From 4516e5b6a2ab9176033956ee197687b5c6574647 Mon Sep 17 00:00:00 2001
From: Chris Hibbert <hibbert@agoric.com>
Date: Wed, 9 Feb 2022 17:52:46 -0800
Subject: [PATCH] fix: when trades for zero are requested don't throw

I thought no one needed to request zero prices, and so I threw in that
case.  https://github.com/agoric/agoric-sdk/issues/4114 reveals a
situation where it will happen in ordinary circumstances.

Change the AMM code to respond with an empty amount when an empty
amount is offered, and vice versa.
---
 .../src/vpool-xyk-amm/constantProduct/swap.js |  8 ++++----
 .../constantProduct/test-checkInvariants.js   | 20 +++++++++++++------
 .../test-compareBondingCurves.js              | 17 +++++++++++++---
 3 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/packages/run-protocol/src/vpool-xyk-amm/constantProduct/swap.js b/packages/run-protocol/src/vpool-xyk-amm/constantProduct/swap.js
index 64111e0b7d1..8aa22402b42 100644
--- a/packages/run-protocol/src/vpool-xyk-amm/constantProduct/swap.js
+++ b/packages/run-protocol/src/vpool-xyk-amm/constantProduct/swap.js
@@ -189,10 +189,10 @@ export const swap = (
   );
   assertGreaterThanZero(poolAllocation.Central, 'poolAllocation.Central');
   assertGreaterThanZero(poolAllocation.Secondary, 'poolAllocation.Secondary');
-  assert(
-    isGreaterThanZero(amountGiven) || isGreaterThanZero(amountWanted),
-    X`amountGiven or amountWanted must be greater than 0: ${amountWanted} ${amountGiven}`,
-  );
+
+  if (!isGreaterThanZero(amountGiven) && !isGreaterThanZero(amountWanted)) {
+    return noTransaction;
+  }
 
   if (!isWantedAvailable(poolAllocation, amountWanted)) {
     return noTransaction;
diff --git a/packages/run-protocol/test/amm/constantProduct/test-checkInvariants.js b/packages/run-protocol/test/amm/constantProduct/test-checkInvariants.js
index 73880770145..a34885a58c1 100644
--- a/packages/run-protocol/test/amm/constantProduct/test-checkInvariants.js
+++ b/packages/run-protocol/test/amm/constantProduct/test-checkInvariants.js
@@ -308,13 +308,21 @@ test('getInputPrice zero input', t => {
     inputReserve: 320n,
     outputReserve: 50n,
     inputValue: 0n,
+    outputValue: 0n,
+  };
+  testGetInputPriceNoTrade(t, input, true);
+  testGetInputPriceNoTrade(t, input, false);
+});
+
+test('getOutputPrice zero output', t => {
+  const input = {
+    inputReserve: 320n,
+    outputReserve: 50n,
+    inputValue: 0n,
+    outputValue: 0n,
   };
-  const messageA =
-    'amountGiven or amountWanted must be greater than 0: {"brand":"[Alleged: BLD brand]","value":"[0n]"} {"brand":"[Alleged: RUN brand]","value":"[0n]"}';
-  testGetInputPriceThrows(t, input, messageA, true);
-  const messageB =
-    'amountGiven or amountWanted must be greater than 0: {"brand":"[Alleged: RUN brand]","value":"[0n]"} {"brand":"[Alleged: BLD brand]","value":"[0n]"}';
-  testGetInputPriceThrows(t, input, messageB, false);
+  testGetOutputPriceNoTrade(t, input, true);
+  testGetOutputPriceNoTrade(t, input, false);
 });
 
 test('getInputPrice big product', t => {
diff --git a/packages/run-protocol/test/amm/constantProduct/test-compareBondingCurves.js b/packages/run-protocol/test/amm/constantProduct/test-compareBondingCurves.js
index 98f176eaf8c..711dd3e9b92 100644
--- a/packages/run-protocol/test/amm/constantProduct/test-compareBondingCurves.js
+++ b/packages/run-protocol/test/amm/constantProduct/test-compareBondingCurves.js
@@ -199,9 +199,20 @@ test('getInputPrice zero input', t => {
     outputReserve: 50n,
     inputValue: 0n,
   };
-  const message =
-    'amountGiven or amountWanted must be greater than 0: {"brand":"[Alleged: BLD brand]","value":"[0n]"} {"brand":"[Alleged: RUN brand]","value":"[0n]"}';
-  getInputPriceThrows(t, input, message);
+
+  const expectedOutput = 0n;
+  testInputGetPrice(t, input, expectedOutput);
+});
+
+test('getOutputPrice zero output', t => {
+  const input = {
+    inputReserve: 320n,
+    outputReserve: 50n,
+    outputValue: 0n,
+  };
+
+  const expectedInput = 0n;
+  testGetOutputPrice(t, input, expectedInput);
 });
 
 test('getInputPrice big product', t => {