go/staking/api: Use go/common/prettyprint.FractionBase10()

Make ConvertToTokenAmount() and PrettyPrintCommissionRatePercentage()
use the newly introduced FractionBase10 function.

Add tests for PrettyPrintCommissionRatePercentage() and move it to
go/staking/api/prettyprint.go.

go/staking/api/commission.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"io"
 	"math/big"
-	"strconv"
 
 	"github.com/oasisprotocol/oasis-core/go/common/prettyprint"
 	"github.com/oasisprotocol/oasis-core/go/common/quantity"
@@ -28,17 +27,6 @@ var (
 	_ prettyprint.PrettyPrinter = (*CommissionSchedule)(nil)
 )
 
-// CommissionRatePercentage returns the string representing the commission rate
-// in percentage for the given commission rate numerator.
-func CommissionRatePercentage(rateNumerator quantity.Quantity) string {
-	rate := big.NewRat(rateNumerator.ToBigInt().Int64(), CommissionRateDenominator.ToBigInt().Int64())
-	// Multiply rate by 100 to convert it to percentage.
-	rate.Mul(rate, big.NewRat(100, 1))
-	// Return string representation of the rate that omits the trailing zeros.
-	rateFloat, _ := rate.Float64()
-	return strconv.FormatFloat(rateFloat, 'f', -1, 64)
-}
-
 // CommissionScheduleRules controls how commission schedule rates and rate
 // bounds are allowed to be changed.
 type CommissionScheduleRules struct {
@@ -66,7 +54,7 @@ type CommissionRateStep struct {
 func (crs CommissionRateStep) PrettyPrint(ctx context.Context, prefix string, w io.Writer) {
 	fmt.Fprintf(w, "%s- Start: epoch %d\n", prefix, crs.Start)
 
-	fmt.Fprintf(w, "%s  Rate:  %s %%\n", prefix, CommissionRatePercentage(crs.Rate))
+	fmt.Fprintf(w, "%s  Rate:  %s\n", prefix, PrettyPrintCommissionRatePercentage(crs.Rate))
 }
 
 // PrettyType returns a representation of CommissionRateStep that can be used
@@ -91,8 +79,8 @@ type CommissionRateBoundStep struct {
 func (crbs CommissionRateBoundStep) PrettyPrint(ctx context.Context, prefix string, w io.Writer) {
 	fmt.Fprintf(w, "%s- Start:        epoch %d\n", prefix, crbs.Start)
 
-	fmt.Fprintf(w, "%s  Minimum Rate: %s %%\n", prefix, CommissionRatePercentage(crbs.RateMin))
-	fmt.Fprintf(w, "%s  Maximum Rate: %s %%\n", prefix, CommissionRatePercentage(crbs.RateMax))
+	fmt.Fprintf(w, "%s  Minimum Rate: %s\n", prefix, PrettyPrintCommissionRatePercentage(crbs.RateMin))
+	fmt.Fprintf(w, "%s  Maximum Rate: %s\n", prefix, PrettyPrintCommissionRatePercentage(crbs.RateMax))
 }
 
 // PrettyType returns a representation of CommissionRateBoundStep that can be

go/staking/api/prettyprint.go

@@ -4,9 +4,8 @@ import (
 	"context"
 	"fmt"
 	"io"
-	"math/big"
-	"strings"
 
+	"github.com/oasisprotocol/oasis-core/go/common/prettyprint"
 	"github.com/oasisprotocol/oasis-core/go/common/quantity"
 )
 
@@ -29,24 +28,7 @@ func ConvertToTokenAmount(amount quantity.Quantity, tokenValueExponent uint8) (s
 		return "", ErrInvalidTokenValueExponent
 	}
 
-	divisor := new(big.Int).Exp(big.NewInt(10), big.NewInt(int64(tokenValueExponent)), nil)
-
-	// NOTE: We use DivMod() and manual string construction to avoid conversion
-	// to other types and support arbitrarily large amounts.
-	var quotient, remainder *big.Int
-	quotient, remainder = new(big.Int).DivMod(amount.ToBigInt(), divisor, new(big.Int))
-
-	// Prefix the remainder with the appropriate number of zeros.
-	remainderStr := fmt.Sprintf("%0*s", tokenValueExponent, remainder)
-	// Trim trailing zeros from the remainder.
-	remainderStr = strings.TrimRight(remainderStr, "0")
-	// Ensure remainder is not empty.
-	if remainderStr == "" {
-		remainderStr = "0"
-	}
-
-	// Combine quotient and remainder to a string representing the token amount.
-	return fmt.Sprintf("%s.%s", quotient, remainderStr), nil
+	return prettyprint.FractionBase10(amount, tokenValueExponent), nil
 }
 
 // PrettyPrintAmount writes a pretty-printed representation of the given amount
@@ -82,3 +64,17 @@ func PrettyPrintAmount(ctx context.Context, amount quantity.Quantity, w io.Write
 		fmt.Fprintf(w, "%s %s", symbol, tokenAmount)
 	}
 }
+
+// PrettyPrintCommissionRatePercentage returns the string representing the
+// commission rate (bound) in percentage for the given commission rate (bound)
+// numerator.
+func PrettyPrintCommissionRatePercentage(rateNumerator quantity.Quantity) string {
+	// Handle invalid commission rate (bound) numerator.
+	if rateNumerator.Cmp(CommissionRateDenominator) > 0 {
+		return fmt.Sprintf("(invalid)")
+	}
+	// Reduce commission rate denominator's base-10 exponent by 2 to obtain the
+	// value in percentage.
+	denominatorExp := commissionRateDenominatorExponent - 2
+	return fmt.Sprintf("%s%%", prettyprint.FractionBase10(rateNumerator, denominatorExp))
+}

go/staking/api/prettyprint_test.go

@@ -89,3 +89,25 @@ func TestPrettyPrintAmount(t *testing.T) {
 			"pretty printing stake amount didn't return the expected result")
 	}
 }
+
+func TestPrettyPrintCommissionRatePercentage(t *testing.T) {
+	require := require.New(t)
+
+	for _, t := range []struct {
+		expectedRate  string
+		rateNumerator *quantity.Quantity
+	}{
+		{"0.0%", quantity.NewFromUint64(0)},
+		{"50.0%", quantity.NewFromUint64(50_000)},
+		{"100.0%", quantity.NewFromUint64(100_000)},
+		{"20.2%", quantity.NewFromUint64(20_200)},
+		{"30.03%", quantity.NewFromUint64(30_030)},
+		{"12.345%", quantity.NewFromUint64(12_345)},
+		// Checks for invalid commission rate numerators.
+		{"(invalid)", quantity.NewFromUint64(100_001)},
+		{"(invalid)", quantity.NewFromUint64(123_456)},
+	} {
+		rate := PrettyPrintCommissionRatePercentage(*t.rateNumerator)
+		require.Equal(t.expectedRate, rate, "obtained pretty print didn't match expected value")
+	}
+}