serai/processor/scheduler/utxo/primitives/src/lib.rs

#![cfg_attr(docsrs, feature(doc_auto_cfg))]
#![doc = include_str!("../README.md")]
#![deny(missing_docs)]

use core::fmt::Debug;

use serai_primitives::{Coin, Amount};

use primitives::ReceivedOutput;
use scanner::{Payment, ScannerFeed, AddressFor, OutputFor};

/// An object able to plan a transaction.
#[async_trait::async_trait]
pub trait TransactionPlanner<S: ScannerFeed> {
  /// An error encountered when determining the fee rate.
  ///
  /// This MUST be an ephemeral error. Retrying fetching data from the blockchain MUST eventually
  /// resolve without manual intervention/changing the arguments.
  type EphemeralError: Debug;

  /// The type representing a fee rate to use for transactions.
  type FeeRate: Clone + Copy;

  /// The type representing a planned transaction.
  type PlannedTransaction;

  /// Obtain the fee rate to pay.
  ///
  /// This must be constant to the finalized block referenced by this block number and the coin.
  async fn fee_rate(
    &self,
    block_number: u64,
    coin: Coin,
  ) -> Result<Self::FeeRate, Self::EphemeralError>;

  /// Calculate the for a tansaction with this structure.
  ///
  /// The fee rate, inputs, and payments, will all be for the same coin. The returned fee is
  /// denominated in this coin.
  fn calculate_fee(
    &self,
    block_number: u64,
    fee_rate: Self::FeeRate,
    inputs: Vec<OutputFor<S>>,
    payments: Vec<Payment<S>>,
    change: Option<AddressFor<S>>,
  ) -> Amount;

  /// Plan a transaction.
  ///
  /// This must only require the same fee as would be returned by `calculate_fee`. The caller is
  /// trusted to maintain `sum(inputs) - sum(payments) >= if change.is_some() { DUST } else { 0 }`.
  ///
  /// `change` will always be an address belonging to the Serai network.
  fn plan(
    &self,
    block_number: u64,
    fee_rate: Self::FeeRate,
    inputs: Vec<OutputFor<S>>,
    payments: Vec<Payment<S>>,
    change: Option<AddressFor<S>>,
  ) -> Self::PlannedTransaction;

  /// Obtain a PlannedTransaction via amortizing the fee over the payments.
  ///
  /// `operating_costs` is accrued to if Serai faces the burden of a fee or drops inputs not worth
  /// accumulating. `operating_costs` will be amortized along with this transaction's fee as
  /// possible. Please see `spec/processor/UTXO Management.md` for more information.
  ///
  /// Returns `None` if the fee exceeded the inputs, or `Some` otherwise.
  fn plan_transaction_with_fee_amortization(
    &self,
    operating_costs: &mut u64,
    block_number: u64,
    fee_rate: Self::FeeRate,
    inputs: Vec<OutputFor<S>>,
    mut payments: Vec<Payment<S>>,
    change: Option<AddressFor<S>>,
  ) -> Option<Self::PlannedTransaction> {
    // Sanity checks
    {
      assert!(!inputs.is_empty());
      assert!((!payments.is_empty()) || change.is_some());
      let coin = inputs.first().unwrap().balance().coin;
      for input in &inputs {
        assert_eq!(coin, input.balance().coin);
      }
      for payment in &payments {
        assert_eq!(coin, payment.balance().coin);
      }
      assert!(
        (inputs.iter().map(|input| input.balance().amount.0).sum::<u64>() + *operating_costs) >=
          payments.iter().map(|payment| payment.balance().amount.0).sum::<u64>(),
        "attempted to fulfill payments without a sufficient input set"
      );
    }

    let coin = inputs.first().unwrap().balance().coin;

    // Amortization
    {
      // Sort payments from high amount to low amount
      payments.sort_by(|a, b| a.balance().amount.0.cmp(&b.balance().amount.0).reverse());

      let mut fee = self
        .calculate_fee(block_number, fee_rate, inputs.clone(), payments.clone(), change.clone())
        .0;
      let mut amortized = 0;
      while !payments.is_empty() {
        // We need to pay the fee, and any accrued operating costs, minus what we've already
        // amortized
        let adjusted_fee = (*operating_costs + fee).saturating_sub(amortized);

        /*
          Ideally, we wouldn't use a ceil div yet would be accurate about it. Any remainder could
          be amortized over the largest outputs, which wouldn't be relevant here as we only work
          with the smallest output. The issue is the theoretical edge case where all outputs have
          the same value and are of the minimum value. In that case, none would be able to have the
          remainder amortized as it'd cause them to need to be dropped. Using a ceil div avoids
          this.
        */
        let per_payment_fee = adjusted_fee.div_ceil(u64::try_from(payments.len()).unwrap());
        // Pop the last payment if it can't pay the fee, remaining about the dust limit as it does
        if payments.last().unwrap().balance().amount.0 <= (per_payment_fee + S::dust(coin).0) {
          amortized += payments.pop().unwrap().balance().amount.0;
          // Recalculate the fee and try again
          fee = self
            .calculate_fee(block_number, fee_rate, inputs.clone(), payments.clone(), change.clone())
            .0;
          continue;
        }
        // Break since all of these payments shouldn't be dropped
        break;
      }

      // If we couldn't amortize the fee over the payments, check if we even have enough to pay it
      if payments.is_empty() {
        // If we don't have a change output, we simply return here
        // We no longer have anything to do here, nor any expectations
        if change.is_none() {
          None?;
        }

        let inputs = inputs.iter().map(|input| input.balance().amount.0).sum::<u64>();
        // Checks not just if we can pay for it, yet that the would-be change output is at least
        // dust
        if inputs < (fee + S::dust(coin).0) {
          // Write off these inputs
          *operating_costs += inputs;
          // Yet also claw back the payments we dropped, as we only lost the change
          // The dropped payments will be worth less than the inputs + operating_costs we started
          // with, so this shouldn't use `saturating_sub`
          *operating_costs -= amortized;
          None?;
        }
      } else {
        // Since we have payments which can pay the fee we ended up with, amortize it
        let adjusted_fee = (*operating_costs + fee).saturating_sub(amortized);
        let per_payment_base_fee = adjusted_fee / u64::try_from(payments.len()).unwrap();
        let payments_paying_one_atomic_unit_more =
          usize::try_from(adjusted_fee % u64::try_from(payments.len()).unwrap()).unwrap();

        for (i, payment) in payments.iter_mut().enumerate() {
          let per_payment_fee =
            per_payment_base_fee + u64::from(u8::from(i < payments_paying_one_atomic_unit_more));
          payment.balance().amount.0 -= per_payment_fee;
          amortized += per_payment_fee;
        }
        assert!(amortized >= (*operating_costs + fee));
      }

      // Update the amount of operating costs
      *operating_costs = (*operating_costs + fee).saturating_sub(amortized);
    }

    // Because we amortized, or accrued as operating costs, the fee, make the transaction
    Some(self.plan(block_number, fee_rate, inputs, payments, change))
  }
}