Reintroduce #1719 random tests (#1791)

* Add joon's random tests

* Move file its in

* Use ctx for v10 state machine compatibility

* Change bounds, comment on rounding behavior

Co-authored-by: Dev Ojha <dojha@berkeley.edu>
Co-authored-by: Dev Ojha <ValarDragon@users.noreply.github.com>

x/gamm/pool-models/balancer/amm_joinpool_test.go

@@ -3,7 +3,9 @@ package balancer_test
 import (
 	"errors"
 	"fmt"
+	"math/rand"
 	"testing"
+	time "time"
 
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
@@ -1004,3 +1006,110 @@ func TestGetPoolAssetsByDenom(t *testing.T) {
 		})
 	}
 }
+
+// Tests selecting a random amount of coins to LP, and then that ExitPool(JoinPool(tokens))
+// preserves the pools number of LP shares, and returns fewer coins to the acter than they started with.
+func (suite *KeeperTestSuite) TestRandomizedJoinPoolExitPoolInvariants() {
+	type testCase struct {
+		initialTokensDenomIn  int64
+		initialTokensDenomOut int64
+
+		percentRatio int64
+
+		numShares sdk.Int
+	}
+
+	const (
+		denomOut = "denomOut"
+		denomIn  = "denomIn"
+	)
+
+	now := int64(time.Now().Unix())
+	rng := rand.NewSource(now)
+	suite.T().Logf("Using random source of %d\n", now)
+
+	// generate test case with randomized initial assets and join/exit ratio
+	newCase := func() (tc *testCase) {
+		tc = new(testCase)
+		tc.initialTokensDenomIn = rng.Int63() % (1 << 62)
+		tc.initialTokensDenomOut = rng.Int63() % (1 << 62)
+
+		// 1%~100% of initial assets
+		tc.percentRatio = rng.Int63()%100 + 1
+
+		return tc
+	}
+
+	swapFeeDec := sdk.ZeroDec()
+	exitFeeDec := sdk.ZeroDec()
+
+	// create pool with randomized initial token amounts
+	// and randomized ratio of join/exit
+	createPool := func(tc *testCase) (pool *balancer.Pool) {
+		poolAssetOut := balancer.PoolAsset{
+			Token:  sdk.NewInt64Coin(denomOut, tc.initialTokensDenomOut),
+			Weight: sdk.NewInt(5),
+		}
+
+		poolAssetIn := balancer.PoolAsset{
+			Token:  sdk.NewInt64Coin(denomIn, tc.initialTokensDenomIn),
+			Weight: sdk.NewInt(5),
+		}
+
+		pool = createTestPool(suite.T(), swapFeeDec, exitFeeDec, poolAssetOut, poolAssetIn).(*balancer.Pool)
+		suite.Require().NotNil(pool)
+
+		return pool
+	}
+
+	// joins with predetermined ratio
+	joinPool := func(pool types.PoolI, tc *testCase) {
+		tokensIn := sdk.Coins{
+			sdk.NewCoin(denomIn, sdk.NewInt(tc.initialTokensDenomIn).MulRaw(tc.percentRatio).QuoRaw(100)),
+			sdk.NewCoin(denomOut, sdk.NewInt(tc.initialTokensDenomOut).MulRaw(tc.percentRatio).QuoRaw(100)),
+		}
+		numShares, err := pool.JoinPool(suite.Ctx, tokensIn, swapFeeDec)
+		suite.Require().NoError(err)
+		tc.numShares = numShares
+	}
+
+	// exits for same amount of shares minted
+	exitPool := func(pool types.PoolI, tc *testCase) {
+		_, err := pool.ExitPool(suite.Ctx, tc.numShares, exitFeeDec)
+		suite.Require().NoError(err)
+	}
+
+	invariantJoinExitInversePreserve := func(
+		beforeCoins, afterCoins sdk.Coins,
+		beforeShares, afterShares sdk.Int,
+	) {
+		// test token amount has been preserved
+		suite.Require().True(
+			!beforeCoins.IsAnyGT(afterCoins),
+			"Coins has not been preserved before and after join-exit\nbefore:\t%s\nafter:\t%s",
+			beforeCoins, afterCoins,
+		)
+		// test share amount has been preserved
+		suite.Require().True(
+			beforeShares.Equal(afterShares),
+			"Shares has not been preserved before and after join-exit\nbefore:\t%s\nafter:\t%s",
+			beforeShares, afterShares,
+		)
+	}
+
+	testPoolInvariants := func() {
+		tc := newCase()
+		pool := createPool(tc)
+		originalCoins, originalShares := pool.GetTotalPoolLiquidity(sdk.Context{}), pool.GetTotalShares()
+		joinPool(pool, tc)
+		exitPool(pool, tc)
+		invariantJoinExitInversePreserve(
+			originalCoins, pool.GetTotalPoolLiquidity(sdk.Context{}),
+			originalShares, pool.GetTotalShares(),
+		)
+	}
+
+	for i := 0; i < 50000; i++ {
+		testPoolInvariants()
+	}
+}

x/gamm/pool-models/internal/cfmm_common/lp.go

@@ -110,6 +110,9 @@ func MaximalExactRatioJoin(p types.PoolI, ctx sdk.Context, tokensIn sdk.Coins) (
 	totalShares := p.GetTotalShares()
 
 	for i, coin := range tokensIn {
+		// Note: QuoInt implements floor division, unlike Quo
+		// This is because it calls the native golang routine big.Int.Quo
+		// https://pkg.go.dev/math/big#Int.Quo
 		shareRatio := coin.Amount.ToDec().QuoInt(poolLiquidity.AmountOfNoDenomValidation(coin.Denom))
 		if shareRatio.LT(minShareRatio) {
 			minShareRatio = shareRatio
@@ -125,6 +128,8 @@ func MaximalExactRatioJoin(p types.PoolI, ctx sdk.Context, tokensIn sdk.Coins) (
 	}
 
 	remCoins = sdk.Coins{}
+	// critically we round down here (TruncateInt), to ensure that the returned LP shares
+	// are always less than or equal to % liquidity added.
 	numShares = minShareRatio.MulInt(totalShares).TruncateInt()
 
 	// if we have multiple share values, calculate remainingCoins