Today we continue the system design discussion on the online retail store  and  look at some of the services in detail. For example, the card payment services is a popular service for all the transactions since they are billed to the card. card numbers are not stored as is because it is a sensitive information. Instead, it is encrypted and decrypted. The function to Encrypt str using pass_str as the password gives a result that is a binary string of the same length as str.  The Decrypt function the encrypted string crypt_str using pass_str as the password.  The card information is associated with the customer which is usually stored in the accounts table. Charges to the card are facilitated through the programmatic access to a third party bank services that enables posting a charge to the card.  Cards can also be specific to the store and these may also have monetary values in which case the numbers for the card are generated randomly. These special cards are stored with their balance Order Processors are usually implemented as message queues processors which are written specific to handle different kinds of transactions based on the incoming messages. The message queue may be its own cluster with journaling and retries specified.  The performance of the message processor must be near realtime and support millions of transactions a day.  It provides an async mechanism to deduct from the card or to reload the card. Point of sale registers send transaction requests to the order processors through API services. The orders flowing through the processors on fulfilment automatically update the  account and the card associated which is ready by the application and the point of sale register. There are some interesting edge cases to be considered for the spend and reload activities on the card such that the balance updated remains accurate. Usually the payment activity on the card is the slowest transaction as it is routed through the third party vendor and with errors handled with user interaction. But this is generally overcome by keeping it as the last step of the user interaction. Many orders are executed within a transaction scope but with states being maintained that move progressively forward as intiated, processed and completed or failed, the operations can be structured for retries and re-entrancy. The stores or the purchases from the store are added to the account holder's history with the help of transactions table. Much of the online shopping experience continues to be driven by simple relational database as the choice data platform. The adoption of cloud services has increased the use of such data storage options in the cloud. Accounts data enables authentication module to keep the user signed in across different services. This is usually achieved with the some form of API authentication and portal membership provider that facilitates a session for the account holder.  Popular mechanisms include OAuth, JWT and such others.  Tokens are issued on successful authentication which are refreshed on expiry.  The transactions with the store are important for customer satisfaction. A variety of machine learning techniques such grouping and collaborative filtering are used to make recommendations to the users in the form of customers who purchased this also purchased those. 
#codingexercise
Calculate the H-index of a sorted array. 
This is the index of the array whose values is greater than or equal to the index.
int GetHIndex(int[] A)
{
var t = new int[A.Length+1];
for (int i = 0; i < A.Length; i++)
      t[Math.min(A.Length, A[i])]++;
int sum = 0;
for (int i = t.Length-1; i >=0; i--){
sum += t[i];
if (sum >=i)
    return i;
}
return -1;
}