Representing sub-satoshi amounts

[sgeisler]

If this library is intended to be used for all rust-bitcoin crates (which I think it should for interoperability reasons) then it should be capable of representing sub-satoshi amounts (e.g. for lightning payments). But unfortunately the smallest amount needed for lightning is pico BTC which leads to a storage requirement of 64.2 bits if we want to represent up to 21*10^6 BTC with that precision.

My proposal would be to add a type parameter to Amount which optionally extends the precision:

trait Precision : Default {}
struct Satoshi ();
struct PicoBtc (u32); // Additional space to represent sub-satoshi amounts

impl Precision for Satoshi;
impl Precision for PicoBtc;

struct Amount<P = Satoshi> where P: Precision (Inner, P);

impl Amount<PicoBtc> {
    pub fn to_satoshi(&self) -> Amount<Satoshi> {...}
}

The default empty Satoshi type would be completely optimized away by the compiler, so this feature should incur no performance penalties if not used.

What do you think about this approach?

[stevenroose]

Why is that u32 there in PicoBtc? IIUC, the 10^-4 offset would be implemented in the context-specific impl block, right?

I suppose sub-satoshi values would be ignored?

Btw, this would probably not work together well with #1, right? Since I don't think it's ergonomically possible to make a print formatter also use these contexts. But the denomination-specific formatter should definitely be implemented in the inner-specific impl, because otherwise MilliSatoshi denomination would be always 0 for less-then-one-satoshi amounts.

I like it though. And since @jeandudey already indicated he doesn't have time to work on his bitcoin crates, so I guess we can go ahead on a fork of this and ultimately work towards having it included either in the rust-bitcoin organization or just as bitcoin::util::amount.

[sgeisler]

Why is that u32 there in PicoBtc?

Random, I didn't want to calculate what's needed and felt quite sure about u32. It's just a design idea, I don't even know if this compiles. Maybe we also want to go lower than pico btc from the beginning to avoid the introduction of a third sub-sat struct down the road. I'd like to coordinate that with the rust-lightning team.

IIUC, the 10^-4 offset would be implemented in the context-specific impl block, right?

Yes.

I suppose sub-satoshi values would be ignored?

One could simply ignore them or allow for different rounding strategies, but I think that could be added as needed.

Btw, this would probably not work together well with #1, right?

I don't think that's a problem: one could implement a function in the Precision trait that returns the sub satoshi amount in something like pico sat or whatever fits into a u64. For Satoshi you would just return 0 and for other structs implementing Precision return the sub satoshi amount converted to the right SI prefix.

Your pretty printing methods could then decide how many sub-sat digits they want to use.