How to calculate the difficulty and target from the bit of bitcoin block header.

The method of obtainig the difficulty of bitcoin.

* Difficulty:  a measure of how difficult it is to find a new block.

    - changed by every 2016 blocks.

    - difficulty = maximum_target / current_target.

    - maximum target in bitcoin is constant.

         ==> 0x00000000FFFF0000000000000000000000000000000000000000000000000000

    - lower current target makes Bitcoin generation more difficult.

       the maximum target is the lowest possible difficulty.

* Difficulty 1 is defined as 0x1d00ffff,

    - maximum target : 0x00ffff * 2**(8*(0x1d - 3))

       = 0x00000000FFFF0000000000000000000000000000000000000000000000000000

* The maximum difficulty:  roughly maximum_target / 1, (about 2^224)