Merge PR #2408: decimal: Add a method to multiply a decimal by an Int

This is for usage within the slash function, to minimize operations
required to slash each unbonded and redelegating account.

PENDING.md

@@ -44,7 +44,8 @@ BREAKING CHANGES
     * [core] [\#1807](https://github.com/cosmos/cosmos-sdk/issues/1807) Switch from use of rational to decimal
     * [types] [\#1901](https://github.com/cosmos/cosmos-sdk/issues/1901) Validator interface's GetOwner() renamed to GetOperator()
     * [x/slashing] [#2122](https://github.com/cosmos/cosmos-sdk/pull/2122) - Implement slashing period
-    * [types] [\#2119](https://github.com/cosmos/cosmos-sdk/issues/2119) Parsed error messages and ABCI log errors to make them more human readable.
+    * [types] [\#2119](https://github.com/cosmos/cosmos-sdk/issues/2119) Parsed error messages and ABCI log errors to make     them more human readable.
+    * [types] \#2407 MulInt method added to big decimal in order to improve efficiency of slashing
     * [simulation] Rename TestAndRunTx to Operation [#2153](https://github.com/cosmos/cosmos-sdk/pull/2153)
     * [simulation] Remove log and testing.TB from Operation and Invariants, in favor of using errors \#2282
     * [simulation] Remove usage of keys and addrs in the types, in favor of simulation.Account \#2384

types/decimal.go

@@ -215,6 +215,16 @@ func (d Dec) Mul(d2 Dec) Dec {
 	return Dec{chopped}
 }
 
+// multiplication
+func (d Dec) MulInt(i Int) Dec {
+	mul := new(big.Int).Mul(d.Int, i.i)
+
+	if mul.BitLen() > 255+DecimalPrecisionBits {
+		panic("Int overflow")
+	}
+	return Dec{mul}
+}
+
 // quotient
 func (d Dec) Quo(d2 Dec) Dec {
 

types/decimal_test.go

@@ -325,3 +325,20 @@ func TestStringOverflow(t *testing.T) {
 		dec3.String(),
 	)
 }
+
+func TestDecMulInt(t *testing.T) {
+	tests := []struct {
+		sdkDec Dec
+		sdkInt Int
+		want   Dec
+	}{
+		{NewDec(10), NewInt(2), NewDec(20)},
+		{NewDec(1000000), NewInt(100), NewDec(100000000)},
+		{NewDecWithPrec(1, 1), NewInt(10), NewDec(1)},
+		{NewDecWithPrec(1, 5), NewInt(20), NewDecWithPrec(2, 4)},
+	}
+	for i, tc := range tests {
+		got := tc.sdkDec.MulInt(tc.sdkInt)
+		require.Equal(t, tc.want, got, "Incorrect result on test case %d", i)
+	}
+}

x/stake/keeper/slash.go

@@ -172,7 +172,7 @@ func (k Keeper) slashUnbondingDelegation(ctx sdk.Context, unbondingDelegation ty
 	}
 
 	// Calculate slash amount proportional to stake contributing to infraction
-	slashAmount = sdk.NewDecFromInt(unbondingDelegation.InitialBalance.Amount).Mul(slashFactor)
+	slashAmount = slashFactor.MulInt(unbondingDelegation.InitialBalance.Amount)
 
 	// Don't slash more tokens than held
 	// Possible since the unbonding delegation may already
@@ -218,7 +218,7 @@ func (k Keeper) slashRedelegation(ctx sdk.Context, validator types.Validator, re
 	}
 
 	// Calculate slash amount proportional to stake contributing to infraction
-	slashAmount = sdk.NewDecFromInt(redelegation.InitialBalance.Amount).Mul(slashFactor)
+	slashAmount = slashFactor.MulInt(redelegation.InitialBalance.Amount)
 
 	// Don't slash more tokens than held
 	// Possible since the redelegation may already