fix(a3p): use makeRatio for mintFeeRatio

rel: Agoric/BytePitchPartnerEng#22

a3p-integration/proposals/z:acceptance/test-lib/ratio.js

@@ -126,3 +126,27 @@ const multiplyHelper = (amount, ratio, divideOp) => {
 export const ceilMultiplyBy = (amount, ratio) => {
   return multiplyHelper(amount, ratio, natSafeMath.ceilDivide);
 };
+
+const PERCENT = 100n;
+
+/**
+ * @param {bigint} numerator
+ * @param {import('@agoric/ertp').Brand} numeratorBrand
+ * @param {bigint} [denominator] The default denominator is 100
+ * @param {import('@agoric/ertp').Brand} [denominatorBrand] The default is to reuse the numeratorBrand
+ * @returns {import('@agoric/zoe/src/contractSupport/types-ambient.js').Ratio}
+ */
+export const makeRatio = (
+  numerator,
+  numeratorBrand,
+  denominator = PERCENT,
+  denominatorBrand = numeratorBrand,
+) => {
+  denominator > 0n ||
+    Fail`No infinite ratios! Denominator was 0 ${q(denominatorBrand)}`;
+
+  return harden({
+    numerator: AmountMath.make(numeratorBrand, numerator),
+    denominator: AmountMath.make(denominatorBrand, denominator),
+  });
+};

a3p-integration/proposals/z:acceptance/test-lib/vaults.js

@@ -10,7 +10,7 @@ import {
   GOV2ADDR,
 } from '@agoric/synthetic-chain';
 import { AmountMath } from '@agoric/ertp';
-import { ceilMultiplyBy } from './ratio.js';
+import { ceilMultiplyBy, makeRatio } from './ratio.js';
 import { getAgoricNamesBrands, getAgoricNamesInstances } from './utils.js';
 import { boardSlottingMarshaller, makeFromBoard } from './rpc.js';
 import { retryUntilCondition } from './sync-tools.js';
@@ -100,10 +100,13 @@ export const calculateMintFee = async (toMintValue, vaultManager) => {
 
   const mintFee = governance.current.MintFee;
   const { numerator, denominator } = mintFee.value;
-  const mintFeeRatio = harden({
-    numerator: AmountMath.make(brands.IST, numerator.value),
-    denominator: AmountMath.make(brands.IST, denominator.value),
-  });
+
+  const mintFeeRatio = makeRatio(
+    numerator.value,
+    brands.IST,
+    denominator.value,
+    brands.IST,
+  );
 
   const toMintAmount = AmountMath.make(brands.IST, toMintValue * 1_000_000n);
   const expectedMintFee = ceilMultiplyBy(toMintAmount, mintFeeRatio);