PackableMemory.java

class PackableMemory
{
	int size;
	public byte mem[] = null;


	public PackableMemory(int size)
	{
		this.size = size;
		this.mem = new byte[size];
	}


	// Pack the 4-byte integer val into the four bytes mem[loc]...mem[loc+3].
	// The most significant portion of the integer is stored in mem[loc].
	// Bytes are masked out of the integer and stored in the array, working
	// from right(least significant) to left (most significant).
	void pack(int val, int loc)
	{
		final int MASK = 0xff;
		for (int i = 3; i >= 0; i--)
		{
			mem[loc + i] = (byte) (val & MASK);
			val = val >> 8;
		}
	}


	// Unpack the four bytes mem[loc]...mem[loc+3] into a 4-byte integer,
	// and return the resulting integer value.
	// The most significant portion of the integer is stored in mem[loc].
	// Bytes are 'OR'ed into the integer, working from left (most significant)
	// to right (least significant)
	int unpack(int loc)
	{
		final int MASK = 0xff;
		int v = (int) mem[loc] & MASK;
		for (int i = 1; i < 4; i++)
		{
			v = v << 8;
			v = v | ((int) mem[loc + i] & MASK);
		}
		return v;
	}


	// Test the above pack and unpack methods by iterating the following
	// over all possible 4-byte integers: pack the integer,
	// then unpack it, and then verify that the unpacked integer equals the
	// original integer. It tests all nonnegative numbers in ascending order
	// and then all negative numbers in ascending order. The transition from
	// positive to negative numbers happens implicitly due to integer overflow.
	public void packTest()
	{

		int i = 0;
		long k = 0;
		do
		{
			this.pack(i, 4);
			int j = this.unpack(4);
			if (j != i)
			{
				System.out.printf("pack/unpack test failed: i = %d, j = %d\n",
					i, j);
				System.exit(0);
			}
			i++;
			k++;
		}
		while (i != 0);
		System.out.printf("pack/unpack test successful, %d iterations\n", k);
	}


	// main routine to test the PackableMemory class by running the
	// packTest() method.
	public static void main(String[] args)
	{
		PackableMemory pm = new PackableMemory(100);
		pm.packTest();
		System.exit(0);
	}
}