From cbb5b5586370c89e334ea85e54c8c7259e38795d Mon Sep 17 00:00:00 2001
From: Chris Hibbert <hibbert@agoric.com>
Date: Mon, 19 Aug 2024 14:18:30 -0700
Subject: [PATCH] feat: ratio.quantize() shouldn't increase precision
 unnecessarily

---
 packages/zoe/src/contractSupport/ratio.js     |  4 ++
 .../unitTests/contractSupport/ratio.test.js   | 41 ++++++++++++++-----
 2 files changed, 35 insertions(+), 10 deletions(-)

diff --git a/packages/zoe/src/contractSupport/ratio.js b/packages/zoe/src/contractSupport/ratio.js
index 46e0281c04da..99ff95734532 100644
--- a/packages/zoe/src/contractSupport/ratio.js
+++ b/packages/zoe/src/contractSupport/ratio.js
@@ -352,6 +352,10 @@ export const ratiosSame = (left, right) => {
 export const quantize = (ratio, newDen) => {
   const oldDen = ratio.denominator.value;
   const oldNum = ratio.numerator.value;
+  if (oldDen < newDen) {
+    return ratio;
+  }
+
   const newNum =
     newDen === oldDen ? oldNum : bankersDivide(oldNum * newDen, oldDen);
   return makeRatio(
diff --git a/packages/zoe/test/unitTests/contractSupport/ratio.test.js b/packages/zoe/test/unitTests/contractSupport/ratio.test.js
index 62b9ce9e56d6..8a4632b5fc5c 100644
--- a/packages/zoe/test/unitTests/contractSupport/ratio.test.js
+++ b/packages/zoe/test/unitTests/contractSupport/ratio.test.js
@@ -501,25 +501,25 @@ const { brand } = makeIssuerKit('moe');
 
 test('ratio - quantize', t => {
   /** @type {Array<[numBefore: bigint, denBefore: bigint, numAfter: bigint, denAfter: bigint]>} */
-  const cases = /** @type {const} */ [
+  const cases = [
     [1n, 1n, 1n, 1n],
     [10n, 10n, 10n, 10n],
     [2n * 10n ** 9n, 1n * 10n ** 9n, 20n, 10n],
 
-    [12345n, 12345n, 100n, 100n],
-    [12345n, 12345n, 100000n, 100000n],
-    [12345n, 12345n, 10n ** 15n, 10n ** 15n],
-
-    [12345n, 123n, 100365854n, 10n ** 6n],
-    [12345n, 123n, 10036585n, 10n ** 5n],
-    [12345n, 123n, 1003659n, 10n ** 4n],
-    [12345n, 123n, 100366n, 10n ** 3n],
     [12345n, 123n, 10037n, 10n ** 2n],
     [12345n, 123n, 1004n, 10n ** 1n],
     [12345n, 123n, 100n, 10n ** 0n],
+
+    [12345n, 12345n, 100n, 100n],
   ];
 
-  for (const [numBefore, denBefore, numAfter, denAfter] of cases) {
+  for (const [
+    numBefore,
+    denBefore,
+    numAfter,
+    target,
+    denAfter = target,
+  ] of cases) {
     const before = makeRatio(numBefore, brand, denBefore, brand);
     const after = makeRatio(numAfter, brand, denAfter, brand);
     t.deepEqual(
@@ -530,6 +530,27 @@ test('ratio - quantize', t => {
   }
 });
 
+test('ratio - quantize - leave it alone', t => {
+  const cases = [
+    [12345n, 123n, 10n ** 5n, 12345n, 123n],
+    [12345n, 123n, 10n ** 4n, 12345n, 123n],
+    [12345n, 123n, 10n ** 3n, 12345n, 123n],
+
+    [12345n, 12345n, 100_000n, 12345n, 12345n],
+    [12345n, 12345n, 10n ** 15n, 12345n, 12345n],
+  ];
+
+  for (const [numPre, denPre, qTarget, numAfter, denAfter] of cases) {
+    const before = makeRatio(numPre, brand, denPre, brand);
+    const after = makeRatio(numAfter, brand, denAfter, brand);
+    t.deepEqual(
+      quantize(before, qTarget),
+      after,
+      `${numPre}/${denPre} quantized to ${qTarget} should be ${numAfter}/${denAfter}`,
+    );
+  }
+});
+
 test('ratio - parse', t => {
   const { brand: moeBrand } = makeIssuerKit('moe');
   const { brand: larryBrand } = makeIssuerKit('larry');