never return negative convert rate

x/photon/keeper/grpc_query_test.go

@@ -5,7 +5,6 @@ import (
 
 	sdk "github.com/cosmos/cosmos-sdk/types"
 
-	"github.com/atomone-hub/atomone/x/photon/keeper"
 	"github.com/atomone-hub/atomone/x/photon/testutil"
 	"github.com/atomone-hub/atomone/x/photon/types"
 	"github.com/stretchr/testify/require"
@@ -23,17 +22,42 @@ func TestParamsQuery(t *testing.T) {
 }
 
 func TestConversionRateQuery(t *testing.T) {
-	k, m, ctx := testutil.SetupPhotonKeeper(t)
-	m.StakingKeeper.EXPECT().BondDenom(ctx).Return("uatone")
-	uatoneSupply := int64(100_000_000_000_000) // 100,000,000atone
-	m.BankKeeper.EXPECT().GetSupply(ctx, "uatone").Return(sdk.NewInt64Coin("uatone", uatoneSupply))
-	uphotonSupply := int64(100_000_000_000) // 100,000photon
-	m.BankKeeper.EXPECT().GetSupply(ctx, types.Denom).Return(sdk.NewInt64Coin("uatone", uphotonSupply))
-
-	resp, err := k.ConversionRate(ctx, &types.QueryConversionRateRequest{})
-
-	require.NoError(t, err)
-	expectedConversionRate := sdk.NewDec(keeper.UphotonMaxSupply - uphotonSupply).
-		QuoInt64(uatoneSupply).String()
-	require.Equal(t, expectedConversionRate, resp.ConversionRate)
+	tests := []struct {
+		name             string
+		uatoneSupply     int64
+		uphotonSupply    int64
+		expectedResponse *types.QueryConversionRateResponse
+	}{
+		{
+			name:          "nominal case",
+			uatoneSupply:  100_000_000_000_000, // 100,000,000atone
+			uphotonSupply: 100_000_000_000,     // 100,000photon
+			expectedResponse: &types.QueryConversionRateResponse{
+				ConversionRate: "9.9",
+			},
+		},
+		{
+			name:          "max supply of photon exceeded",
+			uatoneSupply:  100_000_000_000_000, // 100,000,000atone
+			uphotonSupply: types.MaxSupply + 1,
+			expectedResponse: &types.QueryConversionRateResponse{
+				ConversionRate: "0",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			k, m, ctx := testutil.SetupPhotonKeeper(t)
+			m.StakingKeeper.EXPECT().BondDenom(ctx).Return("uatone")
+			m.BankKeeper.EXPECT().GetSupply(ctx, "uatone").
+				Return(sdk.NewInt64Coin("uatone", tt.uatoneSupply))
+			m.BankKeeper.EXPECT().GetSupply(ctx, types.Denom).
+				Return(sdk.NewInt64Coin("uatone", tt.uphotonSupply))
+
+			resp, err := k.ConversionRate(ctx, &types.QueryConversionRateRequest{})
+
+			require.NoError(t, err)
+			require.Equal(t, tt.expectedResponse, resp)
+		})
+	}
 }

x/photon/keeper/keeper.go

@@ -48,5 +48,9 @@ func (k Keeper) Logger(ctx sdk.Context) log.Logger {
 // photon.
 func (k Keeper) conversionRate(_ sdk.Context, bondDenomSupply, uphotonSupply sdk.Dec) sdk.Dec {
 	remainMintableUphotons := sdk.NewDec(types.MaxSupply).Sub(uphotonSupply)
+	if remainMintableUphotons.IsNegative() {
+		// If for any reason the max supply is exceeded, avoid returning a negative number
+		return sdk.ZeroDec()
+	}
 	return remainMintableUphotons.Quo(bondDenomSupply)
 }