Fix TruncateDecimal (#3913)

.pending/bugfixes/sdk/3870-Fix-DecCoins-TruncateDecimal-to-never-return-zero-coins-in

@@ -0,0 +1,2 @@
+#3870 Fix DecCoins#TruncateDecimal to never return zero coins in
+either the truncated coins or the change coins.

types/dec_coin.go

@@ -114,7 +114,8 @@ func (coin DecCoin) Sub(coinB DecCoin) DecCoin {
 	return DecCoin{coin.Denom, coin.Amount.Sub(coinB.Amount)}
 }
 
-// return the decimal coins with trunctated decimals, and return the change
+// TruncateDecimal returns a Coin with a truncated decimal and a DecCoin for the
+// change. Note, the change may be zero.
 func (coin DecCoin) TruncateDecimal() (Coin, DecCoin) {
 	truncated := coin.Amount.TruncateInt()
 	change := coin.Amount.Sub(truncated.ToDec())
@@ -171,20 +172,20 @@ func (coins DecCoins) String() string {
 }
 
 // TruncateDecimal returns the coins with truncated decimals and returns the
-// change.
-func (coins DecCoins) TruncateDecimal() (Coins, DecCoins) {
-	changeSum := DecCoins{}
-	out := make(Coins, len(coins))
-
-	for i, coin := range coins {
+// change. Note, it will not return any zero-amount coins in either the truncated or
+// change coins.
+func (coins DecCoins) TruncateDecimal() (truncatedCoins Coins, changeCoins DecCoins) {
+	for _, coin := range coins {
 		truncated, change := coin.TruncateDecimal()
-		out[i] = truncated
+		if !truncated.IsZero() {
+			truncatedCoins = truncatedCoins.Add(Coins{truncated})
+		}
 		if !change.IsZero() {
-			changeSum = changeSum.Add(DecCoins{change})
+			changeCoins = changeCoins.Add(DecCoins{change})
 		}
 	}
 
-	return out, changeSum
+	return truncatedCoins, changeCoins
 }
 
 // Add adds two sets of DecCoins.

types/dec_coin_test.go

@@ -253,6 +253,40 @@ func TestDecCoinsIntersect(t *testing.T) {
 		require.NoError(t, err, "unexpected parse error in %v", i)
 
 		require.True(t, in1.Intersect(in2).IsEqual(exr), "in1.cap(in2) != exr in %v", i)
-		// require.Equal(t, tc.expectedResult, in1.Intersect(in2).String(), "in1.cap(in2) != exr in %v", i)
+	}
+}
+
+func TestDecCoinsTruncateDecimal(t *testing.T) {
+	decCoinA := NewDecCoinFromDec("bar", MustNewDecFromStr("5.41"))
+	decCoinB := NewDecCoinFromDec("foo", MustNewDecFromStr("6.00"))
+
+	testCases := []struct {
+		input          DecCoins
+		truncatedCoins Coins
+		changeCoins    DecCoins
+	}{
+		{DecCoins{}, Coins(nil), DecCoins(nil)},
+		{
+			DecCoins{decCoinA, decCoinB},
+			Coins{NewInt64Coin(decCoinA.Denom, 5), NewInt64Coin(decCoinB.Denom, 6)},
+			DecCoins{NewDecCoinFromDec(decCoinA.Denom, MustNewDecFromStr("0.41"))},
+		},
+		{
+			DecCoins{decCoinB},
+			Coins{NewInt64Coin(decCoinB.Denom, 6)},
+			DecCoins(nil),
+		},
+	}
+
+	for i, tc := range testCases {
+		truncatedCoins, changeCoins := tc.input.TruncateDecimal()
+		require.Equal(
+			t, tc.truncatedCoins, truncatedCoins,
+			"unexpected truncated coins; tc #%d, input: %s", i, tc.input,
+		)
+		require.Equal(
+			t, tc.changeCoins, changeCoins,
+			"unexpected change coins; tc #%d, input: %s", i, tc.input,
+		)
 	}
 }

types/decimal.go

@@ -332,6 +332,10 @@ func (d Dec) Format(s fmt.State, verb rune) {
 }
 
 func (d Dec) String() string {
+	if d.Int == nil {
+		return d.Int.String()
+	}
+
 	isNeg := d.IsNegative()
 	if d.IsNegative() {
 		d = d.Neg()
@@ -348,7 +352,6 @@ func (d Dec) String() string {
 	// TODO: Remove trailing zeros
 	// case 1, purely decimal
 	if inputSize <= Precision {
-
 		bzStr = make([]byte, Precision+2)
 
 		// 0. prefix
@@ -377,6 +380,7 @@ func (d Dec) String() string {
 	if isNeg {
 		return "-" + string(bzStr)
 	}
+
 	return string(bzStr)
 }