🚧 market: implement fixed repay position

Author: cruzdanilo

src/index.ts

@@ -31,6 +31,7 @@ export { default as globalUtilization } from "./interest-rate-model/globalUtiliz
 export type { default as IRMParameters, IRMFloatingParameters } from "./interest-rate-model/Parameters.ts";
 
 export { default as fixedRepayAssets } from "./market/fixedRepayAssets.js";
+export { default as fixedRepayPosition } from "./market/fixedRepayPosition.js";
 export { default as floatingDepositRates } from "./market/floatingDepositRates.js";
 
 export { default as abs } from "./vector/abs.js";

src/market/fixedRepayPosition.ts

@@ -0,0 +1,44 @@
+import type { FixedRepaySnapshot } from "./fixedRepayAssets.js";
+import WAD from "../fixed-point-math/WAD.js";
+import divWad from "../fixed-point-math/divWad.js";
+import min from "../fixed-point-math/min.js";
+import mulDiv from "../fixed-point-math/mulDiv.js";
+import mulWad from "../fixed-point-math/mulWad.js";
+
+export default function fixedRepayPosition(
+  {
+    penaltyRate,
+    backupFeeRate,
+    borrowed,
+    supplied,
+    unassignedEarnings,
+    lastAccrual,
+    principal,
+    fee,
+  }: FixedRepaySnapshot,
+  maturity: number,
+  assets: bigint,
+  timestamp = Math.floor(Date.now() / 1000),
+) {
+  const totalPosition = principal + fee;
+  if (totalPosition === 0n) return 0n;
+  if (timestamp >= lastAccrual) {
+    return min(divWad(assets, WAD + BigInt(timestamp - maturity) * penaltyRate), totalPosition);
+  }
+  if (assets >= totalPosition) return totalPosition;
+  if (maturity > lastAccrual) {
+    unassignedEarnings -= mulDiv(unassignedEarnings, BigInt(timestamp) - lastAccrual, BigInt(maturity) - lastAccrual);
+  }
+  if (unassignedEarnings === 0n) return assets;
+  const backupSupplied = borrowed - min(borrowed, supplied);
+  if (backupSupplied === 0n) return assets;
+  const k = divWad(principal, totalPosition);
+  if (k === 0n) return assets;
+  const netUnassignedEarnings = mulWad(unassignedEarnings, WAD - backupFeeRate);
+  if (netUnassignedEarnings === 0n) return assets;
+  const r = mulDiv(netUnassignedEarnings, k, backupSupplied);
+  if (r >= WAD) return min(assets + netUnassignedEarnings, totalPosition);
+  const x = divWad(assets, WAD - r);
+  if (mulWad(k, x) <= backupSupplied && x <= totalPosition) return x;
+  return min(assets + netUnassignedEarnings, totalPosition);
+}