Curtail the occurrence of order books that are blocked by reduced offers
with the implementation of the fixReducedOffersV1 amendment.
This commit identifies three ways in which offers can be reduced:
1. A new offer can be partially crossed by existing offers, so the new
offer is reduced when placed in the ledger.
2. An in-ledger offer can be partially crossed by a new offer in a
transaction. So the in-ledger offer is reduced by the new offer.
3. An in-ledger offer may be under-funded. In this case the in-ledger
offer is scaled down to match the available funds.
Reduced offers can block order books if the effective quality of the
reduced offer is worse than the quality of the original offer (from the
perspective of the taker). It turns out that, for small values, the
quality of the reduced offer can be significantly affected by the
rounding mode used during scaling computations.
This commit adjusts some rounding modes so that the quality of a reduced
offer is always at least as good (from the taker's perspective) as the
original offer.
The amendment is titled fixReducedOffersV1 because additional ways of
producing reduced offers may come to light. Therefore, there may be a
future need for a V2 amendment.
Add an amendment to allow gateways to set a "tick size"
for assets they issue. There are no changes unless the
amendment is enabled (since the tick size option cannot
be set).
With the amendment enabled:
AccountSet transactions may set a "TickSize" parameter.
Legal values are 0 and 3-15 inclusive. Zero removes the
setting. 3-15 allow that many decimal digits of precision
in the pricing of offers for assets issued by this account.
For asset pairs with XRP, the tick size imposed, if any,
is the tick size of the issuer of the non-XRP asset. For
asset pairs without XRP, the tick size imposed, if any,
is the smaller of the two issuer's configured tick sizes.
The tick size is imposed by rounding the offer quality
down to nearest tick and recomputing the non-critical
side of the offer. For a buy, the amount offered is
rounded down. For a sell, the amount charged is rounded up.
Gateways must enable a TickSize on their account for this
feature to benefit them.
The primary expected benefit is the elimination of bots
fighting over the tip of the order book. This means:
- Quicker price discovery as outpricing someone by a
microscopic amount is made impossible. Currently
bots can spend hours outbidding each other with no
significant price movement.
- A reduction in offer creation and cancellation spam.
- More offers left on the books as priority means
something when you can't outbid by a microscopic amount.
Very small payment could fail when STAmount::mulRound underflowed
and returned zero, when it should have rounded up to the smallest
representable value.
