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)
