для задания моментов времени ( std::chrono::timepoint ), интервалов времени ( std::chrono::duration ) и несколько видов часов:

  1. std::chrono::system_clock предназначен для получения данных от системных часов реального времени, моменты времени system_clock::time_point могут быть преобразованы в тип time_t с округлением до секунд при помощи метода system_clock::to_time_t() . Получить текущее время можно с помощью метода system_clock::now() . Недостатком системных часов является их нестабильность — системные часы могут быть подведены пользователем или процессом автоматической синхронизации с внешним сервером;
  2. std::chrono::steady_clock предоставляет возможность работы со стабильными системными часами. Эти часы возвращают не системное время (на них не влияет напрямую изменение системного времени). Эти часы оптимально подходят для замера времени выполнения фрагмента кода. Текущее время можно получить при помощи steady_clock::now() ;
  3. Класс std::chrono::high_resolution_clock позволяет работать с системными часами максимально высокого разрешения (как и для обычные системные часы, он не является стабильным, т.е. допускает подведение времени). При помощи метода high_resolution_clock::now() можно получить самое высокое разрешение из всех видов часов, входящих в библиотеку.

Чтобы получить текущее время от операционной системы нужно использовать один из трех видов часов — вызвать у них метод now() , в результате будет получен объект типа timepoint . Чтобы вывести время на экран вы можете преобразовать его функцией to_time_t в тип time_t , а затем в строку при помощи функции ctime :

Обычно этого достаточно, при преобразовании в тип time_t момент времени округляется до секунд. Если же такое преобразование не выполняется, а лишь вычисляется разница двух моментов времени — можно работать даже с наносекундами — см. Пример с замером времени работы функции.

Вы можете создавать моменты времени ( chrono::timepoint ), преобразованием из time_t , который в свою очередь проще создавать преобразованием из типа std::tm :

Обратите внимание, что несмотря на то, что в структуре tm :

  • нумерация месяцев начинается с нуля (мы пишем седьмой месяц, хотя создаем август);
  • год задается как целое число лет, прошедших с 1900 года (пишем 116 вместо 2016 );
  • поле tm_isdst отвечает за перевод времени на летнее и зимнее (пишем -1 если ничего про это не известно).

Таким образом, вы можете использовать std::chrono для хранения интервалов времени в своей программе, определения наиболее точного времени и замера времени со стабильными часами (без возможности их подведения пользователем), однако при вводе произвольного момента времени в программу или выводе времени на экран/файл наиболее удобным способом является использование структур time_t и std::tm .

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Reorder Point Formula — Know When to Reorder

You have a great new product on the shelves, and it’s selling fast. Every customer purchase means more revenue, but also brings your inventory levels lower. A reorder point formula enables you to reorder your stock at the right time.

What is reorder point?

A reorder point is the unit quantity that triggers the purchase of a particular stock item.

How to calculate reorder point?

  • Calculate your lead time demand in days.
  • Calculate your safety stock in days.
  • Sum your lead time demand and your safety stock to determine your Reorder Point.

Of course you’ll reorder before it goes out of stock, but if you order too early, you’ll need to spend more on storing these excess items. If you order too late, you’ll be facing disappointed customers who’ll look to your competitors.

The question then is: When is the right time to order more stock?

To help you out, we’ve designed a reorder point calculator. It’ll let you know exactly when it’s time to place an order for a new shipment of products.

To understand the maths behind our reorder point calculator, let’s break the formula down.

You’ll need to know the lead time demand, because that’s how long you’ll have to wait before new stock arrives — you’ll want to have enough to satisfy your customers while you wait!

And you’ll need to know your safety stock, because that’ll protect you against any unexpected occurrences. Add your lead time demand to your safety stock… and voila! Once your stock levels hit the total, it’s time to place a new order to replenish your supply.

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Lead time demand — Shipping’s not instant!

New stock doesn’t arrive immediately. Even if your products are in stock, it’ll take your supplier time to pack your order and even more time to ship it over to you. This waiting time is what’s known as “lead time”.

So let’s put things into perspective. Imagine a business (let’s call it J Timewear) in the United States sells watches manufactured in China. Assuming the supplier is always in stock and has a warehouse full of watches ready to ship at a moment’s notice, it’ll probably take the supplier a couple of days to pick and pack the watches. After that, the watches spend another five days in a truck to the port, and from there, it takes about 30 days for a ship to travel from China to the U.S.. Once the watches arrive, they spend another week in customs, and then another three days traveling to J Timewear’s warehouse.

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Calculating the lead time is easy: Just add everything up!

2 + 5 + 30 + 7 + 3 = 47 days of sales

Since it takes J Timewear 47 days to get a new shipment of watches, they’ll need to have enough stock on hand to cover these 47 days of sales.

But knowing the lead time alone isn’t enough. You’ll also need to figure out the demand during this period. Assuming J Timewear sells an average of 310 watches a month (310/31 = 10), they’d be selling about 10 watches a day.

So the lead time demand for J Timewear is (47 x 10 = 470). meaning J Timewear will need 470 watches to tide them over until their next shipment arrives, if nothing unexpected happens.

Safety Stock — When things go wrong

But sometimes, unexpected things happen. This can take the form of a sudden surge in demand after some unexpected celebrity endorsement, and now your product is selling fast. Or perhaps your supplier’s factory has experienced a breakdown and it’ll take a week for them to replace the damaged component and get their machine up and running again.

And here’s where safety stock comes in. Safety stock is buffer stock you carry as a last defense against unpredictable events that either deplete your stock (surge in demand), or unexpected manufacturing time (your lead time skyrockets because the supply chain breaks down). Of course you’d like to have enough safety stock to bring the likelihood of going out of stock down to zero, but most of the time that’s not financially viable. After all, safety stock IS for a rainy day that may never come! So how do we decide then on how much stock to keep on standby?

Here’s a simple formula that you can calculate based off your purchase and sales orders history:

Let’s continue the story of J Timewear. On an average day, they sell 10 watches. But during weekends, they can sell as many as 15. As for lead times, their usual lead time is 47 days, but during typhoon season (yes, in China, they have typhoons) , it can go up to as high as 54 days.

(15 x 54) — (10 x 47) = 340

This means J Timewear needs to have about 340 units of safety stock on hand to guard against the unexpected (especially during typhoon season). Therefore, with 340 units in safety stock, selling about 80 watches on a good week (10 per day on weekdays and 15 on weekends), J Timewear will have enough stock to last a little over 4 weeks.

Your safety stock can cater for all the variations in demand and lead time, providing you with enough stock on hand to weather unexpected occurrences. Everyone and their entire family wants your products? It’s time to sell your safety stock. Supplier needs an extra week because he’s caught in the middle of a typhoon? It’s time to sell your safety stock.

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If your product is seasonal, like school supplies, you’ll have to adjust your safety stock level to cater to peak season demand. Once the peak season’s passed, it’s time to reduce your safety stock levels again, as more safety stock = higher carrying costs. After all, people are a lot less likely to be buying a new set of school stationery in the middle of summer holidays as opposed to right before a new semester begins.

To complete the story of J Timewear, their reorder point formula calculation would be:

470 (Lead time demand) + 340 (safety stock) = 810

So once their stock hits 810 watches, J Timewear will need to place a new order with their supplier. At 810 watches, they’ll have enough to last them as they wait for new stock to arrive (470), while holding enough stock (340) as a buffer against an unexpected surge in demand or supply chain problems.

Planning reorder points are a crucial part of inventory management. Setting your reorder point to the optimum amount lets you cut down on excess spending, while ensuring you’ll have enough stock for your customers even when things take an unexpected turn.

But how can you always ensure you’ll be able to place a fresh order whenever inventory levels hit the reorder point? Keeping tabs on how much you’ve sold every day is easy when you’re starting out with a single store. But as you start selling more and more, across different channels, manually recording every sale becomes a pretty exhausting chore. And if you only tally up your numbers on a weekly basis, missing the reordering point becomes a likely possibility.

Download your Free Reorder Point Calculator!

Work out when to order more stock with this handy tool!
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If missing the reordering point sounds like it’ll be a concern, you may want to look into getting an inventory management software system for your business. It’ll track your inventory movement across all channels, and once your stocks hit the reorder point, you’ll be prompted to place a new purchase order. That way, you won’t have to keep worrying about keeping an eye on your inventory with the stock control software doing your work for you.

Automating your inventory processes means everyone wins. You won’t need to put items on backorder and tell your customers “Oh, I’m sorry, we’re out of stock and we can’t get new stock for another week” and disappointing them. As for your customers, they’ll soon know that you’re a seller that’s always able to deliver, and now that you’ve won their trust, they’ll always be coming back for more.

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