fix: work around huge gas cost when reducing ICR of bottom Trove

Fixes #659.

packages/lib-ethers/src/PopulatableEthersLiquity.ts

@@ -693,7 +693,8 @@ export class PopulatableEthersLiquity
  }
 
  private async _findHintsForNominalCollateralRatio(
- nominalCollateralRatio: Decimal
+ nominalCollateralRatio: Decimal,
+ ownAddress?: string
  ): Promise<[string, string]> {
  const { sortedTroves, hintHelpers } = _getContracts(this._readable.connection);
  const numberOfTroves = await this._readable.getNumberOfTroves();
@@ -733,21 +734,35 @@ export class PopulatableEthersLiquity
 
  const { hintAddress } = results.reduce((a, b) => (a.diff.lt(b.diff) ? a : b));
 
- const [prev, next] = await sortedTroves.findInsertPosition(
+ let [prev, next] = await sortedTroves.findInsertPosition(
  nominalCollateralRatio.hex,
  hintAddress,
  hintAddress
  );
 
- return prev === AddressZero ? [next, next] : next === AddressZero ? [prev, prev] : [prev, next];
+ if (ownAddress) {
+ if (prev === ownAddress) {
+ prev = await sortedTroves.getPrev(prev);
+ } else if (next === ownAddress) {
+ next = await sortedTroves.getNext(next);
+ }
+ }
+
+ if (prev === AddressZero) {
+ prev = next;
+ } else if (next === AddressZero) {
+ next = prev;
+ }
+
+ return [prev, next];
  }
 
- private async _findHints(trove: Trove): Promise<[string, string]> {
+ private async _findHints(trove: Trove, ownAddress?: string): Promise<[string, string]> {
  if (trove instanceof TroveWithPendingRedistribution) {
  throw new Error("Rewards must be applied to this Trove");
  }
 
- return this._findHintsForNominalCollateralRatio(trove._nominalCollateralRatio);
+ return this._findHintsForNominalCollateralRatio(trove._nominalCollateralRatio, ownAddress);
  }
 
  private async _findRedemptionHints(
@@ -775,7 +790,10 @@ export class PopulatableEthersLiquity
  partialRedemptionLowerHint
  ] = partialRedemptionHintNICR.isZero()
  ? [AddressZero, AddressZero]
- : await this._findHintsForNominalCollateralRatio(decimalify(partialRedemptionHintNICR));
+ : await this._findHintsForNominalCollateralRatio(
+ decimalify(partialRedemptionHintNICR)
+ // XXX: if we knew the partially redeemed Trove's address, we'd pass it here
+ );
 
  return [
  decimalify(truncatedLUSDamount),
@@ -942,7 +960,7 @@ export class PopulatableEthersLiquity
 
  const currentBorrowingRate = decayBorrowingRate(0);
  const adjustedTrove = trove.adjust(normalizedParams, currentBorrowingRate);
- const hints = await this._findHints(adjustedTrove);
+ const hints = await this._findHints(adjustedTrove, address);
 
  const {
  maxBorrowingRate,
@@ -1139,7 +1157,7 @@ export class PopulatableEthersLiquity
  await stabilityPool.estimateAndPopulate.withdrawETHGainToTrove(
  { ...overrides },
  compose(addGasForPotentialListTraversal, addGasForLQTYIssuance),
- ...(await this._findHints(finalTrove))
+ ...(await this._findHints(finalTrove, address))
  )
  );
  }