fixedn: allow to parse big decimals

pkg/encoding/fixedn/decimal.go

@@ -0,0 +1,80 @@
+package fixedn
+
+import (
+	"errors"
+	"fmt"
+	"math/big"
+	"strconv"
+	"strings"
+)
+
+const maxAllowedPrecision = 16
+
+// ErrInvalidFormat is returned when decimal format is invalid.
+var ErrInvalidFormat = errors.New("invalid decimal format")
+
+var _pow10 []*big.Int
+
+func init() {
+	var p = int64(1)
+	for i := 0; i <= maxAllowedPrecision; i++ {
+		_pow10 = append(_pow10, big.NewInt(p))
+		p *= 10
+	}
+}
+
+func pow10(n int) *big.Int {
+	last := len(_pow10) - 1
+	if n <= last {
+		return _pow10[n]
+	}
+	p := new(big.Int)
+	p.Mul(_pow10[last], _pow10[1])
+	for i := last + 1; i < n; i++ {
+		p.Mul(p, _pow10[1])
+	}
+	return p
+}
+
+// ToString converts big decimal with specified precision to string.
+func ToString(bi *big.Int, precision int) string {
+	var dp, fp big.Int
+	dp.QuoRem(bi, pow10(precision), &fp)
+
+	var s = dp.String()
+	if fp.Sign() == 0 {
+		return s
+	}
+	frac := fp.Uint64()
+	trimmed := 0
+	for ; frac%10 == 0; frac /= 10 {
+		trimmed++
+	}
+	return s + "." + fmt.Sprintf("%0"+strconv.FormatUint(uint64(precision-trimmed), 10)+"d", frac)
+}
+
+// FromString converts string to a big decimal with specified precision.
+func FromString(s string, precision int) (*big.Int, error) {
+	parts := strings.SplitN(s, ".", 2)
+	bi, ok := new(big.Int).SetString(parts[0], 10)
+	if !ok {
+		return nil, ErrInvalidFormat
+	}
+	bi.Mul(bi, pow10(precision))
+	if len(parts) == 1 {
+		return bi, nil
+	}
+
+	if len(parts[1]) > precision {
+		return nil, ErrInvalidFormat
+	}
+	fp, ok := new(big.Int).SetString(parts[1], 10)
+	if !ok {
+		return nil, ErrInvalidFormat
+	}
+	fp.Mul(fp, pow10(precision-len(parts[1])))
+	if bi.Sign() == -1 {
+		return bi.Sub(bi, fp), nil
+	}
+	return bi.Add(bi, fp), nil
+}