refactor(portfolio-contract): Define a applyHaircut helper function

Author: gibson042

packages/portfolio-contract/tools/plan-solve.ts

@@ -37,6 +37,14 @@ const replaceOrInit = <K, V>(
   map.set(key, callback(old, key, exists));
 };
 
+/**
+ * Reduce a value by a natural number of basis points, rounding the result down
+ * (e.g., 1 bp of 10k is exactly 1, so subtracting 1 bp from any value up to 10k
+ * reduces it by a full unit).
+ */
+export const applyHaircut = (n: bigint, bps: bigint) =>
+  (n * (10000n - bps)) / 10000n;
+
 // eslint-disable-next-line @typescript-eslint/no-unused-vars
 const trace = makeTracer('solve');
 
@@ -453,7 +461,7 @@ export const rebalanceMinCostFlowSteps = async (
           // HACK: subtract 1n to avoid rounding errors in Noble per
           // https://github.com/Agoric/agoric-private/issues/415
           const usdnOut =
-            (BigInt(flow) * (10000n - BigInt(edge.variableFee))) / 10000n - 1n;
+            applyHaircut(BigInt(flow), BigInt(edge.variableFee)) - 1n;
           details = { detail: { usdnOut } };
           break;
         }