Implement fractional difficulty

[vkoskiv]

Swift can't deal with 256 bit UInts natively, so figure out how to implement this.
Without fractional difficulty, difficulty can only double or halve, which is inconvenient given difficulty will be adjusted hourly.

[vkoskiv]

Some reading:
http://www.hashcash.org/bitcoin/
http://www.cypherspace.org/bitcoin/hashcash.html

[johndpope]

https://github.com/CryptoCoinSwift/UInt256

[johndpope]

CryptoCoinSwift/UInt256#8

[vkoskiv]

The project currently has the file UInt+SHA256.swift that implements most of the required functionality for working with 256 bit block hashes. However, I recently discovered it lacks multiple operands, most notably division and multiplication. (You can observe the //TODO lines in the file)
I forgot where I found the file, so we'll have to find another implementation.

[vkoskiv]

I don't particularly like CryptoCoinSwift's implementation, since it relies on C functions to do many operations. I'd only implement it if Swift 3 really has performance issues with those operations.

[johndpope]

glancing through feeds
"if you're doing crypto, you're probably safer using NSDecimalNumber, is what we use for Toshi. " https://github.com/toshiapp/toshi-ios-client/blob/32f05c83b1ba7c383aaa1f0a18565df544688fe1/Toshi/Extensions/NSDecimalNumber%2BAdditions.swift

[vkoskiv]

Personally I'm a bit lost currently on how we'll implement the difficulty. I'm tempted to just go with hashcash's fixed number of bits for now.

[slk333]

you can try with NSDecimalNumber, it has 128 bit precision, and can store number up to 512 bit
https://developer.apple.com/documentation/foundation/nsdecimalnumber

[vkoskiv]

@slk333 We need 256 bits for SHA256