chore: fusdc remove maxVariable from FeeConfig (#10859)

closes: #10858

## Description
remove `maxVariable` (cap on variable fee) from `FeeConfig`

### Security Considerations
None

### Scaling Considerations
None

### Documentation Considerations
None

### Testing Considerations
Adds new unit test.

### Upgrade Considerations
None, part of initial release for FUSDC.

packages/boot/test/fast-usdc/snapshots/fast-usdc.test.ts.md

@@ -114,10 +114,6 @@ Generated by [AVA](https://avajs.dev).
             brand: Object @Alleged: USDC brand {},
             value: 10000n,
           },
-          maxVariable: {
-            brand: Object @Alleged: USDC brand {},
-            value: 5000000n,
-          },
           variableRate: {
             denominator: {
               brand: Object @Alleged: USDC brand {},

packages/builders/scripts/fast-usdc/start-fast-usdc.build.js

@@ -24,7 +24,6 @@ const { keys } = Object;
 const options = {
   flatFee: { type: 'string', default: '0.01' },
   variableRate: { type: 'string', default: '0.01' },
-  maxVariableFee: { type: 'string', default: '5' },
   contractRate: { type: 'string', default: '0.2' },
   net: { type: 'string' },
   oracle: { type: 'string', multiple: true },
@@ -50,7 +49,6 @@ const assetInfoUsage =
  * @typedef {{
  *   flatFee: string;
  *   variableRate: string;
- *   maxVariableFee: string;
  *   contractRate: string;
  *   net?: string;
  *   oracle?: string[];
@@ -161,11 +159,10 @@ export default async (homeP, endowments) => {
   /** @param {string} numeral */
   const toRatio = numeral => parseRatio(numeral, USDC);
   const parseFeeConfigArgs = () => {
-    const { flatFee, variableRate, maxVariableFee, contractRate } = fees;
+    const { flatFee, variableRate, contractRate } = fees;
     return {
       flat: toAmount(flatFee),
       variableRate: toRatio(variableRate),
-      maxVariable: toAmount(maxVariableFee),
       contractRate: toRatio(contractRate),
     };
   };

packages/fast-usdc/src/type-guards.js

@@ -113,7 +113,6 @@ const NatAmountShape = { brand: BrandShape, value: M.nat() };
 export const FeeConfigShape = {
   flat: NatAmountShape,
   variableRate: RatioShape,
-  maxVariable: NatAmountShape,
   contractRate: RatioShape,
 };
 harden(FeeConfigShape);

packages/fast-usdc/src/types.ts

@@ -62,9 +62,11 @@ export interface PendingTx extends CctpTxEvidence {
 }
 
 export type FeeConfig = {
+  /** flat fee charged for every advance */
   flat: Amount<'nat'>;
+  /** proportion of advance kept as a fee */
   variableRate: Ratio;
-  maxVariable: Amount<'nat'>;
+  /** proportion of fees that goes to the contract (remaining goes to LPs) */
   contractRate: Ratio;
 };
 

packages/fast-usdc/src/utils/fees.js

@@ -4,7 +4,7 @@ import { Fail } from '@endo/errors';
 import { mustMatch } from '@endo/patterns';
 import { FeeConfigShape } from '../type-guards.js';
 
-const { add, isGTE, min, subtract } = AmountMath;
+const { add, isGTE, subtract } = AmountMath;
 
 /**
  * @import {Amount} from '@agoric/ertp';
@@ -15,7 +15,7 @@ const { add, isGTE, min, subtract } = AmountMath;
 /** @param {FeeConfig} feeConfig */
 export const makeFeeTools = feeConfig => {
   mustMatch(feeConfig, FeeConfigShape, 'Must provide feeConfig');
-  const { flat, variableRate, maxVariable } = feeConfig;
+  const { flat, variableRate } = feeConfig;
   const feeTools = harden({
     /**
      * Calculate the net amount to advance after withholding fees.
@@ -34,8 +34,7 @@ export const makeFeeTools = feeConfig => {
      * @throws {Error} if requested does not exceed fees
      */
     calculateAdvanceFee(requested) {
-      const variable = min(multiplyBy(requested, variableRate), maxVariable);
-      const fee = add(variable, flat);
+      const fee = add(multiplyBy(requested, variableRate), flat);
       !isGTE(fee, requested) || Fail`Request must exceed fees.`;
       return fee;
     },

packages/fast-usdc/test/config-marshal.test.js

@@ -29,7 +29,6 @@ test('cross-vat configuration of Fast USDC FeeConfig', t => {
   const config = harden({
     flat: make(USDC, 100n),
     variableRate: makeRatio(1n, USDC),
-    maxVariable: make(USDC, 100_000n),
     contractRate: makeRatio(20n, USDC),
   });
   testMatches(t, config, FeeConfigShape);
@@ -51,7 +50,6 @@ test('cross-vat configuration of Fast USDC FeeConfig', t => {
         },
       },
       flat: { brand: '$0', value: '+100' },
-      maxVariable: { brand: '$0', value: '+100000' },
       variableRate: {
         denominator: {
           brand: '$0',

packages/fast-usdc/test/exos/advancer.test.ts

@@ -298,7 +298,7 @@ test('updates status to OBSERVED on insufficient pool funds', async t => {
     [
       'Advancer error:',
       Error(
-        `Cannot borrow. Requested ${q(usdc.make(294999999n))} must be less than pool balance ${q(usdc.make(1n))}.`,
+        `Cannot borrow. Requested ${q(usdc.make(293999999n))} must be less than pool balance ${q(usdc.make(1n))}.`,
       ),
     ],
   ]);

packages/fast-usdc/test/mocks.ts

@@ -89,6 +89,5 @@ export const makeTestFeeConfig = (usdc: Omit<AmountUtils, 'mint'>): FeeConfig =>
   harden({
     flat: usdc.make(1n),
     variableRate: makeRatio(2n, usdc.brand),
-    maxVariable: usdc.units(5),
     contractRate: makeRatio(20n, usdc.brand),
   });

packages/fast-usdc/test/utils/fees.test.ts

@@ -22,7 +22,6 @@ const USDCRatio = (numerator: bigint, denominator: bigint = 100n) =>
 const aFeeConfig: FeeConfig = {
   flat: USDC(10n),
   variableRate: USDCRatio(2n),
-  maxVariable: USDC(100n),
   contractRate: USDCRatio(20n),
 };
 harden(aFeeConfig);
@@ -69,32 +68,6 @@ const feeToolsScenario = test.macro({
   },
 });
 
-test(feeToolsScenario, {
-  name: 'below max variable fee',
-  requested: USDC(1000n),
-  expected: {
-    totalFee: USDC(30n), // 10 flat + 20 variable
-    advance: USDC(970n),
-    split: {
-      ContractFee: USDC(6n),
-      PoolFee: USDC(24n),
-    },
-  },
-});
-
-test(feeToolsScenario, {
-  name: 'above max variable fee',
-  requested: USDC(10000n),
-  expected: {
-    totalFee: USDC(110n), // 10 flat + 100 max
-    advance: USDC(9890n),
-    split: {
-      ContractFee: USDC(22n),
-      PoolFee: USDC(88n),
-    },
-  },
-});
-
 test(feeToolsScenario, {
   name: 'zero variable fee',
   requested: USDC(1000n),
@@ -147,33 +120,14 @@ test(feeToolsScenario, {
   },
 });
 
-test(feeToolsScenario, {
-  // TODO consider behavior where 0 or undefined means "no fee cap"
-  name: 'only flat is charged if `maxVariable: 0`',
-  requested: USDC(10000n),
-  config: {
-    ...aFeeConfig,
-    variableRate: USDCRatio(20n),
-    maxVariable: USDC(0n),
-  },
-  expected: {
-    totalFee: USDC(10n), // only flat
-    advance: USDC(9990n),
-    split: {
-      ContractFee: USDC(2n),
-      PoolFee: USDC(8n),
-    },
-  },
-});
-
 test(feeToolsScenario, {
   name: 'AGORIC_PLUS_OSMO with commonPrivateArgs.feeConfig',
   // 150_000_000n
   requested: USDC(MockCctpTxEvidences.AGORIC_PLUS_OSMO().tx.amount),
   // same as commonPrivateArgs.feeConfig from `CommonSetup`
   config: makeTestFeeConfig(withAmountUtils(issuerKits.USDC)),
   expected: {
-    totalFee: USDC(3000001n), // 1n + min(2% of 150USDC, 5USDC)
+    totalFee: USDC(3000001n), // 1n + 2% of 150USDC
     advance: USDC(146999999n),
     split: {
       ContractFee: USDC(600000n), // 20% of fee
@@ -189,11 +143,11 @@ test(feeToolsScenario, {
   // same as commonPrivateArgs.feeConfig from `CommonSetup`
   config: makeTestFeeConfig(withAmountUtils(issuerKits.USDC)),
   expected: {
-    totalFee: USDC(5000001n), // 1n + min(2% of 300USDC, 5USDC)
-    advance: USDC(294999999n),
+    totalFee: USDC(6000001n), // 1n + 2% of 300USDC
+    advance: USDC(293999999n),
     split: {
-      ContractFee: USDC(1000000n), // 20% of fee
-      PoolFee: USDC(4000001n),
+      ContractFee: USDC(1200000n), // 20% of fee
+      PoolFee: USDC(4800001n),
     },
   },
 });