util: implement FixedN from string

When working with NEP5 contracts we frequently need
to parse fixed-point decimals with arbitrary precision.

pkg/util/fixed8.go

@@ -3,6 +3,7 @@ package util
 import (
 	"encoding/json"
 	"errors"
+	"math"
 	"strconv"
 	"strings"
 
@@ -71,25 +72,36 @@ func Fixed8FromFloat(val float64) Fixed8 {
 // Fixed8FromString parses s which must be a fixed point number
 // with precision up to 10^-8
 func Fixed8FromString(s string) (Fixed8, error) {
+	num, err := FixedNFromString(s, precision)
+	if err != nil {
+		return 0, err
+	}
+	return Fixed8(num), err
+}
+
+// FixedNFromString parses s which must be a fixed point number
+// with precision 10^-d.
+func FixedNFromString(s string, precision int) (int64, error) {
 	parts := strings.SplitN(s, ".", 2)
+	d := int64(math.Pow10(precision))
 	ip, err := strconv.ParseInt(parts[0], 10, 64)
 	if err != nil {
 		return 0, errInvalidString
 	} else if len(parts) == 1 {
-		return Fixed8(ip * decimals), nil
+		return ip * d, nil
 	}
 
 	fp, err := strconv.ParseInt(parts[1], 10, 64)
-	if err != nil || fp >= decimals {
+	if err != nil || fp >= d {
 		return 0, errInvalidString
 	}
 	for i := len(parts[1]); i < precision; i++ {
 		fp *= 10
 	}
 	if ip < 0 {
-		return Fixed8(ip*decimals - fp), nil
+		return ip*d - fp, nil
 	}
-	return Fixed8(ip*decimals + fp), nil
+	return ip*d + fp, nil
 }
 
 // UnmarshalJSON implements the json unmarshaller interface.

pkg/util/fixed8_test.go

@@ -85,6 +85,20 @@ func TestFixed8FromString(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestFixedNFromString(t *testing.T) {
+	val := "123.456"
+	num, err := FixedNFromString(val, 3)
+	require.NoError(t, err)
+	require.EqualValues(t, 123456, num)
+
+	num, err = FixedNFromString(val, 4)
+	require.NoError(t, err)
+	require.EqualValues(t, 1234560, num)
+
+	_, err = FixedNFromString(val, 2)
+	require.Error(t, err)
+}
+
 func TestSatoshi(t *testing.T) {
 	satoshif8 := Satoshi()
 	assert.Equal(t, "0.00000001", satoshif8.String())