CODEWITHSUNDEEP
javabytebit
not_shitashi
not_shitashi

Reputation: 291

What does this binary documentation mean?

I'm trying to decode somebody's byte array and I'm stuck at this part:

&lt state &gt ::= "01" <i>(2 bits) for A</i>
                   "10" <i>(2 bits) for B</i>
                   "11" <i>(2 bits) for C</i>

I think this wants me to look at the next 2 bits of the next byte. Would that mean the least or most significant digits of the byte? I suppose I would just throw away the last 6 bits if it means the least significant?

I found this code for looking at the bits of a byte:

for (int i = 0; i < byteArray.Length; i++)
{
   byte b = byteArray[i];
   byte mask = 0x01;
   for (int j = 0; j < 8; j++)
   {
      bool value = b & mask;
      mask << 1;
   }
}

Can someone expand on what this does exactly?

Upvotes: 0

Views: 104

Answers (1)

Daniel S.
Daniel S.

Reputation: 6640

Just to give you a start:

To extract individual bits of a byte, you use "&", called the bitwise and operator. The bitwise and operation means "preserve all bits which are set on both sides". E.g. when you calculate the bitwise-and of two bytes, e.g. 00000011 & 00000010, then the result is 00000010, because only the bit at the second last position is set in both sides.

In java programming language, the very same example looks like this:

int a = 3;
int b = 2;
int bitwiseAndResult = a & b; // bitwiseAndResult will be equal to 2 after this

Now to examine if the n'th bit of some int is set, you can do this:

int intToExamine = ...;
if ((intToExamine >> n)) & 1 != 0) {
    // here we know that the n'th bit was set
}

The >> is called the bitshift operator. It simply shifts the bits from left to right, like this: 00011010 >> 2 will have the result 00000110.

So from the above you can see that for extracting the n'th bit of some value, you first shift the n'th bit to position 0 (note that the first bit is bit 0, not bit 1), and then you use the bitwise and operator (&) to only keep that bit 0.

Here are some simple examples of bitwise and bit shift operators: http://www.tutorialspoint.com/java/java_bitwise_operators_examples.htm

Upvotes: 1

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