Improve the algorithm for calculating the next bitcoin target.

[yangby-cryptape]

Issue

Causes "arithmetic operation overflow" on Bitcoin dev chain.

Reason

In the following code:

https://github.com/ckb-cell/ckb-bitcoin-spv/blob/a3c43fd5fb70044ad438f703212b255af5024476/verifier/src/utilities/bitcoin.rs#L29-L35

when prev-target (x) is too large, y = x * U256::from(actual) will overflow.

A Proposed Solution

Pseudocode as following:

let x = U512::from_little_endian(&prev_target_le_bytes); 
let y = x * U512::from(actual); 
let z_tmp = y / U512::from(expected); 
if z_tmp > U512::from(U256::MAX) {
    panic!();
}
let z = z_tmp as U256;

Affected Scope

Bitcoin mainnet and Bitcoin testnet don't have such large target.
So, we can ignore it temporary.

But I strongly suggest fixing it when upgrade the production contract next time.

How to Reproduce

Modify this unit test, to calculate the next bitcoin target for following headers, directly.

Header 2016:

0000002088e6d6b787292d12516840a1b970464d4d3e942b5d3d23a337fdbf68762ddf603e6c81d61074df79939b5483aed21e52576ba6a5d723e33b821dc1796d21272c993c1266ffff7f2000000000

Header 4031:

00000020e86a51dbe721a9c266d639427c010c5117518b2da95eea191911f57a7eb994327b0faee0e012aff149c85178ef930559e6c04554a8ff34883aad0c93e4af846742461266ffff7f2001000000

These data could be added to as a unit test after fixing it.

References

• Description in the contract document.