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[wangjia184]

Thanks for joining our test.

https://github.com/XVCoder/CustomerScoreRank/blob/b0485b046e34596ab192066ab5eee8b500c8dcdf/CustomerScoreRank/Services/CustomerService.cs

Thread Safety

        private static readonly List<Customer> _customers = new();

        public async Task<decimal> UpdateScore(long customerId, decimal score)
        {
            decimal finalScore;
            lock (_customers)
            {
                if (_customers.Any(x => x.Id == customerId))
                {// Update
                    var customer = _customers.First(x => x.Id == customerId);
                    customer.Score += score;
                    finalScore = customer.Score;
                }
                else
                {// Add
                    var customer = new Customer { Id = customerId, Score = score };
                    _customers.Add(customer);
                    finalScore = customer.Score;
                }
            }
            return await Task.FromResult(finalScore);
        }

The above code can result in unexpected result when handling simultaneous requests. For example

• Multuple instances could be duplicated for an individual customer
• Multiple threads could obtain the same instance and modify its score at the same time, which could ends in unpredicatable result.

Besides this one, the other two retrieval methods are not safe neither. The list is being accessed without synchronization.

Performance

The retrieval operations (GetCustomersByRank / GetCustomersByCustomerId) sort the entrie list before each query, which is very low efficient. The proper approach is to keep the customers in order, rather than sorting it everytime before each query.

Here are some suggested approaches.

Approach	UpdateScore	GetCustomersByRank	GetCustomersByCustomerId
SortedSet + Dictionary	O(n)	O(n)	O(log(n))
BinarySearch + Dictionary	O(n)	O(1)	O(log(n))
SkipList+ Dictionary	O(log(n))	O(log(n))	O(log(n))

The above table lists the expected time complexity for each approach. You may choose an approach to refactor your code.

Br.

[XVCoder]

@wangjia184 refactored.