An Addressed Byte Array

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Background

A few days ago, I needed to keep track of operator state (on/off) for a large number of operators. For this, I intended to use a BitArray, however when the number of operators was set to Int32.MaxValue, I got an OutOfMemoryException much before the count was finalized. I posted a question here where you can read the complete thread. In the end, I decided to write a class to work around.

The Code

I used a Byte[] and created a method to read and write different bits in any byte.

Let's say that all I need to work with are 16 bits, the class creates a byte[] of 2 elements with each holding value 0xff (all bits are true).

byte[] b = new byte[2]; //b[0] and b[1]

Now let's say that I want to set bit 10 to false. Bit 10 is (obviously) in b[1]. I create a mask 0x04 to 'extract' the relevant bit and work on it.

Using the Code

In the attached project, I used the class to work out Prime Numbers using the Sieve of Eratosthenes method. Incidentally while creating this example, I also tried using BitArray for working on a large set of integers (1 to Int32.MaxValue) and I got the same OutOfMemoryException which I did not when I used the AddressingByte class.

When a instance of the PrimeNumberCalculator class is created, it creates an instance of AddressingByte, passing to it the number of bytes required and the initial value (0xff since I need all bits to be true at startup). To work the sieve, calls to two methods (GetValueAt(Int32 AddressOfBit) and SetValueAt(Int32 AddressOfBit, Boolean BoolValue)) are made...

//Calculation to Find Primes
for (Int32 i = 2; i < _topNumber; i++)
  if (_numbers.GetValueAt(i))
    for (Int32 j = i * 2; j < _topNumber; j += i)
      _numbers.SetValueAt(j, false);

... where _topNumber is the maximum number I want to work to and _numbers is the instance of AddressByte.

Points To Note

A bit of a disclaimer. The solution supplied to the problem described here works. It is not fast but I was not after speed and/or efficiency.

History

• Version 1.0: 28^th April, 2008