concept for capping ustc

proto/terra/market/v1beta1/market.proto

@@ -21,4 +21,9 @@ message Params {
     (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Dec",
     (gogoproto.nullable)   = false
   ];
+  bytes min_ustc_stability_spread = 4 [
+    (gogoproto.moretags)   = "yaml:\"min_ustc_stability_spread\"",
+    (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Dec",
+    (gogoproto.nullable)   = false
+  ];
 }

proto/terra/wasm/v1beta1/genesis.proto

@@ -15,11 +15,11 @@ message Model {
 // Code struct encompasses CodeInfo and CodeBytes
 message Code {
   LegacyCodeInfo code_info  = 1 [(gogoproto.nullable) = false];
-  bytes    code_bytes = 2;
+  bytes          code_bytes = 2;
 }
 
 // Contract struct encompasses ContractAddress, ContractInfo, and ContractState
 message Contract {
-  LegacyContractInfo   contract_info  = 1 [(gogoproto.nullable) = false];
-  repeated Model contract_store = 2 [(gogoproto.nullable) = false];
+  LegacyContractInfo contract_info  = 1 [(gogoproto.nullable) = false];
+  repeated Model     contract_store = 2 [(gogoproto.nullable) = false];
 }

x/market/keeper/params.go

@@ -19,6 +19,12 @@ func (k Keeper) MinStabilitySpread(ctx sdk.Context) (res sdk.Dec) {
 	return
 }
 
+// MinTerraStabilitySpread is the minimum spread applied to swaps to / from Terra.
+func (k Keeper) MinUstcStabilitySpread(ctx sdk.Context) (res sdk.Dec) {
+	k.paramSpace.Get(ctx, types.KeyMinUstcStabilitySpread, &res)
+	return
+}
+
 // PoolRecoveryPeriod is the period required to recover Terra&Luna Pools to the MintBasePool & BurnBasePool
 func (k Keeper) PoolRecoveryPeriod(ctx sdk.Context) (res uint64) {
 	k.paramSpace.Get(ctx, types.KeyPoolRecoveryPeriod, &res)

x/market/keeper/swap.go

@@ -89,6 +89,12 @@ func (k Keeper) ComputeSwap(ctx sdk.Context, offerCoin sdk.Coin, askDenom string
 		}
 
 		spread = tobinTax
+
+		// applied min ustc stability spread for swapping into ustc denom
+		if askDenom == core.MicroUSDDenom && spread.LT(k.MinUstcStabilitySpread(ctx)) {
+			spread = k.MinUstcStabilitySpread(ctx)
+		}
+
 		return retDecCoin, spread, nil
 	}
 

x/market/types/params.go

@@ -19,23 +19,27 @@ var (
 	KeyPoolRecoveryPeriod = []byte("PoolRecoveryPeriod")
 	// Min spread
 	KeyMinStabilitySpread = []byte("MinStabilitySpread")
+	// Min spread for USTC denom
+	KeyMinUstcStabilitySpread = []byte("MinUSTCStabilitySpread")
 )
 
 // Default parameter values
 var (
-	DefaultBasePool           = sdk.NewDec(1000000 * core.MicroUnit) // 1000,000sdr = 1000,000,000,000usdr
-	DefaultPoolRecoveryPeriod = core.BlocksPerDay                    // 14,400
-	DefaultMinStabilitySpread = sdk.NewDecWithPrec(2, 2)             // 2%
+	DefaultBasePool               = sdk.NewDec(1000000 * core.MicroUnit) // 1000,000sdr = 1000,000,000,000usdr
+	DefaultPoolRecoveryPeriod     = core.BlocksPerDay                    // 14,400
+	DefaultMinStabilitySpread     = sdk.NewDecWithPrec(2, 2)             // 2%
+	DefaultMinUstcStabilitySpread = sdk.ZeroDec()                        // 0%
 )
 
 var _ paramstypes.ParamSet = &Params{}
 
 // DefaultParams creates default market module parameters
 func DefaultParams() Params {
 	return Params{
-		BasePool:           DefaultBasePool,
-		PoolRecoveryPeriod: DefaultPoolRecoveryPeriod,
-		MinStabilitySpread: DefaultMinStabilitySpread,
+		BasePool:               DefaultBasePool,
+		PoolRecoveryPeriod:     DefaultPoolRecoveryPeriod,
+		MinStabilitySpread:     DefaultMinStabilitySpread,
+		MinUstcStabilitySpread: DefaultMinUstcStabilitySpread,
 	}
 }
 
@@ -57,6 +61,7 @@ func (p *Params) ParamSetPairs() paramstypes.ParamSetPairs {
 		paramstypes.NewParamSetPair(KeyBasePool, &p.BasePool, validateBasePool),
 		paramstypes.NewParamSetPair(KeyPoolRecoveryPeriod, &p.PoolRecoveryPeriod, validatePoolRecoveryPeriod),
 		paramstypes.NewParamSetPair(KeyMinStabilitySpread, &p.MinStabilitySpread, validateMinStabilitySpread),
+		paramstypes.NewParamSetPair(KeyMinUstcStabilitySpread, &p.MinUstcStabilitySpread, validateMinUstcStabilitySpread),
 	}
 }
 
@@ -117,3 +122,20 @@ func validateMinStabilitySpread(i interface{}) error {
 
 	return nil
 }
+
+func validateMinUstcStabilitySpread(i interface{}) error {
+	v, ok := i.(sdk.Dec)
+	if !ok {
+		return fmt.Errorf("invalid parameter type: %T", i)
+	}
+
+	if v.IsNegative() {
+		return fmt.Errorf("min spread must be positive or zero: %s", v)
+	}
+
+	if v.GT(sdk.OneDec()) {
+		return fmt.Errorf("min spread is too large: %s", v)
+	}
+
+	return nil
+}