How should I increment the number for the least concurrent streaming cyclization scenario?

If many threads are called GetNextNumber

concurrently with the following code, it GetNextNumber

will return 1 more time than any other numbers.

private class RoundRobbinNumber
{
    private int _maxNumbers = 10;
    private int _lastNumber;

    private RoundRobbinNumber(int maxNumbers)
    {
        _maxNumbers = maxNumbers;
    }

    public int GetNextNumber()
    {
        int nextNumber = Interlocked.Increment(ref _lastNumber);
        if (_lastNumber > _maxNumbers)
        {
            Interlocked.CompareExchange(ref _lastNumber, 1, _maxNumbers);
            nextNumber = 1;
        }
        return nextNumber;
    }
}

      

Is there a way to reset it _lastNumber

back to one and reliably return the incremented number for each calling thread GetNextNumber()

without having to use a lock?

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3 answers


The trick is to loop through the operation until it is successful. I offer a general pattern for this approach in my answer here .



public int GetNextNumber()
{
  int initial, computed;
  do
  {
    initial = _lastNumber;
    computed = initial + 1;
    computed = computed > _maxNumbers ? computed = 1 : computed;
  } 
  while (Interlocked.CompareExchange(ref _lastNumber, computed, initial) != initial);
  return computed;
}

      

+2


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Not sure if anyone other than this could be even simpler:



class RoundRobinNumber
{
    private int _maxNumbers = 10;
    private int _lastNumber = 0;

    public RoundRobinNumber(int maxNumbers)
    {
        _maxNumbers = maxNumbers;
    }

    public int GetNextNumber()
    {
        int nextNumber = Interlocked.Increment(ref _lastNumber);

        int result = nextNumber % _maxNumbers;  

        return result >= 0 ? result : -result;
    }
}

      

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Andrey answers without conditional statements:

using System;
namespace Utils
{
    public class RoundRobinCounter
    {
        private int _max;
        private int _currentNumber = 0;

        public RoundRobinCounter(int max)
        {
            _max = max;
        }

        public int GetNext()
        {
            uint nextNumber = unchecked((uint)System.Threading.Interlocked.Increment(ref _currentNumber));
            int result = (int)(nextNumber % _max);
            return result;
        }
    }
}

      

And here is the .NET fiddle running this code.

+1


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