02a2b43b2b Remove Default impl for Target and Work (Tobin C. Harding)
cb9893c4a9 Add Target and Difficulty types (Tobin C. Harding)
Pull request description:
Ugh! 1600 lines of green and 1100 of red - my apologies.
Currently we use the `Uint256` type for proof-of-work calculations. It was observed in #1181 that providing a public 256 bit integer type like this implies that it is a general purpose integer type. We do not want to provide a general purpose integer type (see the 1000 arithmetic functions on stdlib integer types for why not :)
Add two new opaque integer types `Target` and `Work`. These are the inverse of each other, both conceptually and mathematically.
There is a lot of code in this PR, sorry about that. At a high level the PR does:
- Add a `pow` module.
- Put a modified version of `Uint256` in `pow`, making it private.
- Add two new wrapper types `Target` and `Work` that provide a very limited API specific to their use case. In particular there are methods on each to convert to the other.
- Only implement methods that we use on each type.
### Note
During development I got mixed up with the word "difficulty", I have discovered this has a very specific meaning in Bitcoin. Please see rustdocs on the `Target::difficulty` function for explanation of this term. For this reason we use the type `Work` defined as the inverse of target, and reserve "difficulty" for the Bitcoin concept.
ACKs for top commit:
apoelstra:
ACK 02a2b43b2b
Kixunil:
ACK 02a2b43b2b
Tree-SHA512: 4d701756a42b832f03b8d542f3a5278b4ca1d5983ffd7d4630577ebd4cc8f47029719f9018185e01fa459d8fb32426b5cb4d6b8d8b588ebbb7b65e4aeee94412